The polishing agent of the invention has polishing grains suspended in a solution. The polishing grains consist essentially of a first substance with a glass transition temperature T.sub.G, and the polishing grains contain a dopant. The concentration of the dopant is set so that the glass transition temperature T.sub.G ' of the doped substance is lower than the glass transition temperature T.sub.G of the undoped first substance. The polishing agent is advantageously used for the microscratch-free planarization of a semiconductor substrate or of layers applied on it.
For example, aluminum oxide (Al.sub.2 O.sub.3) has a hard hexagonal phase (corundum) and a soft cubic y-phase (alumina). There are actually eight different known polymorphs of silicon dioxide (SiO.sub.2): quartz, cristobalite, tridymite, coesite, stishovite, keatite, melanophlogite, and fibrous silicon dioxide. All these materials each have a defined hardness and density. The possibilities for varying the hardness of the polishing grains used are restricted by the selection of polymorphs available. This does not allow a continuous spectrum of properties. The grinding process can hence be matched to the hardness and composition of the material to be planarized only within limitations.
SUMMARY OF THE INVENTION
The object of the invention is to provide a polishing agent for semiconductor substrates, which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which allows polishing at the highest possible erosion rate, yet avoids the creation of microscratches.
With the above and other objects in view there is provided, in accordance with the invention, a polishing agent, comprising:
a solution and polishing grains suspended in the solution;
the polishing grains consisting essentially of a first substance with a given glass transition temperature T.sub.G ;
a dopant in the polishing grains to form a doped substance;
the dopant having a concentration set to lower a glass transition temperature T.sub.G ' of the doped substance to below the given glass transition temperature T.sub.G of the undoped first substance.
The invention hence proposes, through the controlled addition of one or more materials, to modify the substance, of which the polishing grains consist, in such a way that its glass transition temperature is lowered. As a consequence, the substance becomes softer.
For particles in the size range of abrasives, with a diameter of between 50 and 500 nm, a macroscopic value such as hardness can no longer be meaningfully used. Nevertheless, it has been shown that the hardness relevant to the grinding process depends monotonically on the glass transition temperature.
It is particularly expedient to limit the addition of the dopants so as to avoid any phase transition of the first substance. In this case, the addition of the dopant merely causes a slight structural modification of the first substance, but not a phase transition.
The addition of the dopant causes the long-range order and therefore the crystal structure to be weakened. By varying the dopant concentration, the degree of hardness of the polishing grains can in this way be adjusted precisely. Transition to the vitreous state is not necessary for this softening.
Even though transition to the vitreous state is not necessary for the desired softening of the polishing grains, the glass transition temperature nevertheless constitutes a criterion which can be used to find suitable soft compositions. In this context, the term glass transition temperature refers to the ideal glass transition temperature, i.e. the temperature at which a transition to the vitreous state would take place with infinitely slow cooling. In terms of thermodynamics, this temperature T.sub.G is wherein the entropy S.sub.glass (T) is higher than the entropy S.sub.crystal (T) at that level. According to the invention, however, it is not the glass transition per se that matters, since even a crystal structure modified in the manner described above can meet the requirement of being softer in a controlled way.
An upper limit for the addition of dopants is formed by those concentrations at which there is a risk of phase separation (efflorescence) or inverted viscosity (i.e. system A doped with system B converts to system B doped with system A).
In accordance with an added feature of the invention, the concentration of the dopant is set to lower the glass transition temperature T.sub.G ' of the doped substance by at least 10% relative to the given glass transition temperature T.sub.G of the undoped first substance.
In accordance with an additional feature of the invention, the first substance is an oxide of a metal or a semimetal.
In accordance with another feature of the invention, the first substance is selected from the group consisting of SiO.sub.2, Al.sub.2 O.sub.3, and Ce.sub.2 O.sub.3. The erosion behavior of these substances is known, and so is their interaction with the chemical additives contained in the polishing agent.
In accordance with a further feature of the invention, the dopant contains at least one element selected from the group consisting of B, P, As, Sb, Si, and Al. Since, apart from its being weakened, the crystal structure is preserved when the dopant(s) is added, the interaction in the total system, consisting of the layer to be processed, the solvent and the additives which it contains and the polishing grains, is changed only to the extent that the creation of microscratches is prevented by the softening of the polishing grains. Adjustment of the other parameters of the planar polishing process can be avoided. Complete new development of the polishing process is therefore unnecessary. For a correspondingly produced polishing agent, there is therefore a high potential for use in a chemical mechanical polishing process.
