Isolation passageway including annular region

A passageway which includes an annular region, the passageway adapted to isolate the gaseous contents of one of a pair of adjacent, vacuumized environments from the other of the pair while providing for the movement of a substrate therebetween.

 

We claim:

1. An isolation passageway for substantially preventing the diffusion of gases from the first of a pair of adjacent vacuumized environments into the second of said pair of vacuumized environments, said first environment differing from the second by the presence of at least one element:

said passageway:

(1) defined by closely spaced walls;

(2) adapted to provide for the movement of a substrate therethrough along a path interconnecting said environments;

(3) being substantially annular in a cross-section of a central region of said passageway, said cross-section taken in a plane extending in the direction of said path; one boundary of the annular region being formed by a cylindrical drum;

(4) adapted to maintain at least a 10.sup.4 ratio of the concentration of said at least one element in said first environment as compared to the concentration thereof in said second environment;

(5) including means for urging one surface of the substrate traveling through the passageway into contact with the circumferential surface of said drum; and

(6) the circumferential surface of said drum having a plurality of circumferential grooves spacedly positioned across the longitudinal extent thereof.

2. A passageway as in claim 1, wherein the circumferential surface of said drum which contacts the surface of the substrate is fabricated from a low friction, low thermal conductivity material.

3. A passageway as in claim 2, wherein said circumferential surface of said drum is formd of borosilicate glass.

4. A passageway as in claim 3, wherein the substrate is formed from a magnetic material and the substrate is urged into contact with the glass by magnetic attraction.

5. A passageway as in claim 1, wherein each of the adjacent environments is developed and maintained in a dedicated chamber, each chamber adapted to deposit thim film layers of semiconductor alloy material onto the substrate.

6. A passageway as in claim 1, further including means for vacuumizing the environments to a pressure of about 0.25 to 1 torr.

7. A passageway as in claim 1, further including roller means for urging the substrate into contact with said circumferential surface.

8. A passageway as in claim 1, further including means for introducing sweep gas into a first flow channel formed between the grooves and the substrate.

9. A passageway as in claim 8, wherein the introducing means is adapted to initiate and sustain a flow of sweep gas at a velocity sufficient to substantially prevent the diffusion of process gas from the first environment to the second environment through said first channel.

10. A passageway as in claim 9, wherein the introducing means comprises a source of relatively inert gas and an introductory manifold; and said first channel includes aperture means for receiving sweep gas from said introductory manifold.

11. A passageway as in claim 8, further including means for introducing sweep gas into a second flow channel formed between the substrate and the surface of the passageway opposite said circumferential surface.

12. A passageway as in claim 11, wherein the introducing means is adapted to initiate and sustain a flow of sweep gas through the second channel of the passageway at a velocity sufficient to substantially prevent the diffusion of process gas from the first to the second environment through said second channel.

13. A passageway as in claim 12, wherein the introducing means comprises a source of relatively inert gas and an introductory manifold; and said second channel includes aperture means for receiving sweep gas from said introductory manifold.

14. A passageway as in claim 1, further including means for subjecting the surface of the substrate not urged into contact with said circumferential surface to a plasma as said substrate moves through said passageway.

15. A passageway as in claim 14, wherein the first environment is a first chamber adapted for the deposition of a first layer of semiconductor alloy material and the second environment is a second chamber adapted for the deposition of a second layer of semiconductor alloy material.

16. A passageway as in claim 15, wherein the subjecting means is adapted to develop and sustain a hydrogen plasma for capping the surface of the previously deposited layer of semiconductor alloy material.

17. A passageway as in claim 16, wherein the subjecting means includes a cathode operatively coupled to an r.f. power supply for developing said plasma.

18. A passageway as in claim 16, wherein the subjecting means includes an antenna operatively coupled to a source of microwave energy for developing said plasma.

19. A passageway as in claim 1, wherein the ends of the cylindrical drum are leak-proofed by an annularly-shaped end seal.

20. A passageway as in claim 19, wherein each of the end seals include a pair of spaced O-rings.

21. A passageway as in claim 20, wherein a pump is provided to evacuate the space between each of the O-rings.

FIELD OF THE INVENTION

This invention relates generally to apparatus adapted to isolate a pair of adjacent environments from one another and more particularly to an improved isolation passageway operatively interconnecting adjacent chambers, at least one chamber of which is adapted to deposit a layer of thin film material in such a manner as to substantially eliminate contamination of the gaseous atmosphere present in one chamber caused by the diffusion of gases from the gaseous atmosphere present in the adjacent chamber.

BACKGROUND OF THE INVENTION

In its most specific embodiment, this invention relates to apparatus specially adapted to produce semiconductor devices on a continuously moving substrate by depositing successive layers of thin film semiconductor alloy material in each of at least two adjacent interconnected deposition chambers. The composition of each layer of said thin film material is dependent upon the particular reaction gas constituents introduced into each of the deposition chambers. While the constituents introduced into the first deposition chamber are carefully controlled and isolated from the constituents introduced into the adjacent deposition chamber, the apparatus must be constructed so as to provide for the continuous passage of said substrate between those chambers. Therefore, the deposition chambers are designed to be operatively interconnected by a relatively narrow passageway (1) through which the substrate may continuously pass and (2) adapted to isolate the reaction gas constituents introduced into the first deposition chamber from the reaction gas constituents introduced into the adjacent deposition chamber.

Applicants' assignee has invented and patented "gas gates" such as those disclosed in U.S. Pat. Nos. 4,438,724 and 4,450,786, which gas gates were operatively designed to prevent dopant gas constituents introduced into a first deposition chamber from diffusing into an adjacent second deposition chamber, thereby contaminating the layer of intrinsic semiconductor alloy material deposited in said second deposition chamber. It is therefore one important feature of the present invention to reduce the size of the isolation passageway of prior art gas gates so as to correspondingly reduce the diffusion of dopant gas constituents present in the dopant gaseous environment from contaminating the intrinsic gas constituents present in the intrinsic gaseous environment.

