Semiconductor wafer cleaning method

Semiconductor wafers are positioned in a cleaning tank and subjected to sequential flows of one or more highly diluted cleaning solutions that are injected into the lower end of the tank and allowed to overflow at the upper end. One solution comprises one part ammonium hydroxide, two parts hydrogen peroxide, and 300-600 parts deionized water together with a trace of high purity surfactant. Rinsing water is flowed through the tank after the first solution is dumped. A second solution comprises highly dilute hydrofluoric acid. A third solution is more dilute than the first solution. A fourth solution contains hydrochloric acid greatly diluted with deionized water. The solutions are initiated either by injecting the chemicals into an incoming DI water line or directly into the tank. The cleaning tank is provided with a megasonic generator in its lower portion for selective application of megasonic energy. Quick dump valves in the tank bottom enable the solutions to be quickly dumped followed by one or more rinse steps, including a quick refill while spraying and then dumping of the rinsing water.

 

What is claimed is:

1. A method for cleaning semiconductor wafers comprising:

positioning one or more wafers in a cleaning tank;

flowing dionized water into the tank to cover the wafers;

injecting into the tanks separately from the water, ammonium hydroxide and hydrogen peroxide to create a highly dilute cleaning solution comprising:

one part ammonium hydroxide, two parts hydrogen peroxide and about 300-600 parts deionized water;

dumping said solution from the tank after cleaning the wafers for a short time;

filling the tank to rinse the wafers and the tank with deionized rinsing water;

spraying the wafers and interior walls of the tank while the tank is being filled with the rinsing water; and

applying megasonic energy to the solution and water in the tank.

2. The method of claim 1, wherein the water, the ammonium hydroxide and the hydrogen peroxide are introduced into a lower end of the tank.

3. A method for cleaning semiconductor wafers comprising the steps of:

positioning one or more wafers in a cleaning tank;

sequentially forming one or more highly diluted cleaning solutions in said tank by introducing deionized water into the tank and separately introducing active components into the tank to create solutions wherein each of the active components of the solutions are diluted with deionized water at ratios greater than 200 to 1 and allowing the solutions to flow out of the tank at a level above the wafers in said solutions;

dumping each of said solutions from the tank and rinsing said wafers with deionized rinsing water after the dumping step and flowing clean deionized water through the tank; and

selectively applying megasonic energy into the solutions and water in the tank.

4. A method for cleaning semiconductor wafers comprising the steps of:

positioning one or more wafers in a cleaning tank;

flowing hot deionized water into a lower end of the tank and allowing excess water to overflow a top of the tank to carry contaminants out of the tank;

simultaneously with the flowing step injecting a dilute surfactant directly into the tank;

simultaneously with the flowing step injecting directly into the tank a small amount of ammonium hydroxide at a location spaced from an inlet for said water; and

injecting hydrogen peroxide directly into the tank at a location spaced from said inlet after initiating the injection of the ammonium hydroxide.

5. A method of cleaning semiconductor wafers comprising the steps of:

positioning a plurality of wafers in a tank with the wafers being supported in spaced generally parallel relation with flat side surfaces of the wafers extending generally vertically with the tank having a width only slightly larger than a diameter of the wafers, and with a tank having sidewalls which taper downwardly and inwardly towards the lower end of the tank which is considerably narrower than the diameter of the wafers and with a megasonic transducer array positioned in a lower portion of the tank directly beneath the wafers;

introducing a high flow of deionized water into the lower portion of the tank in a manner to cause the water level to rise substantially uniformly in the tank and to quickly fill the tank and overflow an upper end of the tank;

injecting directly into the tank, separate from said water, ammonium hydroxide and hydrogen peroxide to create a solution in the tank comprising approximately one part ammonium hydroxide, two parts hydrogen peroxide, and about 300-600 parts deionized water; and

applying megasonic energy to this solution while the water and the ammonium hydroxide and hydrogen peroxide are being injected into the tank, and continuing the application of megasonic energy into the tank until the solution is dumped from the tank.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for cleaning semiconductor wafers and the like.

BACKGROUND OF THE INVENTION

In the fabrication of semiconductor wafers used in making a variety of semiconductor circuit devices, the importance of minimizing contamination on the wafers has been recognized since the early days of the industry. However, as the end product devices have become more and more miniaturized and complex, the cleanliness requirements have become increasingly more stringent so that the devices will function properly. With the reduced size of the devices, a contaminant occupies an increased percentage of the available space for current elements, and hence cleanliness of the materials becomes far more critical.

As the devices become more complex, they also become more valuable, such that unsatisfactory products represent a very significant loss of revenue. A cassette load of large diameter wafers may have an end process value of as much as a million dollars. Also, there is the cost incident to unusable end products that might arise as a result of the discovery of unsatisfactory semiconductor devices after their combination with other components.

In addition to the foregoing cost, there are the major expenses associated with the cleaning processes themselves. One of the major capital expenditure is in the cost of cleaning and drying equipment and associated plumbing, heating and cooling equipment, robotic wafer handling apparatus, computerized control equipment, apparatus for storing and disposing cleaning solutions, and the clean room space required for the apparatus. Of course, there is the cost of the cleaning solutions and the cost of heating, cooling and filtering the solutions, as well as the cost of storing and disposing of them. In view of environmental concerns and regulations, the cost of disposal of certain materials can be greater than the cost of the material being discarded.

Still the most common system for cleaning semiconductor wafers utilizes a series of tanks containing the necessary cleaning solutions, with the tanks being positioned in a "wet bench" in a clean room. A batch of semiconductor wafers, is moved in sequence through the series of tanks, usually by means of computer controlled automated apparatus. A major concern with this type system is that of contamination occurring as the batch of wafers is transferred from one tank to another. Also of significance is the possible contamination introduced by the handling apparatus itself. Further, whenever wafers are moved, there is the risk of damage to the wafers due to mishandling.

Another system, rather than utilizing tanks which are open to the surrounding clean room, utilizes a full, continuous flow of cleaning solutions through a pipe-like construction. A supposed advantage of that system is that by keeping the wafers immersed in cleaning fluids throughout the process, the risk of contaminants being on the wafers is decreased. The effectiveness of this system, however, is somewhat controversial, and the apparatus is relatively expensive to purchase and to operate.

Still the most commonly used cleaning solutions are those developed by RCA many years ago employing hydrogen peroxide chemistry, particularly those referred to as "standard clean 1" or "SC-1" and "standard clean 2" or "SC-2." SC-1 typically comprises ammonium hydroxide, hydrogen peroxide and deionized water in the following ratios: 1 NH.sub.4 OH:1 H.sub.2 O.sub.2 :5 H.sub.2 O. SC-2 usually comprises 6 H.sub.2 O:1 H.sub.2 O.sub.2 :1 Hcl. Typically wafers are immersed in these solutions for 10 minutes at 25.degree.-80.degree. C. for each solution. Intermediate and final rinses of deionized water are used between chemical steps. If the wafers are particularly contaminated, there is an initial cleaning step utilizing a solution known as "Caros" or "Pirhana," typically comprised of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 in ratios varying from 2-5:1. Following the use of Pirhana there is frequently an additional etching step employing DHF (dilute hydrofluoric acid).