In accordance with again a further feature of the invention, the diameter of the polishing grains is from 50 to 500 nm.
Owing to technical, that is to say physical and chemical constraints, the selection of suitable substances as the first substance for the abrasive is restricted. The addition of one or more dopants makes it possible to tailor the hardness of the abrasive to the respective substrate to be polished, without it being necessary to change the abrasive.
The above-described polishing agents can be used for the microscratch-free planarization of semiconductor substrates or of layers applied on them. With the above and other objects in view there is provided, therefore, in accordance with the invention, a planarizing method, which comprises the following method steps:
providing a polishing agent as described above; and
planarizing a surface of a semiconductor substrate or of layers applied on the semiconductor substrate substantially without microscratches by polishing the surface with the above-described polishing agent.
In accordance with a concomitant feature of the invention, a first substance is used with a hardness such that, when the surface is polished with the first substance in the undoped state, microscratches are created in the surface, and the concentration of the dopant is chosen high enough that no microscratches are created in the surface on the semiconductor substrate during the planarizing polishing with the doped substance.
With this special use, the hardness of the polishing agent can be tailored precisely to the hardness and composition of the semiconductor substrate to be processed.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a polishing agent for semiconductor substrates, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the relationship between glass transition temperature and melting point; and
FIG. 2 is a graph showing the relationship between melting point and the hardness of the polishing grains.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a relationship between glass transition temperature and melting point that can be derived from the following Table 1:
TABLE 1
Glass Transition
Temperature [K] Melting Point [K]
S 246 342
Se 318 490
As.sub.2 Te.sub.3 379 633
ZnCl.sub.2 380 590
As.sub.2 Se.sub.3 468 633
As.sub.2 S.sub.3 478 573
B.sub.2 O.sub.3 530 793
BeF.sub.2 598 821
GeSe 695 980
GeS.sub.2 765 1073
GeO.sub.2 853 1388
SiO.sub.2 1453 2003
FIG. 1 displays a monotonic increase in melting point with the glass transition temperature for the substances described.
FIG. 2 represents the Mohs scratching hardness as a function of the melting point. The values are taken from Table 2 below:
TABLE 2
Melting Mohs Hardness
Point [K] (scratching hardness)
KCl 770 2.2
NaCl 800 2.5
NaF 992 3.2
BaO 1925 3.3
SrO 2430 3.9
CaO 2570 4.5
MgO 2642 6.5
The relationship represented in FIG. 2 between melting point and scratching hardness shows a monotonic increase in scratching hardness with the increase in the melting point.
Accordingly, this gives a monotonic increase in hardness with rising glass transition temperature as well.
The lowering of the glass transition temperature through the addition of dopants is given, with reference to the example of PSG (phosphorus silicate glass) and BPSG (borophosphorus silicate glass), according to the following Table 3:
TABLE 3
Glass
Composition Transition
Mass % B.sub.2 O.sub.3 Mass % P.sub.2 O.sub.5 Temperature
T.sub.G
PSG 800
BPSG-A 11.9 4.2 645
BSG 18.9 515
PSG 7.5 750
PSG 6.9 725
The preparation of the colloidal systems suitable for the polishing agent will be explained below with reference to the example of SiO.sub.2 as the first substance of the polishing grains. It is, of course, equally possible to carry out these processes with other first substances and other dopants.
It is possible to produce colloidal SiO.sub.2 with a dopant using various methods.
In a pyrogenic process, a SiCl.sub.4 /AlCl.sub.3 mixture is reacted in an oxyhydrogen gas flame and the reaction products are hydrolyzed. In the mixed oxides created by this process, aluminum oxide Al.sub.2 O.sub.3 is incorporated as dopant in the primary structure of the host oxide silicon dioxide SiO.sub.2. The incorporation of other dopants, for example boron or phosphorus, is correspondingly possible through the reaction of SiCl.sub.4 with BCl.sub.3 or PCl.sub.5.
Another possible way of producing colloidal SiO.sub.2 involves the dealkalisation of sodium orthosilicate solution. In this process, orthosilicic acid molecules condense to form spherical SiO.sub.2 aggregates with a diameter of from 50 to 500 nm. If, during the condensation, the dopants are added in a compound such as (NH.sub.4).sub.2 HPO.sub.4 or H.sub.3 BO.sub.4 with a corresponding ratio and pH, it is likewise possible to incorporate the dopant into the primary structure of the host oxide.