Another and equally important feature of this disclosure will become apparent from the description presented in the following paragraphs. It is to be noted that the assignee of the subject invention is recognized as the world leader in photovoltaic technology. Photovoltaic devices produced by said assignee have set world records for photoconversion efficiency and long term stablility under operating conditions (the efficiency and stability considerations will be discussed in greater detail hereinbelow). Additionally, said assignee has developed commercial processes for the continuous roll-to-roll manufacture of large area photovoltaic devices. Such continuous processing systems are disclosed in the following U.S. patents, disclosures of which are incorporated herein by reference: No. 4,400,409, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; No. 4,410,588, for Continuous Amorphous Solar Cell Production Systems; and No. 4,438,723, for Multiple Chamber Deposition and Isolation System And Method. As disclosed in these patents, a web of substrate material may be continuously advanced through a succession of operatively interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific layer of semiconductor alloy material onto the web or onto a previously deposited layer. In making a photovoltaic device, for instance, of n-i-p type configurations, the first chamber is dedicated to the deposition of a layer of an n-type semiconductor alloy material, the second chamber is dedicated to the deposition of a layer of substantially intrinsic amorphous semiconductor alloy material, and the third chamber is dedicated to the deposition of a layer of a p-type semiconductor alloy material. The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to, photovoltaic devices which include one or more cascaded n-i-p type cells. By making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained. Note, that as used herein the term "n-i-p type" will refer to any sequence of n and p or n, i and p layers of semiconductor alloy material operatively disposed and successively deposited to form a photoactive region wherein charge carriers are generated by the absorbtion of photons from incident radiation.

The concept of utlizing multiple stacked cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed were limited to the utilization of p-n junctions formed by single crystalline semiconductor devices. Essentially the concept espoused by Jackson was to employ different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell, a large band gap material absorbs only the short wavelength light, while in subsequent cells, smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. Such tandem cell structures can be economically fabricated in large areas by employing thin film semiconductor alloy materials (with or without crystalline inclusions), in accordance with the principles of the instant invention. It should be noted that Jackson employed crystalline semiconductor materials for the fabrication of his stacked cell structure; however, since it is virtually impossible to match lattice contents of differing crystalline materials, it is not possible to fabricate such crystalline tandem cell structures in a commercially feasible manner. In contrast thereto, and as the assignee of the instant invention has shown, such tandem cell structures are not only possible, but can be economically fabricated over large areas by employing the thin film semiconductor alloy materials and the deposition techniques discussed and briefly described herein.

More particularly, the assignee of the instant invention is presently able to manufacture stacked large area photovoltaic devices on a commercial basis by utilizing the previously referenced, continuous deposition, roll-to-roll processor. That processor is characterized by the assignee as a 1.5 megawatt capacity machine insofar as its annual output of photovoltaic devices is capable of producing 1.5 megawatts of electrical power. Said 1.5 megawatt processor, as presently configured, is adapted to produce tandem photovoltaic cells which comprise two stacked n-i-p type photovoltaic devices disposed optically and electrically in series upon a stainless steel web of substrate material. The processor currently includes six operatively interconnected, dedicated deposition chambers, each deposition chamber adapted to sequentially deposit one of the layers of semiconductor alloy material from which the tandem device is fabricated. The deposition chambers vary in length depending upon the thickness of the particular layer of semiconductor alloy material to be deposited therein.

In order to better understand the manner in which the length of the processor is determined, note that the thicknesses of individual layers of semiconductor alloy material vary from approximately 100 angstroms for the doped layers to approximately 3500 angstroms for the lowermost intrinsic layer. Since the processor operates by developing an r.f. plasma which is adapted to decompose the process gases and deposit a layer of semiconductor alloy material and since the thickness of the deposited layer is directly dependent upon the residence time of the web of substrate material in the deposition chamber, the approximately 3500 angstrom thick layer of intrinsic semiconductor alloy material requires a deposition chamber of over six feet in length in order to provide an annual output of 1.5 megawatts of electrical power. The 1.5 megawatt processor also includes additional chambers for (1) the payoff and takeup of the web of substrate material, (2) the cleaning of the web of substrate material and (3) preventing interdiffusion of the gaseous constituents of the adjacent deposition environments, said interdiffusion prevention preferably occurring in the form of discrete isolation passageway chambers (such as external gas gates). With the addition of all of these chambers, the total length of the 1.5 megawatt processor comes to approximately 40 feet. Accordingly, it must be appreciated that, while this 1.5 megawatt processor is the first apparatus capable of commercially fabricating photovoltaic devices; it is a complex, elongated piece of machinery.

The assignee of the instant invention is now designing and constructing a new and improved semiconductor processing machine for the production of significantly higher annual quantities of photovoltaic energy, i.e., about 25 megawatts of electrical power. It must be noted that in order to produce an annual output of 25 megawatts, the length of the machine must be increased so that the length of this 25 megawatt processor will be at least an order of magnitude longer than the present 1.5 megawatt machine. Since not all of the reasons for this increased length are readily apparent, they will be enumerated in the following paragraphs.

A first reason for the elongation is that the new processor will be configured to fabricate tandem photovoltaic devices which comprise at least 3 and possibly 4 stacked cells; therefore the processor will require 9 to 12 dedicated deposition chambers instead of the six dedicated deposition chambers required by the present processor. Another factor in determining the length of the processor, mentioned previously, is that the length of each of the individual deposition chambers is dependent upon the thickness of each of the layers of semiconductor alloy material to be deposited therein. The thickness of that material is, in turn, dependent upon, the rate of deposition of particular mixtures of precursor process gases and the speed of the web of substrate material passing through that chamber of the processor. Consequently, if the rate of deposition of the precursor gas mixture remains constant (and Applicants' assignee finds that significantly increasing the rate of deposition of semiconductor alloy material tends to deleteriously affect the photovoltaic conversion properties of that material), the web speed will also have to be kept constant and the deposition chambers in the 25 megawatt processor will have to be over sixteen times longer than in the 1.5 megawatt processor in order to deposit a sufficient quantity of semiconductor alloy material for fabricating photovoltaic devices which would provide an annual output of 25 megawatts of electrical power.

Even assuming that the presently employed one foot wide web of substrate material was to be increased in size to a two foot width, a scaled-up version of the present processor which is designed to have a 25 megawatt capacity would still total approximately 400 feet in length. Even more significantly, note that in a deposition apparatus of this size, the cathode utilized for the deposition of the thickest layer of semiconductor alloy material, i.e., the bottommost intrinsic layer of semiconductor alloy material of the tandem photovoltaic device, would have to be approximately 60 feet in length.