While those solutions contain the most commonly used chemicals and those are the most common ratios, solutions with other ingredients and solutions with different ratios have been utilized, including some with relatively dilute solutions of the active ingredients. A modified SC-1 mixture of 0.01 NH.sub.4 OH:1H.sub.2 O.sub.2 :5H.sub.2 O has been reported to help reduce surface roughening.

An additional technique for loosening particles, is that referred to as megasonic cleaning. In this technique, highly effective non-contact scrubbing action on both front and back side surfaces of the wafers is achieved by extremely high-frequency sonic energy, while the wafers are submerged in liquid. By utilizing the megasonic system, with standard cleaning solutions, films and adsorbed contaminants are removed at the same time that particles are being removed by the megasonic energy. Sonic waves of 850-900 Khz are generated by an array of piezoelectric transducers. Particles ranging in size from several micrometers down to about 0.2 micrometers have been efficiently removed with input power densities of 25 watts/in. Megasonic cleaning systems are available from VERTEQ, INC., assignee of the present invention.

As noted above, because of the advances in the miniaturization and functions of semiconductor circuit devices, improved semiconductor cleaning techniques are highly desirable. Some of the goals or industry needs are to reduce particulate levels to less than 0.1 micron, to reduce defect density levels to less than 0.001 particles/cm.sup.2, and to reduce surface metallic contamination levels to 1E.sup.8 atoms/cm.sup.2. In addition, it is desirable to eliminate chemical cross-contamination from transfer of cassettes from one tank to another, as with traditional systems. Further, an important goal is to control the cleaning processes to prevent or minimize surface microroughness of the finished product. Another goal is to reduce the high cost of ownership associated with wet chemistry processing, which includes the cost of cleaning solutions and their disposal, and many other elements. It is, of course, always desirable to lower the initial cost of equipment and to improve the reliability of the equipment.

SUMMARY OF THE INVENTION

The two priority applications referred to above contain claims directed to improved processes employing dilute chemistry, and also claims to the improved apparatus and method of use. The claims of the present application are directed to the apparatus and method of using the apparatus. However, the processes concerning the dilute chemistries are also summarized here so as to better understand the apparatus. Briefly stated, the improved semiconductor cleaning processes disclosed herein utilize highly dilute cleaning solutions different from the commonly used SC-1 and SC-2 solutions, and megasonic energy is applied to the wafers selectively during the cleaning steps and during the rinsing steps. The cleaning and rinsing steps are preferably all performed in a single tank or without moving the wafers, utilizing a combination of continuous flow, quick dump and spray rinse techniques.

Such a process accomplishes the aforementioned objectives of improved cleanliness and reduced costs of equipment and costs of ownership. The use of highly dilute cleaning solutions not only reduces the cost of materials required, but also eliminates the need for the handling of toxic materials in that most of the highly dilute solutions can be disposed of without special handling equipment or techniques. It has also been found that the excellent cleaning results are obtained in a much shorter time than that required for the conventional process of moving a batch of wafers through a sequence of cleaning tanks. Rather than using conventional SC-1 and SC-2 chemistries using highly dilute cleaning solutions enables the effective, high purity chemicals to be injected to an incoming water flow, thereby eliminating other previously used batch mixing techniques for such materials. Further, the use of dilute solutions reduces the volume of rinsing water and the amount of time required for rinsing.

The preferred cleaning system of the invention which may be referred to as "VcS," includes several steps or cycles, some of which have optional aspects, and each of which includes the use of a preferred dilute solution. Cycle 1, which may be referred as Vcl, includes a solution comprising 300-600 H.sub.2 O:2 H.sub.2 O.sub.2 :1 NH.sub.4 OH and about 14 ppm of a suitable surfactant. A second cycle, which may be referred as Vc2 employs a highly diluted buffered oxide etch (BOE). In a third cycle, which may be identified as Vc3, a solution is employed having 1000 H.sub.2 O:5 H.sub.2 O.sub.2 :1 NH.sub.4 OH and 20 ppm surfactant. In a fourth optional cycle, Vc4, a solution is employed having 1,000 parts H.sub.2 O to 1 part Hcl. These unique solutions are provided at certain preferred temperatures, in a particular sequence and manner, and also, megasonic energy is applied in certain stages.

In a preferred sequence of the Vc1 cycle, a tank containing the wafers to be cleaned is quickly filled with hot deionized water. In one example, the tank was filled in about a minute with hot DI water. The wafers are then inserted. As the hot water is entering, cold DI water is also introduced, at a low rate, to carry chemical and to cause tank overflow after the tank is filled. The diluted surfactant and the ammonium hydroxide are injected at the beginning of the cycle at rates that cause them to reach the desired ratios in the solution. Hydrogen peroxide is introduced into the tank after a delay period of about 25 seconds at a rate to cause that component to reach its desired strength in the final solution at the same time as the ammonia. All the chemical additives for the Vc1 solution are in the tank within about a minute and a half, such that the above-mentioned ratios are attained, except to the extent that a minor amount of the chemical is continuously lost due to the continued low flow of cold DI water into the tank and its overflow at the top.

In one form of the invention, each of the chemical is introduced through a manifold into the DI water flow entering the tank. The inflowing DI water and chemical are circulated throughout the lower portion of the tank and then rise as a uniform mixture. In a preferred form of the invention, a diffusing element is positioned below the wafers being cleaned to prevent splashing of incoming liquid and to cause the solution to rise uniformly in a laminar flow pattern within the tank so that all the wafers are treated in a similar manner.

In another form of the invention, each of the chemicals are injected directly into the tank rather than with the DI water. The injection ports are in the lower portion of a side wall of the tank adjacent the water inlet port and below the diffuser. Megasonic energy is applied to the interior of the tank after an initial delay of about 15 seconds, but prior to the insertion of the wafers.

One of the significant advantages of the invention is that it has been found that the wafers only need to be subjected to the cleaning solutions for a short period of time, even though they are very dilute. Thus, after the chemicals have been added in about the first 90 seconds, it is only necessary to continue the flow of cold DI water and the application of megasonic energy for about an additional 90 seconds. This creates a total cleaning cycle time of about 3 minutes.

The tank is then quickly emptied and cool DI water is sprayed onto the wafers and the interior sides of the tank. Simultaneously, the megasonic energy power level is reduced. The tank is then refilled with cold DI water at a high flow rate, while the cold spray continues. Once the level of liquid in the tank covers a megasonic energy transducer array in the lower part of the tank, the megasonic energy is once more applied at full power until the cold DI flow is terminated and the tank is once more dumped. The cold DI spray is interrupted near the end of the cold DI flow, but then is continued when the dump valve is opened. These steps of dumping and rinsing are repeated as needed.

