Source: http://www.google.com/patents/US6037277?dq=6462713
Timestamp: 2016-07-25 22:32:14
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Matched Legal Cases: ['Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60']

Patent US6037277 - Limited-volume apparatus and method for forming thin film aerogels on ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsAn apparatus and method for forming thin film aerogels on semiconductor substrates is disclosed. It has been found that in order to produce defect˜free nanoporous dielectrics with a controllable high porosity, it is preferable to substantially limit evaporation and condensation of pore fluid in the...http://www.google.com/patents/US6037277?utm_source=gb-gplus-sharePatent US6037277 - Limited-volume apparatus and method for forming thin film aerogels on semiconductor substratesAdvanced Patent SearchPublication numberUS6037277 APublication typeGrantApplication numberUS 08/746,697Publication dateMar 14, 2000Filing dateNov 14, 1996Priority dateNov 16, 1995Fee statusPaidPublication number08746697, 746697, US 6037277 A, US 6037277A, US-A-6037277, US6037277 A, US6037277AInventorsAlok Masakara, Teresa Ramos, Douglas M. SmithOriginal AssigneeTexas Instruments IncorporatedExport CitationBiBTeX, EndNote, RefManPatent Citations (27), Non-Patent Citations (9), Referenced by (60), Classifications (21), Legal Events (5) External Links: USPTO, USPTO Assignment, EspacenetLimited-volume apparatus and method for forming thin film aerogels on semiconductor substrates
US 6037277 AAbstract
An apparatus and method for forming thin film aerogels on semiconductor substrates is disclosed. It has been found that in order to produce defect˜free nanoporous dielectrics with a controllable high porosity, it is preferable to substantially limit evaporation and condensation of pore fluid in the wet gel thin film, e.g. during gelation, during aging, and at other points prior to obtaining a dried gel. The present invention simplifies the atmospheric control needed to prevent evaporation and condensation by restricting the atmosphere in contact with the wet gel thin film to an extremely small volume. In one embodiment, a substrate 26 is held between a substrate holder 36 and a parallel plate 22, such that a substantially sealed chamber 32 exists between substrate surface 28 and chamber surface 30. Preferably, the average clearance between surfaces 28 and 30 is less than 5 mm, or more preferably, less than 1 mm. Temperature control means 34 may optionally be used to control the temperature in chamber 32. In operation, the atmosphere in chamber 32 becomes saturated by an extremely small amount of pore fluid evaporated from a wet gel thin film on surface 28, thus preventing further evaporation or condensation. This invention is ideally suited for rapid aging of thin film wet gels.
1. A method for post-deposition processing of a wet gel thin film deposited on a semiconductor substrate, said method comprising:determining a first temperature between 0 degrees C. and 200 degrees C.; and providing a substrate having a wet gel thin film upon a surface, the wet gel thin film wetted at least by a first solvent, the substrate in a substantially sealed chamber, wherein the substrate is at the first temperature, and the atmospheric volume of the chamber is less than the volume which would be saturated--at the first temperature--by an amount of the first solvent equivalent to about 5% of the volume of the first solvent in the wet gel thin film; whereby evaporation of the first solvent is substantially limited by the atmospheric volume of the chamber. 2. The method of claim 1, further comprising removing at least part of said first solvent from said chamber atmosphere, thereby allowing said thin film to dry after aging.
3. The method of claim 1, wherein said first temperature is selected in a range of between 80� C. and 150� C.
4. The method of claim 1, wherein said first solvent principally comprises a polyol.
5. The method of claim 1, further including controlling the chamber temperature.
6. The method of claim 5, wherein said controlling said chamber step comprises ramping said chamber to said first temperature during a first time period and holding said chamber at said first temperature for a second time period.
7. The method of claim 5, further comprising down-ramping said substrate and said chamber to room temperature.
8. The method of claim 5, wherein during at least a portion of said down-ramping step, a temperature differential is maintained between said substrate and said chamber, said chamber being cooler than said substrate.
9. The method of claim 5, wherein during said controlling said chamber step, adjustment of the atmosphere within said chamber is accomplished only by changing the temperature of said chamber and said substrate.
10. The method of claim 1, wherein said first temperature is between 25� C. and 200� C.
11. The method of claim 1, wherein the substrate temperature is ramped from a temperature below the first temperature to a temperature above the first temperature.
12. The method of claim 1, wherein the atmospheric volume of the chamber is less than the volume which would be saturated--at the first temperature--by an amount of the first solvent equivalent to about 1% of the volume of wet gel thin film.