Clearly, a 400 foot long processor which requires the incorporation of a 60 foot long cathode presents many problems. The physical space required to house a machine approximately the length of 11/2 football fields presents problems in plant design, location and cost. Additionally, the mechanical design and operation of such a large, complex machine creates engineering problems related to the maintenance of the required optical, electrical and structural characteristics of the deposited semiconductor alloy material. The length and weight of the 400 foot span of the web of substrate material, which continuously moves through the deposition apparatus, makes web handling and steering difficult, which, in turn, provides for numerous problems in maintaining substrate tracking, alignment and support. Likewise, maintenance of preselected vacuum conditions and deposition parameters within the 400 foot long vacuum envelope which the web of substrate material must traverse is, at best, quite difficult. Similarly, physical maintenance, i.e., disassembly, cleaning, etc. of the deposition apparatus becomes a nightmare.

Even more importantly (because it directly relates to the deposition of uniform, high quality semiconductor alloy material), the large areas covered by some of the deposition cathodes in such a scaled-up 25 megawatt processor creates problems of plasma uniformity and gas utilization within the cathode and deposition regions. Of the foregoing, plasma uniformity poses the most significant problem. Due to the large area plasma regions created by such large area cathodes, nonuniformities in the ionized precursor process gas mixtures are likely to arise. More specifically, varying compositions of the activated process gas mixture along the length of a large area cathode will give rise to irregular and nonhomogeneous plasma sub-regions, which irregularities and nonhomogeneties will result in the deposition of nonuniform, nonhomogeneous layers of semiconductor alloy material.

It should be abundantly clear from the foregoing discussion that, as the 1.5 megawatt continuous photovoltaic device production machine is scaled up to higher throughput capacities, it becomes an economic necessity to substantially reduce the overall length thereof. Such improvements would result in a substantial savings of deposition time, floor space, the cost of building the machine and the operating cost for the production of photovoltaic devices therein.

The Assignee of the instant application has previously disclosed the concept of utilizing a non-horizontally disposed cathode plate in order to simultaneously deposit semiconductor alloy material in discrete plasma regions developed adjacent both of the opposed faces of that cathode plate. This concept is described in U.S. Pat. No. 4,423,701 filed Mar. 29, 1982 entitled "Glow Discharge Deposition Apparatus Including A Non-Horizontally Disposed Cathode", which patent is assigned to the assignee of the instant invention. Prior to the disclosure of said patent, only one-half (one face) of the potential surface area (two faces) of an r.f. powered cathode plate was utilized from which to develop a plasma, thereby limiting to one the number of substrates on which layers of thin film semiconductor alloy material could be simultaneously deposited. The vertical orientation of the cathode plate, as described in said patent provided the further advantage that deposition debris which is generated during the decomposition of the precursor gaseous mixture could not as readily come to rest on the vertically disposed surface of the substrate. Therefore, a continuous processor, utilizing such a generally vertically disposed cathode plate arrangement, would require less down time for dismantling, cleaning and reassembling. Finally, said above-referenced patent recognized the possibility of utilizing two webs of substrate material for the simultaneous and continuous deposition onto each of the webs of successive layers of semiconductor alloy material as said webs moved through the discrete plasma regions, developed on both faces of the cathode plates in each of the deposition chamber (in a generally linear path of travel).

However, while the deposition apparatus generally disclosed in U.S. Pat. No. 4,423,701 described a process of and apparatus for developing a plasma region adjacent each of the opposed faces of a generally vertically disposed cathode plate in order to continuously and simulataneously deposit layers of semiconductor alloy material onto each of two webs of substrate material as those webs passed through a plurality of interconnected deposition chambers, that process still failed to solve the problem of reducing the length of the continuous processor so as to provide a commercially viable deposition process capable of depositing successive layers of semiconductor alloy material for fabricating triple or four (quad) cell tandem photovoltaic devices and having an annual capacity of up to 25 megawatts of electrical power.

Finally, Applicants' assignee, in U.S. Pat. No. 4,601,260 entitled "Vertical Semiconductor Processor", was able to substantially reduce the length of such a 25 megawatt semiconductor processing apparatus by vertically orienting the path of travel of the web of substrate material through the deposition chambers thereof. More particularly, that application is directed to apparatus for the continuous vapor deposition of successive layers of semiconductor alloy material. The apparatus includes a plurality of discrete chambers, each chamber of which is dedicated to the deposition of a layer of semiconductor alloy material of a preselected conductivity type. Pumps are provided for vacuumizing each of the chambers and a web of substrate material is continuously advanced through each of those chambers for the glow discharge deposition of semiconductor alloy material thereonto. The glow discharge structure includes (1) a conduit for introducing a precursor mixture of process gases, (2) a conduit for exhausting nondeposited gases of the precursor mixture and (3) a means for decomposing the precursor mixture in a plasma region. As in the earlier generations of continuous processing machines referred to hereinabove, an isolation chamber is operatively disposed between each of the adjacent discrete deposition chambers for isolating the gaseous environments of adjacent chambers from one another while providing for the passage of the web of substrate material therebetween. The improvement in the apparatus resides in direction of the substrate material through at least one of the deposition chambers in a non-linear path of travel and the operative disposition of the decomposing means so as to develop a plurality of plasma regions in those chambers through which the substrate material is non-linearly directed. In the preferred embodiment, at least two of the plurality of plasma regions are disposed in different non-linear portions of the path of travel through which the substrate material is advanced so that the total length of the deposition apparatus may be substantially foreshortened. In other words, while the web of substrate material must still traverse about 400 feet of real estate in order to have the requisite thickness of semiconductor alloy material deposited thereupon a high percentage of that real estate is traversed in the vertical direction and the aforementioned problems regarding machine length are significantly ameliorated.

In the previously mentioned patent applications, wherein the semiconductor deposition systems are primarily concerned with the production of photovoltaic cells, isolation between the deposition chambers is accomplished either by employing gas gates which pass or "sweep" an inert gas, such as argon or hydrogen, about the substrate as it passes therethrough; by gas gates which establish unidirectional flow of the reaction gas mixture introduced into the intrinsic deposition chamber into the dopant deposition chambers; or by magnetic gas gates which result in a reduced passageway opening between adjacent deposition chambers, said magnetic gates adapted to attract the metallic substrate material moving therethrough so as to reduce the size of the passageway opening and thereby effect a correspondingly decreased amount of "contaminants" diffusing from the dopant deposition chambers into the adjacently disposed intrinsic deposition chamber. It should be noted that any of these gas gates could also be operably connected between non-deposition chambers, as, for example, a chamber in which the transparent conductive oxide layer (discussed hereinafter) is added atop the uppermost layer of semiconductor alloy material. Since it is clearly undesirable to have gaseous constituents from the transparent conductive oxide chamber (or from any chamber in which non-semiconductor gaseous precursors are present) diffuse into the semiconductor deposition chambers, such prior art gas gates were also employed between the transparent conductive oxide chamber and the final chamber in which layers of doped semiconductor alloy material were deposited. In a like manner, any of these types of gas gates could be employed between each and every chamber which is operatively interconnected for continuously producing thin film photovoltaic devices.