If a diluted hydrofluoric acid (DHF) treatment of the Vc2 cycle is employed, HF in the form of diluted buffered oxide etchant (BOE) is applied to the wafers. This may be accomplished by dumping the tank contents, raising the cassette of wafers, filling the tank and then immersing the wafers in the tank. To prevent streaking, the wafers should be quickly immersed in the HF bath. This solution acts to strip the oxide, removing metals which are less electronegative than silicon. The megasonic energy is not applied during this period of time in that it has been found that the dilute buffered hydrofluoric acid treatment is better without megasonic treatment. Utilizing the megasonic treatment when the wafer surface is hydrophobic tends to cause microroughening of the surface. The wafers are only treated for about a minute before the solution is quickly dumped to a discharge tank or recycling unit.

The above method of controlling HF is one of three techniques disclosed. A second method of utilizing diluted HF is to remove the wafers from the overflowing and megasonically active DI wafer tank and move them to a free-standing recirculated and filtered diluted hydrofluoric acid (DHF) tank for the time required to remove the oxide. At the end of the time period the wafers are quickly transferred back to an overflowing DI water bath without megasonic energy. This technique is best suited for more concentrated HF solutions where greater than 100 .ANG. of oxide is required to be removed from the surface.

Another technique which is preferred for HF-last applications or applications in which only native oxides of 20-30 angstroms (.ANG..degree.) need to be removed is to inject small amounts of HF or BOE into the cold DI water stream to create the desired concentration of HF. At the end of the etch period (approximately 2 minutes), the HF injector is turned off and high flow DI water rinse is begun, removing the chemistry from the wafers without exposing them to an air interface. This reduces the particulate contamination associated with this interface.

A combination of the latter two techniques is also possible when moderate to large amounts of oxide (>300 .ANG..degree.) need to be removed and it is desired to terminate the DHF sequence with the injection techniques to reduce contamination levels for HF-last requirements. This approach would be accomplished by starting the etch in the separate recirculated and filtered HF tank for a sufficient time to remove all but tile last 50-75 .ANG..degree. of oxide. The wafers would then be quickly transferred to the overflowing DHF mixture already prepared prior to the transfer. The remaining oxide would then be removed at a much lower rate. The process is terminated by stopping injection of HF. High flow DI water then replaces the chemistry in situ. The overflowing chemistry and DI water are collected in the overflow weir and directed to a dedicated HF waste treatment drain. Once the dilution is adequate, the diverter valve can be switched to the normal plenum drain.

Rinse DI water is applied to the tank and allowed to overflow for about 5 minutes, without spray or dump. In some situations, the quick dump procedure is followed to rinse the wafers more quickly. The wafers are then dried for HF last application. Or if the Vc3 cycle is desired, surfactant is injected into the cold DI line; and after 15-30 seconds, a small amount of hydrogen peroxide is injected into the cold DI stream. During this phase, cold DI is flowing continuously through the tank at high rate, causing overflow. The flow of cold DI water, the H.sub.2 O.sub.2, and the surfactant is then stopped for about 1 minute.

Hot water flow then starts with surfactant, followed by the introduction of hydrogen peroxide through the cold DI line. Ammonium hydroxide is introduced once the bath temperature reaches 40-45.degree. C. and is continued for about 15 seconds. The flow of hydrogen peroxide and surfactant continues for about a minute and is stopped about when the NH.sub.4 OH stops, while the LOW-FLOW COLD DI water continues.

In a final cleaning step of the Vc3 cycle, hot DI water at a high flow rate is once more injected into the tank, causing a high rate of overflow at the same time that surfactant is introduced into the cold DI line. After a short delay, additional ammonium hydroxide and hydrogen peroxide are introduced for about a minute. Once the temperature reaches 50-55.degree. C., the megasonic energy is once more applied. After the introduction of the hot DI and the chemicals, only cold flow DI continues for a short period, while the application of megasonic energy continues. The tank is then subjected to a series of dump-and-rinse cycles, with the megasonic energy applied when the transducer area is covered with liquid.

If the Vc4 cycle is desired, hot DI water is introduced to displace cold DI water in the tank. After the hot water has been flowing for a short period, a small amount of Hcl is injected into the cold DI flow line. The megasonic energy is applied throughout this period of time. The tank is then subjected to a final series of dump and rinse cycles. The wafers are then ready to be dried.

The above-described process provides surprisingly good results compared to conventional "wet bench," multiple tank techniques. The particle removal capability of the diluted chemical components is remarkable, as is the minimization of undesirable metals and surface microroughness. Prior systems which were thought to be adequate were usually not even tested for the presence of particles below 0.3 microns. This process is capable of removing particles below 0.11 .mu.. In addition, the overall time required for the entire operation is much less than that required by conventional techniques. Also, less chemicals and less water are employed, resulting in reduced costs. In addition, the equipment is less expensive and the space required is reduced.

Another major advantage of the present invention is that the chemicals are used in such dilute quantities that they can, except for the HF, simply be sent to drain along with the DI water. The amount of chemical in the water may be less than that which is flushed to drain when cleaning a tank having had a more concentrated chemical component. Less chemical means less contaminants, particularly since the chemicals are the major cause of undesirable metals. Another advantage of the system is that all cleaning steps can be performed in a single tank.

Although possibly unnecessary, current environmental considerations require that even very dilute HF be stored for special handling for disposal or else to be recycled. Thus, when the liquids used during the Vc2 cycle are drained from the cleaning tank, they are transferred to a special tank rather than down a regular drain. One preferred alternative is to provide one or more reservoirs that enable the Vc2 liquid to be used again within the same tank. This can be done with a lower reservoir and an upper reservoir arranged so that the process tank can be quickly dumped and refilled, employing large valves in the side walls of the process tank.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic end view of a system utilizing the invention, including a cross-sectional view of a cleaning tank with a cassette of wafers positioned therein.

FIG. 2 is a schematic side elevational view of the tank of FIG. 1.

FIG. 3 is a schematic plan view of the tank of FIG. 1 with the liquid flow lines.

FIG. 4 is a timing and flow diagram schematically illustrating the steps of the Vc1 through Vc4 cycles.

FIG. 5 is a chart showing the composition of the cleaning solutions used.

FIG. 6 is a schematic view of a process tank with quick-dump and quick-fill reservoirs useful for an alternate arrangement for the Vc2 cycle.

FIG. 7 is a graph illustrating the effectiveness of the rinse and quick-dump technique used in the VcS process.

FIG. 8 is a pictorial graph illustrating particle removal performance comparisons of the process of the invention with conventional and state-of-the-art RCA cleans.