While the aforementioned magnetic gas gates proved effective in limiting contamination (relative to similarly constructed but non-magnetic gas gates) by providing for a passageway opening of reduced size through which contaminants could diffuse, the temperature gradients to which the web of substrate material is continually subjected (since deposition parameters require an elevated temperature of approximately 175.degree.-275.degree. C.) tend to warp the web to such a degree that the magnets (which are spacedly positioned throughout the length of the gas gate) are unable to fully flatten the web into a completely planar configuration. Since the web can not be held in a planar configuration, the size of the passageway opening must be designed to provide for sufficient tolerance to prevent contact of a wall of that opening with the deposition surface of the web. The added tolerance means the passageway opening allows for a correspondingly greater degree of diffusion between adjacent chambers than would be necessary if the web could be made to assume a substantially planar configuration while passing therethrough. Further the greater the degree of diffusion a passageway permits, the greater the length of the passageway must be in order to prevent the diffusing gaseous contaminants from one deposition chamber from reaching and entering the adjacent deposition chamber. Therefore, the added tolerance necessitated by the non-planar configuration of the web results in a longer gas gate passageway than would otherwise be necessary and a longer passageway results in a lengthier semiconductor processing apparatus.

It is therefore yet another object of the present invention to provide an isolation passageway which is adapted to maintain the web of substrate material passing therethrough in a substantially planar attitude (relative to the wall of the passageway opening against which it is urged) for decreasing the size of the passageway opening and thereby allowing for a decrease in the length of the passageway due to a decrease in the percentage of contaminants which are permitted entry into that passageway opening.

While previously described patent application Ser. No. 718,571 dealt with the problem of foreshortening the overall length of a semiconductor deposition apparatus while simultaneously increasing the annual electrical output of photovoltaic cells produced therein, no attention was paid to the existing length of the external isolation passageways deployed between every chamber which was adapted to deposit one of the successive layers of semiconductor alloy material. This oversight becomes important when realizing the fact that, and as pointed out hereinabove, the number of discrete layers of semiconductor alloy material which must be deposited in said next generation processor will be increased from six in the present tandem (two cell) configuration to nine in a triple (3 cell) configuration or twelve in a quadruple (4 cell) configuration. For the incorporation of each additional layer of semiconductor alloy material, an additional isolation passageway will also have to be incorporated in order to achieve the requisite degree of isolation between adjacent deposition chambers. Accordingly, it can now be appreciated that a reduction in the length of the isolation passageways would result in a further and appreciable reduction in the total length of the processor. It is therefore another object of the present invention to design an isolation passageway which is foreshortened over the length of previous gas gates so as to achieve a further reduction in the total length of photovoltaic processors which are designed to deposit successive layers of semiconductor alloy material.

One further aspect (of providing isolation through the use of sweep gases) must be touched upon in order to fully appreciate the technology involved. This aspect deals with the degree of isolation which is necessary in order to fabricate a highly efficient photovoltaic device. More particularly, it must be realized just how seriously and deleteriously "contaminants" can affect the efficiency of the semiconductor device produced in the vacuum envelope of the semiconductor processor. For instance, if a gas gate passageway is dimensioned to be approximately 0.4 inches high, 16 inches wide and 6 inches long with the pressure in a first chamber being 0.6 torr and the pressure in an adjacent second chamber being 0.57 torr, a flow rate of 500 SCCM of the precursor gaseous constituents passing through the gas gate passageway interconnecting those chambers will result in the presence of sufficient gaseous precursor constituents to sustain the plasma in the deposition chambers as well as to provide a concentration ratio of the dopant species from the first deposition chamber to the intrinsic species present in the adjacent second deposition chamber of about 10.sup.4. This ratio represents a concentration approximately sufficient to produce an intrinsic thin film semiconductor alloy material in the second deposition chamber of high purity. It must be understood that the flow rates, slot dimensions, and chamber pressure stated hereinabove represent but one example of parameters which are sufficient for the practice of the present invention. Other flow rates, slot dimensions and chamber pressures may also be utilized for providing effective isolation of the intrinsic semiconductor alloy material deposited in one of the chambers from the dopant semiconductor alloy material deposited in the adjacent deposition chamber.

It is further to be noted that Applicants' gas gates, discussed hereinabove, are effective in maintaining at least a 10.sup.4 concentration ratio of the element absent in the intrinsic deposition chamber relative to the element present in the dopant deposition chamber by establishing a substantially viscous flow of gases through the gas gate slot. It must be noted that gases moving within the deposition system of the subject application, which system is maintained at a pressure of approximately 5.times.10.sup.-1 torr and above, are in the viscous flow regime, whereas gases moving through a deposition system which is maintained at a pressure of approximately 5.times.10.sup.-2 to 5.times.10.sup.-3 torr are in a transition flow regime known as the Knudsen flow regime, and gases moving through a deposition system which is maintained at a pressure of approximately 5.times.10.sup.-3 torr and below are in the molecular flow regime.

In a molecular flow regime, a flow of gases in a first direction cannot limit the back diffusion of gases. This is because, at the pressure which gives rise to molecular flow, the molecules of the oppositely directed process and sweep gases are so widely separated that relatively few diffusion limiting collisions can occur therebetween. Applicants' glow discharge deposition system, since it operates at approximately 0.5 torr, clearly operates in the viscous flow regime. It is in this viscous flow regime that, the molecules of oppositely directed process and sweep gases realize a sufficient number of intermolecular collisions so as to effectively limit back diffusion from one of the pair of chambers to the adjacent chamber.

It should thus be realized that Applicants' improved isolation passageway, as described hereinafter, while particularly adapted for use in systems operating at below atmospheric pressures, it is effective only in pressure regimes which give rise to the viscous flow of gases. It is therefore only in these viscous pressure regimes that the isolation passageway of the subject invention is operable to limit contamination in the aforedescribed 10.sup.4 contamination level.

While the magnetic gas gates disclosed apparatus (namely ceramic magnets positioned above the gas gate passageway opening for urging the magnetic substrate upwardly) by which the height dimension of the passageway opening in the gas gate could be reduced (the reduction in the height dimension of the passageway opening correspondingly reducing the diffusion of dopant gases for a given flow rate, thereby decreasing the contamination of the process gases introduced into and, consequently, the layer of semiconductor alloy material deposited in the intrinsic deposition chamber), Applicant's assignee has reported in U.S. Pat. No. 4,450,786 entitled "Grooved Gas Gate", the disclosure of which is incorporated by reference, that when the web of substrate material is urged by the magnets against the upper wall of the gas gate passageway, the passageway is divided by the web of substrate material into a relatively wide lower slit and a realtively narrow upper slit. For purposes of the instant application the term "upper slit" shall be defined as the spacing, however irregular it may be, between the upper surface of the substrate and the upper wall of the gas gate passageway. Irregular spacing between the web and the upper passageway wall may be present because waffling of the web of substrate material cannot be completely liminated by the attractive force of the magnets. The process gases, being inherently viscous, are unable to travel through the narrow upper slit with sufficient velocity to prevent the diffusion of process gases from the dopant deposition chamber into the adjacent intrinsic deposition chamber.

More particularly, note that gas may be introduced into the passageway opening to "sweep" diffusing contaminants back into the dopant deposition chambers. In order to effect this "sweep", it is required that the velocity of the inert sweep gases and residual process gases traveling through the passageway opening be selected to be sufficiently great to substantially prevent the back diffusion of process gases from the dopant deposition chamber to the intrinsic chamber. However, and as detailed in said '786 patent, due to the fact that the sweep gases employed in the gas gates are viscous, which viscosity becomes more pronounced at the elevated temperatures required for the glow discharge deposition of thin film layers of semiconductor alloy material onto the substrate, the drag on the sweep gases along (1) the upper passageway wall and (2) the unlayered surface of the substrate, which define the relatively narrow upper slit, results in a relatively low velocity flow therethrough. Accordingly, an undesirably high amount of dopant process gas is able to diffuse into the intrinsic deposition chamber through that narrow upper slit.

The velocity profile of the sweep gases flowing from the intrinsic deposition chamber to the dopant deposition chamber through the relatively wide lower passageway slit may be depicted by a generally parabolically shaped curve in which the velocity of the sweep gases is greatest at the center of the slit and at a minimum along the walls thereof. The velocity profile of the sweep gases flowing from the intrinsic deposition chamber to the dopant deposition chamber through the relatively narrow upper passageway slit may also be depicted by a generally parabolically shaped curve, similar to the curve for the wide passageway slit. However, a comparison of the two velocity profiles reveals that the velocity of the sweep gases flowing through the lower, relatively large passageway slit is far greater than the velocity of the sweep gases flowing through the upper, relatively narrow passageway slit. Further, since the height dimension of the narrow upper slit is permitted to vary with the random warping and canoeing of the relatively thin substrate material, the degree of contamination due to back diffusion of dopant process gases is able to correspondingly fluctuate.

At this point, and in order to better understand the relationship of the counter flow of sweep gas to the diffusion of process gas between adjacent deposition environments, it is necessary to discuss the pressure differential which is developed between the adjacent deposition chambers operatively connected by a gas gate. If one was to plot the number of atoms of a gas per second flowing through the narrow passageway opening as a function of the size of that opening (assuming a constant pressure differential is maintained on both ends of the opening) it would be apparent that as the size of the passageway opening is increased, the volume of gases flowing therethrough in order to maintain the constant pressure differential must correspondingly increase. This represents a desirable gas gate characteristic because the greater the velocity of sweep gas flowing from the intrinsic deposition chamber to the dopant deposition chamber, the more difficult it becomes for dopant gases to diffuse against the counter and prevailing flow from the dopant deposition chamber to the intrinsic chamber. The functional dependency of back diffusion, relative to the size of the gas gate passageway opening is represented by the equation (a) (e.sup.-a2 ) where "a" represents the passageway opening. That functional dependency, as evidenced by the amount of back diffusion, reaches a maximum when "a" is about 200 microns or about 10 mils. It is therefore essential that both, the size of the slit above, as well as below, the web of substrate material be kept at or above the 200 micron level at which gas flow is maximized. Through the application of the principles of the subject invention, there is no problem in creating a sufficiently large opening below the web of substrate material since the substrate material is urged under tension against the upper cylindrically-shaped wall of the passageway opening.

However, it is further necessary to prevent the back diffusion of dopant gases through the narrow opening above the web of substrate material (in those instances in which tension on the web is relaxed and dopant gases "seep" into the narrow upper slot) by providing a plurality of circumferential grooves about the surface of the cylindrical drum of the isolation passageway. In this manner, a plurality of spaced, relatively high velocity flow channels are provided in the space defined between the unlayered surface of the web of substrate material and the upper cylindrical wall of the passageway opening. Because the channels are relatively deep, the sweep gases and residual process gases are adapted to flow therethrough at substantial velocities despite the drag incurred as said gases contact the oppositely disposed passageway wall and the substrate surface. Although relatively narrow slits still exist between adjacent high velocity flow channels established by the elongated grooves, it is much more probable for molecules of dopant process gases to enter the high velocity channels during their traverse of the passageway opening separating the dopant deposition chamber from the intrinsic deposition chamber, than to have those molecules remain in the narrow slit between the high velocity flow channels for the entire length of that migration. In order to further insure that back diffusion is prevented, additional sweep gas may be introduced into each of the high velocity flow channels at a point intermediate the length of the gas gate passageway opening. Because of the velocity which the sweep gas can attain in each of the "roomy" flow channels and because of intermolecular collisions which occur between the dopant gases and the sweep gas in the viscous flow regime present in the isolation passageway, the amount of back diffusion from the dopant deposition chamber to the intrinsic deposition chamber is substantially reduced and the production of a more efficient photovoltaic device may be accomplished.

These and the many other objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which follow hereinafter.