FIG. 9 is a timing diagram of a post CMP cleaning procedure for wafers, having a thermal or a deposited oxide film.

FIG. 10 is a timing diagram of a post CMP cleaning procedure for "bare" silicon wafers.

FIG. 11 is a side elevation schematic view of an alternate arrangement for introducing the chemicals into the tank.

FIG. 12 is a cross-sectional view of the arrangement of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, there is illustrated a process tank 10 having spaced vertical end walls and side walls, with the side walls having spaced upper portions that taper to more closely spaced lower portions. On the exterior upper edge of the tank 10 is a lip 13 having a generally triangular cross section. The lip has an upper side which tapers outwardly and downwardly preferably at about a 15.degree. with respect to horizontal and a lower wall which slopes downwardly and inwardly at about a 75.degree. angle with respect to horizontal. Such an arrangement promotes skimming across the liquid surface causing the surface liquid to be "pulled" down the exterior walls of the tank. Optionally, an overflow weir 17 may be employed to collect the overflow and direct it to a desired drain 23. This is important during the direct injection method for the diluted HF or BOE overflow. Alternatively, the entire tank may be positioned in a compartment having a suitable drain in its lower wall.

The tank is preferably sized to handle batches of 25 or 50 wafers 14 shown supported therein, or to receive a cassette supporting the wafers. The wafers are preferably positioned with their large flat sides vertically oriented and spaced in generally parallel aligned relation. As may be seen, the tank is sized to roughly conform to the shape of a group of wafers so as to minimize the quantity of liquid required.

A pair of inlet conduits 16 is illustrated near the lower portion of the tank for introducing cleaning and rinsing solutions from opposite ends of the tanks. The inlet, are oriented to produce a circular flow of the inlet liquids to facilitate mixing. A large diameter dump valve 18 is shown in the tank bottom on opposite ends of the tank. Each valve outlet is closed by a valve member mounted on a pneumatic piston 21 for quickly retracting the valve member to dump the contents of the tank quickly. A third dump valve 19 is shown in the central portion of the tank for dumping DHF since such material cannot be dumped directly through the regular valves 18.

Referring to FIG. 3, a manifold 20 is schematically illustrated connected to a small diameter conduit 27 for ducting cold deionized water into the tank at a low metered rate. This conduit feeds into a larger conduit 29 through an adductor venturi 22 connected to suitable sources of metered hot or cold DI water and the tank inlet 16a. A plurality of manifold inlets 31 are schematically shown connected to the manifold central passage for injecting precisely metered amounts of chemicals or rinse water into the conduit 27. The manifold inlets 31 are shown greatly enlarged in FIG. 3 merely for visibility, but the passages are actually very small relative to the conduit 27. Suitable commercially available injector valves, schematically illustrated at 33, may be utilized for opening and closing the injectors at the desired times. Preferably, the valves are pneumatically controlled. The inlet to the manifold 20 is connected to a high flow manifold flush conduit 51 and a precisely metered low flow conduit 53. A conduit 35 is utilized so that hot or cold mixed chemicals or pure DI during rinsing can be directed into the tank from opposite ends of the tank. A separate cold water line may be connected to inlet 16c to create a rapid fill during rinsing. A spray inlet 37 in one end of the tank is also connected to a source of DI water.

A megasonic energy generator or transducer assembly 24 is shown schematically extending across the tank above the outlets 18 and below the wafers 14. The preferred assembly includes a tube 26 extending across the tank with its ends extending through the tank end walls with suitable sealing. One or more piezoelectric transducers 28 having thin arcuate shapes are coupled by a suitable bonding epoxy or other suitable means to the interior upper wall of the tube. One suitable bonding material is an epoxy sold in sheet form by Ablestick of Gardena, Calif., and identified as ECF 550. Electrical connections 30 extend through the ends of the tube exterior of the tank. Very high frequency energy in the range of 800-900 Khz and above is applied to the transducers causing them to vibrate, which in turn transmits such vibration into the cleaning solution.

In a preferred arrangement, the tube is formed of aluminum and is provided with a Teflon sheath or layer on its exterior so that the assembly can be used with cleaning solutions that would otherwise attack aluminum. Because of the highly dilute chemistries utilized by this invention, the use of a Teflon sheath covering the aluminum is very satisfactory. Stronger concentrations, typical of SC-1 and SC-2 could be a problem due to the penetration potential into Teflon of these chemicals, especially Hcl. Making the tube of quartz is also a good alternative. Quartz is not as strong as aluminum, and it may require the protective Teflon layer for some uses. Other materials are currently being evaluated to replace the aluminum and the sheath.

An elongated partition 34 divides the tube interior into an upper chamber and a lower chamber, as best seen in FIG. 1. Cooling water is ducted through the lower chamber by an inlet 36 and an outlet 38 to maintain the temperature of the transducer within an acceptable limit. An inlet 40 and an outlet 42 are provided for the upper chamber for the ducting of nitrogen or other inert gas to prevent other gas from entering the tank. A sensor 25 senses the liquid level in the tank and will de-energize the assembly, or cause it to switch to a low power setting, when the level is below that. This keeps the transducers at a satisfactory temperature. The assembly 24 is the subject of U.S. patent application Ser. No. 042,889, filed Apr. 5, 1993, now U.S. Pat. No. 5,365,960, issued Nov. 22, 1994, which is incorporated herein by reference.

Operation

Vc1 Cycle

In operation of the VcS system, one or more wafers 14 are positioned in the tank 10. In a preferred form of the invention, the wafers are first subjected to the cleaning steps of the Vc1 cycle, as illustrated in the timing diagram in FIG. 4. As may be seen, the time in minutes is shown on the top line of the diagram, and the various liquid flows and process steps are set forth on the left side of the diagram. The time lines for these items are raised when "on" and down when "off."

Hot deionized water is introduced to the tank 10 by way of conduit 29 and inlets 16a and b, as shown on the hot DI line in FIG. 4. In one form of the invention, the water was introduced at a rate of a little over 16 liters per minute at 80.degree. C. During the first 10 seconds, the dump valves 18 are left open as the water is being brought up to temperature and the conduits are being flushed. In a prototype form of the invention, a tank having a liquid capacity of approximately 16 liters was employed, with the result that the tank was filled in about a minute after the dump valves were closed. The water then flows over the top of the tank, and the water is shut off in a little less than a minute and a half.

At the same time that the precisely metered hot DI water is being introduced through conduit 39, a precisely metered low flow cold DI trickle is also introduced through the conduit 53, the manifold 20, and conduit 27 to the conduit 29. The flow rate of the cold DI water is very small, only about 379 cc per minute in a prototype system, with a temperature of about 30.degree. C.