BRIEF SUMMARY OF THE INVENTION

There is disclosed herein an isolation passageway for substantially preventing the diffusion of gases from one of a pair of adjacent vacuumized environments into the other of said pair of vacuumized environments. The first environment differs from the second by the presence of at least one elemental contaminant. The improved isolation passageway is (1) defined by closely spaced walls, (2) adapted to provide for the movement of a substrate therethrough, (3) substantially annular in a central cross-sectional region and rectangular in the two regions adjacent thereto, said cross-section taken in a plane extending in the direction of the path of the substrate and (4) adapted to maintain at least a 10.sup.4 ratio of the concentration of the at least one element in said first environment as compared to the concentration in said second environment. By further urging one surface of the substrate traveling through the passageway into contact with one of the passageway walls, an isolation passageway of reduced height and length dimensions is provided which is adapted to both decrease the diffusion of gases between said chambers and decrease the length of the passageway so as to correspondingly decrease the length of the deposition machine in which said passageway is incorporated.

The passageway wall which contacts the unlayered surface of the substrate is fabricated from a low friction, low thermal conductivity material such as borosilicate glass. The substrate may be formed from a magnetically attractable material and the substrate may be urged into contact with the glass through magnetic attraction. In a preferred embodiment, each of the adjacent environments are developed and maintained in a dedicated chamber, each chamber adapted to deposit thin film layers of semiconductor alloy material. The chambers are vacuumized to a pressure of about 0.25 to 1 torr. In the most preferred embodiment, the nonlayered surface of the substrate is urged into contact with the passageway wall through the use of roller means which place said nonlayered substrate surface under tension against a passageway wall.

Also in the most preferred embodiment, (1) the passageway is annular in a central cross-sectional region, one boundary of the annular cross-sectional configuration of the passageway formed by a cylindrical drum, the cross-section taken in a plane extending in the direction of the path of the substrate, and (2) the substrate is an elongated web and the nonlayered surface of the web is urged against the circumferential surface of the drum. A plurality of circumferential grooves are spacedly positioned across the entire longitudinal extent of the cylindrical drum for accepting and guiding sweep gas into a first series of flow channels formed between the grooves and the substrate web. The grooves are adapted to sustain a flow of sweep gas at a velocity sufficient to substantially prevent the diffusion of process gas from the first to the second chamber through said first channels.

Sweep gas is also introduced into a second flow channel formed between the substrate web and the surface of the passageway opposite the surface against which said web is urged. The flow of sweep gas through the passageway in the second channel is at a velocity sufficient to substantially prevent the diffusion of process gas from the first to the second chamber. The passageway may further include structure for subjecting the surface of the substrate not urged into contact with the passageway wall to a plasma as said substrate moves through said passageway. The plasma is preferably a hydrogen plasma which is useful in capping the surface of the previously deposited layer of semiconductor alloy material. The first environment is preferably a first chamber adapted for the deposition of a first layer of semiconductor alloy material and the second environment is a second chamber adapted for the deposition of a second layer of semiconductor alloy material differing in conductivity type from the conductivity type of the first layer. Both the semiconductor alloy material and the hydrogen plasma may be accomplished by either r.f. or microwave energy systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary, cross-sectional view of a tandem photovoltaic device, said device comprising a plurality of n-i-p type cells, each layer of the cells formed from a thin film semiconductor alloy material;

FIG. 2 is a schematic depiction of a vertically oriented apparatus adapted to continuously deposit a plurality of successive, thin film layers of semiconductor alloy material upon a continuously advancing web of substrate material;

FIG. 3 is a perspective view of the improved circularly-shaped isolation passageway of the instant invention, said passageway particularly adapted for operative deployment in the apparatus of FIG. 2;

FIG. 4 is a cross-sectional view of the interior configuration of two randomly selected deposition chambers depicted in FIG. 2 as operatively interconnected by the improved isolation passageways of the instant invention and as depicted in FIG. 3;

FIG. 5 is a fragmentary cross-sectional view taken along line 5--5 of the FIG. 3 isolation passageway of the instant invention and illustrating an embodiment thereof in which peripheral grooves are spacedly positioned about the cylindrical drum of the passageway;

FIG. 6 is a partial cross-sectional view taken in a plane extending in the direction of the path of the substrate and depicting an embodiment of the isolation passageway of the instant invention in which gas is swept between the passageway wall and the substrate for preventing the back diffusion of gaseous reactants between adjacent deposition chambers;

FIG. 7A is a partial cross-sectional view similar to FIG. 6 and illustrating the operative disposition of a source of alternating current, relative to the isolation passageway of the instant invention, said source adapted to generate a hydrogen plasma over the layered surface of the substrate; and

FIG. 7B is a partial cross-sectional view similar to FIG. 7A and illustrating the operative disposition of said alternating current source relative to another preferred embodiment of the isolation passageway of the instant invention, said alternating current source disposed between a pair of spaced rotary drums; and

FIG. 8 is a partial cross-sectional view taken along line 8--8 of FIG. 3 and illustrating the operative disposition of a vacuum-tight end seal for preventing leakage between the interior of the isolation passageway of the subject invention and atmosphere.

DETAILED DESCRIPTION OF THE DRAW-NGS

I. The Photovoltaic Cell

Referring now to the drawings and particularly to FIG. 1, a photovoltaic cell, formed of a plurality of successive n-i-p layers, each of which is formed from, preferably, a thin film semiconductor alloy material as shown generally by the reference numeral 10.

More particularly, FIG. 1 shows a p-i-n type photovoltaic device such as a solar cell made up of individual p-i-n type cells 12a, 12b and 12c. Below the lowermost cell 12a is a substrate 11 which may be transparent or formed from a metallic material such as stainless steel, aluminum, tantalum, molybdenum, chrome, or metallic particles embedded within an insulator. Although certain applications may require a thin oxide layer and/or a series of base contacts prior to the application of the amorphous material, for purposes of this application, the term "substrate" shall include not only a flexible film, but also any elements added thereto by preliminary processing. Also included within the scope of the present invention are substrates formed of synthetic polymers, glass or a glass-like material on which an electrically conductive electrode is applied.

Each of the cells 12a, 12b and 12c are preferably fabricated with a thin film semiconductor body containing at least a silicon alloy. Each of the semiconductor bodies includes a p-type conductivity semiconductor layer 20a, 20b and 20c; a substantially intrinsic semiconductor layer 18a, 18b and 18c and an n-type conductivity semiconductor layer 16a, 16b and 16c. Note that the intrinsic layer may include traces of n-type or p-type dopant material without forfeiting its characteristic neutrality, hence it may be referred to herein as a "substantially intrinsic layer". As illustrated, cell 12b is an intermediate cell and, as indicated in FIG. 1, additional intermediate cells may be stacked atop the illustrated cells without departing from the spirit or scope of the present invention. Also, although n-i-p photovoltaic cells are illustrated, the methods and materials described herein may also be and are preferably utilized to produce single or multiple p-i-n cells, accordingly, the term "n-i-p type" as used herein is meant to include any aggregation of n, i and p layers operatively disposed to provide a photoactive region for generating charge carriers in response to the absorption of photon energy. Additionally, the disclosed deposition apparatus may be readily adapted to produce p-n cells, Schottky barrier cells, as well as other semiconductor or devices such as diodes, memory arrays, photoconductive devices and the like.