A suitable surfactant diluted with DI water at about 100:1 is introduced through the conduit 46 and into the manifold 20, at a very small rate, such as about 32 cubic centimeters per minute to provide an ultra dilute concentration in the tank. While various surfactants may be employed, a preferred one found to be of high purity is that sold under the trade name WAKO, NCW-601A, sold by Wako Chemicals. As shown in FIG. 4, the surfactant is introduced at the beginning of the cycle and continues for about one minute. The surfactant protects the wafer to some extent from the NH.sub.4 OH and also reduces surface tension of the DI water.

Ultra high purity, parts-per-billion (ppb) grade NH.sub.4 OH is also introduced at the beginning of the cycle through line 48 at a slow rate of about 53 cubic centimeters per minute, in a particular system. The surfactant and ammonium hydroxide are introduced to the cool deionized water flow in the manifold because the manifold valves 33 are more reliable at the temperature of unheated deionized water. Also, if the injected chemicals are in gaseous form, better mixing is obtained.

After the ammonium hydroxide and surfactant have been flowing for about 25 seconds, seconds, ultra high purity, ppb grade hydrogen peroxide is injected through the conduit 50 into the manifold. Although the H.sub.2 O.sub.2 is usually injected first to grow an oxide to minimize surface roughness that is caused by the NH.sub.4 OH, it has been found that microparticle loosening is enhanced by allowing the ammonium hydroxide, surfactant mixture to perform initial cleaning before the hydrogen peroxide is injected. The hydrogen peroxide is introduced at a rate double that of NH.sub.4 OH, or of approximately 106 cubic centimeters per minute. This occurs for only about one minute, with the NH.sub.4 OH flow terminating at that time also.

The flow rates employed are intended to provide a mix ratio in the process tank of approximately 300 H.sub.2 O:2 H.sub.2 O.sub.2 :1 NH.sub.4 OH with only about 10 to about 20 parts per million surfactant. This is the approximate concentration after the cycle has been underway for about 85 seconds. The hot deionized water and the H.sub.2 O.sub.2 and NH.sub.4 OH components are all stopped at that point, the surfactant introduction having already stopped. Prior uses of surfactant in the cleaning of wafers have indicated that 100 parts per million was the minimum amount. In instances of minimum particulates on the wafers, even less surfactant may be sufficient. Also, more surfactant could be used and may be desired, for particularly "dirty" wafers. However, the range mentioned above is preferred for most situations. Too much surfactant can require longer rinse time.

Although the final mix ratio of the chemicals and the deionized water is as expressed above, it should be recognized that the introduction of chemicals and the mixing of them with deionized water is somewhat of a dynamic situation. The NH.sub.4 OH was introduced before the H.sub.2 O.sub.2, but the initial portions of the NH.sub.4 OH and the DI water flow out of the top of the tank before the flow is stopped. The rate of flow for the two chemicals is the same as that in the final relationship. As chemical is introduced into the bottom of the tank, the lower portions of the wafers are subjected to the DI water mixed with chemical before the upper portions, even though the mixing within the tank is effective. As the time elapses, perhaps the lower portions of the wafers are subjected to a slightly more dilute solution than the upper portions. On balance, however, the final mix ratio is believed to be fairly uniform for about 90 seconds after the chemical injection has ceased.

As may also be seen from the RF line in the timing diagram, the megasonic energy is applied to the tank contents after about 20 seconds from the start, the water level having submerged the transducer assembly. The megasonic energy greatly enhances the effectiveness of the cleaning operation and synergistically enables the outstanding results to be obtained with such dilute solutions. This is in contrast to the much longer times required for conventional wet processing utilizing more concentrated solutions and not employing megasonic energy on the wafers.

While the dilution ratio of 300 DI H.sub.2 O to 1 NH.sub.4 OH and 2 H.sub.2 O.sub.2 is preferred for the greatest cleaning power, increasing the dilution to 600 parts H.sub.2 O has been successfully used on easier to clean contaminants. The actual concentration is determined by the contamination on the "feed" wafer.

The chemistry used in the Vc1 cycle replaces the use of hot sulfuric acid and hydrogen peroxide, commonly referred to as "Piranha," and acts to remove light to medium organic residues and particulates prior to a step employing hydrofluoric acid. The Piranha treatment may still be needed initially for the stripping of wafers having heavy organic deposits such as photoresist.

As mentioned above, in situations in which the tank is not filled with deionized water and a solution is being introduced to the tank, undesirable splashing can occur under the wafers, or out of the tank onto an operator. Thus, the diffuser or straightening device 43 is shown positioned above the inlet 16 to prevent the splashing and to direct the incoming solution generally vertically upwardly in the tank. Also, in cases in which the tank is initially filled with water, the vanes of the diffuser help uniform displacement of the untreated water.

Allowing liquid to overflow the upper end of the tank carries contaminants loosened from the wafers out of the tank. The angled lip 13 on the tank provides an outward component to the flow which has been found to improve the overflow action.

As may be seen from the timing diagram, the cleaning of the Vc1 cycle is complete after 3 minutes. The remaining 2.5 minutes of the cycle are utilized for a series of rinsing steps, which include quickly dumping the contents of the tank, quickly refilling the tank, as shown by the HI-FLOW COLD DI line and the Quick fill D1 line in FIG. 4, while spraying the wafers and the tank walls through nozzles 19. The spraying and the high flow DI continue throughout the rinse phase, as shown. All of the chemicals except HF can be directed into a conventional drain inasmuch as the solutions are so diluted.

Megasonic energy is applied to the liquid during rinsing as soon as the transducer assembly 24 is covered. The rinse water is allowed to overflow the top of the tank for about three seconds and then the liquid is dumped to drain. That rinsing step is then repeated as necessary for a particular operation. Four rinse and dump cycles are shown on the timing diagram and have been found necessary in initial testing employing a cassette holding the wafers. In some situations such as with no cassette, as shown in FIG. 1, fewer quick dump and rinse cycles may be satisfactory. Also, a smaller tank may be utilized without a cassette, and that, in turn, would slightly reduce the cycle times.

The rinse sequence is very fast (20 seconds for each fill) inasmuch as the high flow cold DI water, a quick fill of fine manifold flush, and the spray are functioning. Thus, DI water is flowing at a rate of approximately 48 liters per minute, and a 16-liter tank is quickly filled to a level to cover the wafers in approximately 20 seconds. The actual time is dependent on water pressure which varies considerably. Much less water is used than that in conventional rinsing, since only 3-5 seconds of overflow is used as compared to other systems, wherein water flows through the system for a much longer period of time. Deionized water is sprayed on the wafers by means of spray nozzles to prevent contaminants from settling on the wafer while deionized water is being introduced from below. The spray preferably provides relatively large droplets as opposed to mist. This is important to reduce the potential of electrostatic discharge associated with high resistivity (18.2 megohm cm.sup.-1) DI water spray systems.

During rinsing, the manifold 20 is flushed with cold DI water to eliminate traces of the chemical that were added. This is injected through line 51 of FIG. 3 and is shown on the "Manifold Flushed" line of FIG. 4.

Vc2 Cycle

At the completion of the Vc1 cycle, it is usually desirable to subject the wafers to DHF in the Vc2 cycle. Because under current environmental controls a DHF solution cannot be simply dumped directly to a conventional drain, it is most practical to filter, store and reuse the solution. This is especially true for more concentrated DHF mixtures such as 30:1 or 50:1. The wafers may be subjected to the solutions in several different ways. The wafers may be transferred from the tank 10 to an adjacent tank (not shown) containing the desired DHF solution, and then returned to the tank 10 for rinsing. This is necessary if >100 .ANG..degree. of oxide are to be removed. More concentrate DHF may be required to reduce the etch time.

However, preferably the wafers are treated in the same process tank 10. It is also desirable that the solution be applied quickly to the wafers to prevent streaking or lining. Thus, in a second approach, the wafers may be lifted from the tank 10 as the tank is filling and then quickly lowered into the tank solution. In a third approach, discussed below in connection with FIG. 6, the solution is quickly added to and drained from the tank without moving the wafers.

All deionized water low flow is interrupted for at the commencement of the Vc2 cycle, so as not to change the concentration of the solution to be added. Megasonic energy is not applied during the Vc2 cycle, as noted in the RF line of FIG. 4.

Assume that the wafers are lifted above the tank 10, the DHF solution is quickly introduced to the tank through conduit 54. The solution comprises 100 parts DI H.sub.2 O to one part of buffered oxide etchant (BOE). The etchant, however, is purchased in liquid form having a composition of over 50% water to 1 HF:5 NH.sub.4 F, and a surfactant buffering agent. The final mix in the tank is thus, in effect, 1 HF:5 NH.sub.4 F:200+ DI H.sub.2 O. A suitable etchant is sold by Ashland Electronic Chemical Division at Columbus, Ohio under the trade name ULTRA ETCH. The etchant has an assay range of 33.6-34.6% for the NH.sub.4 F and a range of 7.13-7.23% for the HF.

The Vc2 cycle solution acts to strip the oxide from the wafers, removing metals which are less electro-negative than silicon. The BOE material creates very little surface roughening.

In a prototype system it takes about 30 seconds to fill the tank with the Vc2 cycle solution and about 30 seconds to drain the solution. The wafers are raised when the tank is draining, and should be transferred to a DI H.sub.2 O holding tank with such slow filling, to prevent drying. The BOE time line in FIG. 4 shows the BOE "on" for about one minute, without showing the filling and draining time, since only a few seconds are required to transfer the wafers to a different tank. Also, the main tank can be filled and emptied quickly as discussed below. The solution is transferred to a separate tank to be filtered, stored and reused.

As mentioned above in the Summary of the Invention, another technique which is preferred, if the HF step will be the last or if only native oxides 20-30 .ANG..degree. need to be removed, is to inject small amounts of HF or BOE through the manifold conduit 52 into the cold DI flow from conduit 53 into the manifold 20 to create the desired concentration of HF. At the end of the etch period (approximately 2 minutes), the HF injector is turned off and high flow DI water rinse is begun, removing the chemistry from the wafers without exposing them to an air interface. This reduces the particulate contamination associated with this interface. The combined technique mentioned above may also be used.

Assuming the Vc3 cycle is to follow, the wafers in the tank can be treated by a series of quick dump and refill cycles without spray. The rinse water can flow directly to a conventional drain. Spray-rinsing is commenced as the cleaning solution drains. The tank is filled with cooled DI water, when the cycle ends.

If the wafer cleaning is to end with the HF treatment, the wafers are rinsed with DI H.sub.2 O for about 5 minutes, as shown in FIG. 4, the water being allowed to overflow. The wafers are subjected to a drying procedure as soon as the wafers are removed. The reason for not using the dump and spray rinse procedure is that dumping and spraying cause contamination on the wafers. This is not as much a problem if the Vc3 cycle is employed in that it removes such contamination; however, even in this sequence, the overflow is utilized to minimize contamination and possible damage to the wafer by exposing it to air water interfaces.

In order to improve the particulate levels of the HF rinse sequence in either scenario above, point-of-use filtered DI water is employed. The filter of choice is made by Pall Corp., and identified as a "Posidyne N-66" filter, with an absolute pore size of 0.045 micron. The Posidyne line of filters has a positive "Zeta potential" which has a strong attraction for colloidal silica in the DI water stream. If the filter is not used, the colloidal silica particles are attracted to the positive surface of the silicon wafer directly after the HF treatment in which all of the native oxide is removed. All of the DI water used in obtaining the VcS system results disclosed herein was filtered in this manner.

Vc3 Cycle A third cleaning solution is then created for the Vc3 cycle which is similar to the solution used in the Vc1 cycle, but is even more dilute, as well as being delivered in a unique manner. As a first step of the Vc3 cycle, a dilute surfactant, such as the WAKO product discussed above, is introduced in the cold DI H.sub.2 O LOW-FLOW conduit 27 at a rate of about 32 cubic centimeters per minute for about 80 seconds, as shown in FIG. 4, creating a very dilute surfactant solution. At the same time, the controlled flow cold DI water is flowing into the tank through the conduit 41, causing it to overflow. The surfactant reduces the surface tension of the wafer and coats the surface with a thin organic film prior to the introduction of the chemicals. This acts to reduce the chemical attack of the delicate, extremely reactive surface immediately after the dilute HF treatment.

About 20 seconds after the surfactant flow is initiated, ultra high purity, ppb grade hydrogen peroxide is added through the injector 50 into the manifold 20 at a rate of about 106 cubic centimeters per minute for about 1 minute to begin the slow regrowth of native oxide on the surface, together with the low flow cold DI water through the manifold. The addition of the H.sub.2 O.sub.2 ends at the same time as the surfactant. These rates create a dynamic mixture of about 400 H.sub.2 O 2 H.sub.2 O.sub.2 and about 20 parts per million surfactant, considering that cold DI water is flowing into the tank as the chemicals are being added, both at the high flow rate through conduit 29 and at the low flow rate through conduit 27. Although the chemical is very dilute, dilution ratios up to 800 parts H.sub.2 O have proven effective. After the H.sub.2 O.sub.2 is introduced, the controlled flow DI water injection through conduit 41 is interrupted, leaving only the LOW-FLOW DI through conduit 27 to assist in the overflow of contaminants. This step is continued for about 1 minute, as can be seen from the timing diagram. The solution temperature during this step remains at about 30.degree. C. The purpose for this first step of the cycle is to slowly grow native oxide on the wafers in such a manner as to minimize surface microroughness, with no megasonic energy applied.