It is to be understood that following the deposition of the semiconductor alloy layers, a further deposition process may be either performed in a separate environment or as a part of a continuous process. In this step, a TCO (transparent conductive oxide) layer 22, preferably formed of indium tin oxide, is added. An electrode grid 24 may be added to the device where the cell is of a sufficiently large area, or if the conductivity of the TCO layer 22 is insufficient. The grid 24 is adapted to shorten the carrier path and increase the conductive efficiency.

II. The Multiple Chamber Apparatus

Turning now to FIG. 2, a generally diagrammatic representation of the multi-chambered glow discharge deposition processor for the continuous production of tandem or cascade-type photovoltaic cells is illustrated generally by the reference numeral 26. Due to the elongated nature of the processor 26 (the illustrated processor has a 25 megawatt capacity and is about 140 feet in length), it has been necessary to cut away and continue the longitudinal extent thereof in a plurality of rows across the sheets of drawings. However, and as should be readily apparent, in actual construction and operation, the processor 26 is preferably aligned so that each of the deposition chambers thereof is arranged in a generally linear arrangement. The processor 26 includes a plurality of isolated and dedicated deposition chambers. The term "dedicated" as used herein, will mean the precursor gaseous mixtures of each adjacent deposition chamber are substantially prevented from cross contaminating one another. Moreover, each deposition chamber has introduced thereinto a particular precursor gaseous mixture of process gases which is protected by an external isolation passageway module from (1) contaminating the precursor gaseous mixture introduced into adjacent deposition chambers and (2) being contaminated by environmental conditions.

The processor 26 is particularly adapted to deposit a high volume of large area amorphous triple tandem photovoltaic cells having a generally n-i-p-type configuration onto the deposition surface of the web of substrate material 11 which is continually fed therethrough. In order to deposit the semiconductor alloy material required for producing a tandem photovoltaic device of such an n-i-p-type configuration, the processor 26 includes: a first deposition chamber 28 in which an n-type conductivity layer of semiconductor alloy material is deposited onto the deposition surface of the web of substrate material 11 as said web passes therethrough; a second deposition chamber 30 in which a layer of substantially intrinsic semiconductor alloy material is deposited atop the layer of n-type semiconductor alloy material on the deposition surface of the web of substrate material 11 as the web 11 passes therethrough; a third deposition chamber 32 in which a layer of p-type conductivity semiconductor alloy material is deposited atop the layer of intrinsic semiconductor alloy material on the deposition surface of the web of substrate material 11 as the web passes therethrough; a fourth deposition chamber 34 in which a second n-type conductivity layer of semiconductor alloy material is deposited atop the layer of p-type semiconductor alloy material on the deposition surface on the web of substrate material as the web 11 passes therethrough; a fifth deposition chamber 36 in which a second layer of intrinsic amorphous semiconductor alloy material is deposited atop the second layer of p-type semiconductor alloy material on the deposition surface on the web of substrate material 11 as the web 11 passes therethrough; a sixth deposition chamber 38 in which a second layer of p-type conductivity semiconductor alloy material is deposited atop the second layer of intrinsic semiconductor alloy material on the deposition surface of the web of substrate material 11 as the web 11 passes therethrough; a seventh deposition chamber 40 in which a third layer of n-type conductivity semiconductor alloy material is deposited atop the second layer of n-type semiconductor alloy material on the deposition surface of the web of substrate material 11 as the web 11 passes therethrough; an eighth deposition chamber 42 in which a third layer of intrinsic semiconductor alloy material is deposited atop the third layer of n-type semiconductor alloy material on the deposition surface of the web of substrate material 11 as the web 11 passes therethrough; and a ninth deposition chamber 44 in which a third layer of p-type conductivity semiconductor alloy material is deposited atop the third layer of intrinsic semiconductor alloy material on the deposition surface of the web of substrate material 11 as the web 11 passes therethrough.

It should be apparent that, although nine discrete deposition chambers (three triads of the three deposition chambers) have been described, additional triad deposition chambers or individual deposition chambers may be added to the processor 26 to provide the machine with the capability of producing any number of tandem photovoltaic cells having p-i-n-type or n-i-p-type or p-n-type or n-p-type configuration. It should further be understood that, although, in the preferred embodiment, the substrate is formed as a continuous, electrically conductive web of substrate material 11, the concept of the present invention is equally adapted for depositing the successive layers of semiconductor alloy material atop a continuous, electrically non-conductive substrate or atop discrete, electrically conductive or non-conductive substrate plates which are continuously fed through the plurality of deposition chambers thereof. It should also be apparent that since the length of the path of travel of the web of substrate material 11 through the individual deposition chambers is proportional to the thickness of the n-type, or the intrinsic, or the p-type layer of semiconductor alloy material to be deposited in any one of the given chambers, the length of the path of travel of the web of substrate material 11 through an individual deposition chamber must be increased (if the speed of the web of substrate material 11 is kept constant) in order to deposit a thicker layer thereupon. This can best be illustrated with reference to the first triad of deposition chambers in which the path of travel of the web 11 through the multiple plasma regions developed within the intrinsic deposition chamber 30 can be seen to be much longer than the path of travel thereof through the plasma regions developed within either of the doped deposition chambers 28 and 32 because the intrinsic deposition chamber 30 is adapted for the deposition of a 3500 angstrom thick layer of intrinsic semiconductor alloy material while the doped deposition chambers 28 and 32 are adapted to only deposit layers of approximately 100 angstrom thick semiconductor alloy material.