As seen from the timing diagram, at about 21/2 minutes from the start of the Vc3 cycle, controlled flow hot DI water begins to be flowed through the tank from the conduit 39 for about 11/2 minutes. Simultaneously, surfactant is introduced at about the 32 cc/min. flow rate for that same time period. After about 25 seconds, hydrogen peroxide is injected into the cold DI stream in the manifold at the rate of about 106 cc/min. The NH.sub.4 OH flow begins about 45 seconds after the H.sub.2 O.sub.2 and continues for about 15 seconds at the rate of about 53 cc/min. As may be seen from the timing diagram, the controlled flow hot DI water flow, the surfactant flow NH.sub.4 OH, and the H.sub.2 O.sub.2 flow stop at the same time. The solution temperature during this step has increased from about 30.degree. C. to 45.degree. C. The wafers are therefore subject to this warm bath with the highly diluted chemical ratios expressed. The purpose for this step is to further build up the oxide by increasing the liquid temperature, followed by the injection of minute amounts of NH.sub.4 OH to remove particulates. This procedure is important to prevent increasing surface roughening.

At the completion of that two-minute step a third injection step occurs, which is the same as that of step 2 of the Vc3 cycle, except that the NH.sub.4 OH flow continues longer. That is, controlled hot DI water is flowed through the tank along with injection of surfactant, and after about 25 seconds, NH.sub.4 OH and H.sub.2 O.sub.2 are injected into the tank for about 1 minute, at about the 53 and 106 cc/min. rate respectively. After that injection, the hot DI water and the surfactant are also interrupted and the tank is left with only the low flow cold DI water for a period of about 1 minute. The temperature of the bath has increased to about 60.degree..

As a very significant addition in the third step of the Vc3 cycle, the megasonic energy is once more applied to the tank at the time when the bath temperature reaches 50-55.degree. C., assuming the adequate growth of native oxide. The megasonic energy was not applied during the first two steps of the Vc3 cycle, so as not to interfere with the formation of oxide on the wafers, which is very critical to minimize surface microroughening.

After this 1 minute treatment, the tank is subjected to the quick dump, spray and refill cycles, including flow through the manifold. Megasonic energy is also applied when the tank is sufficiently filled, as with the dump and rinse cycles of Vc1.

The purpose of the Vc3 cycle is to grow a chemically pure, metal-free native oxide on the wafers without increasing the surface microroughness, as well as to remove particles attached to the surface during the DHF sequence of the Vc2 cycle. The unique sequence of the Vc3 cycle is designed to achieve the lowest surface microroughness possible utilizing a high Ph cleaning chemistry which is very conducive to removing particles. It also controls the deposition of metallic species typical of high Ph peroxide solutions by controlling the purity of the chemicals used. This is done by the extreme dilutions of the already ultra pure ppb grade chemicals, as well as by the short exposure time of such chemicals.

Vc4 Cycle

At the completion of the Vc3 cycle, the rinsed wafers remain in cool DI water, with the low flow cool DI water continuing and with the megasonic energy being applied to the tank. At the commencement of the Vc4 cycle, hot DI water is added for a period of time, about 90 seconds. After the temperature of the tank has increased for about 20 seconds, Hcl is introduced through interjector line 56 for about one minute. This results in a final mix ratio of about 1,000 H.sub.2 O to 1 Hcl. After the hot DI flow and the Hcl flow are interrupted, the wafers continue to be subjected to this highly dilute Hcl treatment, as the low flow cold DI and the megasonic energy continues for about one minute. The solution temperature created by the hot DI water flow is preferably about 45.degree. C. to obtain maximum particle removal efficiency. The purpose for the cleaning of the Vc4 cycle is to remove additional traces of metallic species.

As seen from FIG. 5, a final series of quick dump, spray, and refill cycles are then initiated with the megasonic energy applied whenever the transducer is sufficiently immersed. The Vc4 cycle is then complete, as is the cleaning of the wafers. It is then normally necessary to dry the wafers. This can be done in a variety of known ways, either in the same tank or by being moved to an adjacent tank.

If the wafers are to be dried in the same tank, the rinse liquid is drained from the tank in a manner consistent with the drying technique. If the wafers are to be removed to another device for drying, the tank should be left full with rinse water overflowing, preferably with the megasonic energy "on." This ensures that the wafers are not contaminated while waiting for drying.

As may be seen from the timing diagram of FIG. 4, the Vc1 cycle required about 51/2 minutes, the Vc2 cycle about 6 minutes, the Vc3 cycle a little more than 91/2 minutes, and the final cycle being a little less than 41/2, for a total time of about 26 minutes. Further, as noted above, the Vc4 step may not be needed in many instances, and hence only about 21 minutes are needed. By contrast, the conventional wet bench technique employing a separate tank for each step takes about 90 minutes to process a single batch of wafers. With conventional techniques, additional batches of wafers may be started through the early stages of a multiple tank cleaning operation to clean about 300 wafers per hour, utilizing nine different tanks and a series of batches of wafers. The process of the present invention can provide comparable results with fewer tanks. Further, less megasonic apparatus and less handling equipment is needed.

In addition to the savings concerning time and the number of tanks, there are significant savings in the volume of deionized water utilized and the volume of concentrated chemicals. For example, a two-minute rinse and quick dump step is contrasted with a typical 10-20 minute rinse step for conventional techniques to achieve equivalent rinse quality. The reduction in time is due to the use of dilute chemicals which require less time for the removal of chemicals from the wafers and tank walls, and the selected use of megasonic energy during cleaning and while rinsing to prevent particles from reattaching to the walls and wafers. The megasonic energy also moves rinse water through the molecular boundary layer at the surface of the wafer, cassette, and tank walls, which is extremely difficult, if not impossible, to achieve in methods not employing sonic activated methods. For this reason, increased efficiency in rinsing occurs, as can be seen in Diagram 7. In addition to the cost of deionized water, there is an increased cost for the energy required to cool it or heat it. Thus, by reducing the quantities of heated or cooled water, the energy saving steps are in addition to the water saving.

The effectiveness of the rinse and quick dump technique as used in the VcS cleaning process may be better appreciated with reference to FIG. 7. The deionized water has a Ph level of slightly less than 7. The addition of the process chemicals to the DI water changes the Ph level. Rinsing the chemicals from the wafers and the tank walls moves the Ph level back towards that of deionized water. Hence, graphing the pH level of the cleaning liquid provides a convenient technique for monitoring the effectiveness of the rinsing process. The diagram of FIG. 7 shows the pH value of the liquid during the VcS cleaning process, and the elapsed time during the process is shown on the horizontal axis. Also, the various cycles of the process and the rinse dumps are indicated on the graph.