Still referring to FIG. 2, the processor 26 further includes a plurality of external isolation modules 46a-46l for isolating the particular precursor gaseous mixture introduced into a particular deposition chamber from the mixtures introduced into adjacent chambers, each of said mixtures being operative to deposit a particular layer of semiconductor alloy material of a preselected conductivity type. The isolation modules 46a-46l are preferably disposed externally of the deposition chambers and are adapted to permit the web of substrate material 11 to travel between the discrete deposition chambers which they interconnect while substantially preventing said inter-diffusion of said precursor gaseous mixture from one of a pair of adjacent chambers into the other of the pair. External isolation modules of this type are fully disclosed in U.S. Pat. No. 4,480,585 entitled "External Isolation Module", filed June 23, 1983, the disclosure of which is incorporated herewith by reference and the assignee of which is the same as the assignee of the present invention. Generally, the isolation modules 46a-46l are schematically illustrated as including a pair of elongated, horizontally-disposed, passageway-forming plates, said plates adapted to be spacedly positioned in substantially parallel planes for defining the passageway therebetween. The web of substrate material 11 passing through the passageway divides the passageway into a pair of flow channels, i.e., an upper relatively narrow and a lower, relatively wide channel. Sweep gas is uniformly introduced into each of the channels to prevent the diffusion of the precursor gaseous mixtures between the adjacent deposition chambers.

Positioned on the side of the first deposition chamber 28 opposite the second deposition chamber 30, and in operative interconnection therewith, is a substrate cleaning chamber 50 in which the web of substrate material continuously moving therethrough is subjected to high temperature (on the order of 450.degree. C.) so as to bake out contaminants therefrom. A substrate cleaning plasma may also be developed within that chamber if it is deemed necessary to further rid the web of substrate material 11 of contaminants.

On the side of the cleaning chamber 50 opposite the first deposition chamber 30 is the substrate pay-off chamber 52 from which a roll of substrate material 11 is supplied, under tension, from a pay-off roll 11a to the deposition chambers of the processor 26. As the web 11 is unwound from the roll 11a, a sheet of protective interleaf sheeting 9 is wound about interleaf take-up roller 52b. Also present in the pay-off chamber 52 are a pair of idler turning rollers 76a for initially directing the web 11 in a generally horizontal path of travel through the processor 26.

Positioned on the side of the ninth deposition chamber 44 opposite the eighth deposition chamber 42 is a post deposition take-up chamber 54 in which the web of substrate material 11, with the layers of semiconductor alloy material deposited thereupon, is wound about a take-up core 11b. As the web 11 is wound about the take-up roll 11b, a sheet of protective interleaf sheeting 9 from an interleaf pay-off roller 54b is would thereabout. Also present in the take-off chamber 54 are a pair of idler turning rollers 76 for directing the web 11 from its normally horizontal path of travel into winding engagement with the take-up roll 11b.

The first and last external isolation modules 46a and 46l both include a bellows section 56a and 56b, respectively, which bellows are adapted to compensate for expansion or contraction which occurs during operation of the processor 26. Intermediate at least the third deposition chamber 32 of the first triad and the first deposition chamber 34 of the second triad is an intermediate web controller chamber 58 in which a spring tensioning roller 58a is adapted to cooperate with a pair of turning rollers 58b for maintaining the proper tension on the web of substrate material 11. Although only one controller chamber 58 is depicted, it should be apparent that additional controller chambers may be added at any point along the path of travel of the web of substrate material 11 without departing from the spirit or scope of the instant invention. It is also to be noted that each of the deposition chambers, external isolation modules and pay-off and take-up chambers are raised off of the floor and supported by a heavy-duty scaffolding generally depicted by the reference numeral 60. By raising the processor 26 from the floor, said processor is not as responsive to changes in environmental conditions such as heat or cold.

Referring now to FIG. 4, there is illustrated the interior configuration of two of the deposition chambers, such as the deposition chambers 40 and 42 in which the third layer of n-type semiconductor alloy material and the third layer of intrinsic semiconductor alloy material, respectively, are to be deposited and through which the web of substrate material 11 is adapted to move in a non-linear path of travel. It is to be understood that the deposition chambers 40 and 42 are merely intended to be representative of any of the deposition chambers of the processor 26 and that the third n-type and intrinsic deposition chambers have been selected for purposes of illustration only since those deposition chambers require the web of substrate material 11 to make only one non-linear pass for the deposition thereonto of the third n-type layer and the third intrinsic layer of semiconductor alloy material. An explanation of the operation of any of the other deposition chambers of the processor 26, such as the first intrinsic deposition chamber 30 in which the web of substrate material 11 is adapted to make multiple non-linear passes, may be easily understood from the explanation of the operation of the deposition chamber 40 which follows.

Chambers 40 and 42 are discrete one of the plurality of isolated dedicated deposition chambers operatively interconnected by external isolation modules, such as 46i. Such an external isolation module is also operatively disposed, in the preferred embodiment, between any of the chambers of the processor 26 which are not adapted to deposit semiconductor alloy material, but which cannot be allowed to contaminate the deposition chambers adjacent thereto. Note that the reference numeral 46i' is employed because the external isolation module depicted in FIG. 4 is of the type disclosed in detail hereinafter as the novel isolation passageway of the instant invention.

The deposition chambers 40 and 42 include a cathode plate 62 having a plurality of apertures formed therethrough so as to perforate same for the uniform mixing of process gases introduced into one side of the plate with the process gases introduced onto the other side of the cathode plate 62; an upper, transversely elongated generally U-shaped cathode shield 68 which is adapted to restrict the precursor gaseous mixture entering the cathode region from a elongated apertured introductory manifold 64 from exiting the plasma region; and a lower transversely elongated cathode shield 66b which is adapted to prevent the non-deposited precursor gaseous mixture from leaving the cathode region after said mixture has passed through the plasma region developed between the web 11 and both of the faces of the cathode plate 62. The lower cathode shield 66b includes a conically-shaped portion which is operatively interconnected with the exhaust conduit 66 from which the non-deposited precursor gaseous mixture is exhausted from the deposition chamber 40. The lower cathode shield 66b further includes a plurality of apertures 66a disposed on both sides of the longitudinal extent thereof. Since both the upper cathode shield 68 and lower cathode shield 66b are elongated members which extend across the full transverse extent of the deposition chamber and are coextensive with the transverse extent of the web of substrate material 11 and the cathode plate 62, the precursor gaseous mixture introduced into the plasma regions is substantially confined within those regions.

It is to be noted at this point that the precursor gaseous mixture introduced through the apertured introductory manifold 64 is adapted to assume a generally vertical path of travel as depicted by arrow A, said path of travel being generally parallel to the path of movement of the web of substrate material through the plasma region. A plurality of banks of substrate heaters 72a, including heat reflecting