As seen for the Vc1 cycle, the pH level rises sharply to almost the 10 level as the chemicals are added. After the first rinse and dump step, the pH level has been reduced to almost the 9 level. After the second rinse and dump step, the pH level has dropped to about 8.5, and after the third rinse and dump step, the level has dropped to about 7.6. The wafers are almost adequately rinsed at that point, but a fourth rinse and dump step is desired. As can be seen from the graph, this final step drops the pH level below the rinse target and attains that of deionized water, thus indicating that the wafers and the tank are thoroughly rinsed. This outstanding performance was obtained in only two minutes. As noted above, overflow rinsing for systems employing conventional concentrated chemicals would require 10 to 20 minutes to obtain comparable quality, even those using megasonic energy.

As further seen from the graph, the effectiveness of the rinse and dump steps after the Vc3 cycle is even more dramatic in that the pH level is almost at the rinse target after a third rinse and dump step.

The dilute HCl of the Vc4 step lowers the pH value to about 2.4, as may be seen from the graph. The pH level is raised to the target level after three rinse and dump steps in the two minute rinse procedure.

As noted above, the rinse and dump step is not utilized after the pH level has been lowered by the HF of the Vc2 cycle. Instead, the overflow technique is utilized. As can be seen from the graph, the pH level after 5 minutes has been raised considerably but it is still not quite to the target range even though the chemical in the solution is very dilute. Note that BOE acid has a neutral pH. Hence, the graph is illustrating a dilute HF mixture.

It may be useful to consider some of the other differences between the standard wet bench technique and the present invention in order to appreciate and better understand some of its advantages. While the Piranha solution is useful for removing significant quantities of organic material, it leaves particles on wafers and is difficult to rinse. Rinse time of 15-20 minutes with hot DI water is typical in the industry with conventional systems. The solution of the Vc1 cycle is not only effective to remove light organic materials, but due to the dilute quantities of chemicals and surfactant utilized, and due to the use of megasonic energy in both the clean and rinse steps, the resulting wafers have fewer remaining contaminants and much faster rinse rates. A conventional HF acid cleaning step removes oxides and metals, but adds significant particle levels, as well as possible surface microroughening. The use of the cleaning solution of Vc2 accomplishes similar results but with less HF and reduced surface microroughening. Typically, SC-1 is utilized after the HF step. It is effective in removing particles above 0.2 microns, however, below that, the effectiveness drops significantly. Also SC-1 tends to add trace metallic contaminants and significant surface roughness. The use of the cleaning solution of Vc3, however, efficiently reduces particles down to 0.1 micron, does not add metals, nor cause significant microroughness. The SC2 solution removes metals, but it tends to add particles. The solution of Vc4 is probably not even needed in many instances, but it will remove trace metals and actually removes, rather than adds, particles.

In determining the effectiveness of the VcS process, tests have been performed to compare the particle removal capability with that of the well-known SC-1 and SC-2 systems on clean wafers. This is illustrated in a three-dimensional graph in FIG. 8, wherein one horizontal axis indicates the different particle sizes of 0.11 micron, 0.15 micron, and 0.20 micron; the other horizontal axis indicates the various cleaning cycles; and the vertical axis indicates the "particles," that were added to so-called clean wafers when subjected to the processes indicated. The numerical values shown in the boxes are the average of these tests. It should be noted that the "particles" are identified by a laser light technique, and are commonly referred to in the industry as light-point defects or LPDs. Thus, the vertical axis actually indicates the light points observed, which in many instances are particles, particularly the large sizes. Even if some of the light points are actually roughness points on the wafer, that too is an undesirable defect added by the cleaning processes.

The back row of the graph shows the results obtained with the RCA clean system with its conventional SC-1 ratio of 1 part H.sub.2 O.sub.2, one part NH.sub.4 OH, and five parts water, and SC-2 ratio of 6 H.sub.2 O:1 H.sub.2 O.sub.2 :1 HC1. Such system uses the conventional overflow rinsing steps, as discussed above, without the use of megasonic energy during the rinse. However, megasonic energy could be used during the SC-1 sequence. As can be seen, 132 defects at the 0.2-micron level were observed as being added to clean wafers, 318 defects above the 0.15-micron level were observed, and 2,057 defects were detected above the 0.11-micron level. Frequently, defects are measured no lower than 0.25 microns, and the number of defects detected at that level may be quite small, such that the wafer may be considered clean by old standards.

The second row from the back of the graph employs the same RCA clean approach, except that the NH.sub.4 OH component in SC-1 is diluted to 0.05 in relation to one part H.sub.2 O.sub.2 and five parts water. As may be seen, the results are slightly better than the standard system at the 0.2-micron and 0.15-micron levels and are considerably better at the 0.11-micron level where only about one-third as many defects were added as with the normal concentrated approach. This graph then illustrates the value of the more dilute concentration of NH.sub.4 OH with regard to surface microroughening, which is thought to be what the Tencor 6200 is identifying at the 0.11 .mu. sensitivity, as has been indicated by Professor Ohmi and others.

The row on the graph second from the front also uses the dilute form of the RCA clean, but in addition, megasonic energy and the quick-dump rinse treatment was utilized during the final rinse cycle. As can be seen, this produced much better results in that only six defects were added at the 0.2-micron level and only 19 were added at the 0.15-micron level. The reduction to 435 defects at the 0.11-micron level in contrast to 657 without the megasonic is also a significant improvement, but not as dramatic as at the 0.15- and 0.20-micron levels. Thus, this test indicates the value of the quick-dump rinse with megasonic energy being applied.

As can be seen from the front row of the graph, the VcS system actually reduced the number at defects of the 0.2 micron level by 29. Likewise, it reduced the number of defects at the 0.15 micron level by 31. Further, even at the extremely small particle size of 0.11 microns, only 68 defects were added. The chart of FIG. 8 thus illustrates the dramatic difference between the VcS system and the standard RCA clean system. Further, it illustrates the importance in the present invention of the use of dilute chemistries, the use of megasonic energy, the megasonic quick dump and rinse cycles, as well as the sophistication of the Vc3 sequence, which is what is responsible for the extremely low defect count at 0.11 .mu..

Similar tests have also been conducted with regard to the cleaning efficiency of merely the Vc1 cycle, which is the primary particle removal cycle. In cleaning wafers having heavy contamination of silicon nitride, the Vc1 step eliminated most of the particles at 0.15 microns and above. In a specific test, the contaminated wafers had an average of over 2200 particles, and after cleaning, they had an average of about 15 particles. This represented the removal of more than 99 percent of the particles. With the megasonic off, the cleaning capability was not very effective (less than 3% removed).

In another test, silicon nitride particles were deposited on "clean" wafers having an average of 258 particles at 0.11 .mu.. This raised the particle count to about 1500. After the Vc1 clean, the average dropped to about 130 at 0.11 .mu. for a cleaning efficiency of >100% removal. That is, the wafers were cleaned to a level better than that before the deposition.

A similar test was conducted with respect to wafers contaminated with HF. The