13. The method of claim 1, wherein the substrate surface that has a wet gel thin film is not oriented upward.
14. A method for aging a wet gel thin film deposited on a semiconductor substrate, said method comprising:providing a substrate having a wet gel thin film thereon, the gel having a pore fluid including at least a first solvent, in a substantially sealed chamber having a chamber atmosphere, the chamber atmosphere volume being no greater than about 5000 times the wet gel thin film volume; and controlling said chamber at a temperature in a range of between 0� C. and 200� C. 15. The method of claim 14, wherein the chamber atmosphere volume is no greater than about 1000 times the wet gel thin film volume.
16. The method of claim 14, wherein said first solvent principally comprises a polyol.
17. The method of claim 16, wherein the chamber includes a reservoir of first solvent.
18. A method for aging a wet gel thin film deposited on a semiconductor substrate, said method comprising:providing a substrate having a wet gel thin film upon a surface of the substrate, the gel having a pore fluid including at least a first solvent, in a substantially scaled chamber having an interior wall, the wall offset no more than about 5 millimeters from the substrate surface; and controlling the chamber at a temperature in a range between 0� C. and 200� C. 19. The method of claim 18, further comprising controlling the temperature of the substrate to a temperature higher than the chamber temperature, thereby substantially avoiding condensation on the thin film.
20. The method of claim 18, wherein the substrate surface that has a wet gel thin film is not oriented upward.
This application claims benefit of Provisional Application No. 60/006,852, filed Nov. 16, 1995 and provisional Application No. 60/006,853 filed Nov. 16, 1995 and Provisional Application No. 60/012,764 filed Mar. 4, 1996, and Provisional Application No. 60/014,005 filed Mar. 25, 1996 and Provisional Application No. 60/012,800, filed Mar. 4, 1996 and Provisional Application No. 60/022,842 filed Jul. 31, 1996.
Nanoporous dielectrics are some of the most promising new materials for semiconductor fabrication. These dielectric materials contain a solid structure, for example of silica, which is permeated with an interconnected network of pores having diameters typically on the order of a few nanometers. These materials may be formed with extremely high porosities, with corresponding dielectric constants typically less than half the dielectric constant of dense silica. And yet despite their high porosity, it has been found that nanoporous dielectrics may be fabricated which have high strength and excellent compatability with most existing semiconductor fabrication processes. Thus nanoporous dielectrics offer a viable low˜dielectric constant replacement for common semiconductor dielectrics such as dense silica.
The preferred method for forming nanoporous dielectrics is through the use of sol˜gel techniques. The word sol˜gel does not describe a product but a reaction mechanism whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. One theory is that through continued reactions within the sol, one or more molecules in the sol may eventually reach macroscopic dimensions so that it/they form a solid network which extends substantially throughout the sol. At this point (called the gel point), the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term "gel" as used herein means an open˜pored solid structure enclosing a pore fluid.
Si(OEt)4 +H2 O&#8594;HO&#732;Si(OEt)3 +EtOH
(OEt)3 Si&#732;OH+HO&#732;Si(OH)3 &#8594;(OEt)3 Si&#732;O&#732;Si(OH)3 +H2 O
(OEt)3 Si&#732;OEt+HO&#732;Si(OEt)13 &#8594;(OEt)3 Si&#732;O&#732;Si(OEt)5 +EtOH
to form an oligomer and liberate a molecule of water or ethanol. The Si˜O˜Si configuration in the oligomer formed by these reactions has three sites available at each end for further hydrolysis and condensation. Thus, additional monomers or oligomers can be added to this molecule in a somewhat random fashion to create a highly branched polymeric molecule from literally thousands of monomers. An oligomerized metal alkoxide, as defined herein, comprises molecules formed from at least two alkoxide monomers, but does not comprise a gel.
Sol˜gel reactions forms the basis for xerogel and aerogel film deposition. In a typical thin film xerogel process, an ungelled precursor sol may be applied (e.g., spray coated, dip˜coated, or spin˜coated) to a substrate to form a thin film on the order of several microns or less in thickness, gelled, and dried. The precursor sol often comprises a stock solution and a solvent, and possibly also a gelation catalyst that modifies the pH of the precursor sol in order to speed gelation. During and after coating, the volatile components in the sol thin film are usually allowed to rapidly evaporate. Thus, the deposition, gelation, and drying phases may take place simultaneously (at least to some degree) as the film collapses rapidly to a dense film. In contrast, an aerogel process differs from a xerogel process largely by avoiding pore collapse during drying of the wet gel. Some methods for avoiding pore collapse include wet gel treatment with condensation˜inhibiting modifying agents (as described in U.S. Pat. No. 5,470,802, A Low Dielectric Constant Material For Electronics Applications, issued on Nov. 28, 1995 to Gnade, Cho and Smith), supercritical pore fluid extraction, and freeze˜drying.
The present invention, in its simplest form, overcomes the evaporation rate control problem by not attempting to actively control pore fluid vapor concentration above a wafer at all; instead, the wafer is processed in an extremely low˜volume chamber, such that through natural evaporation of a relatively small amount of the pore fluid contained in the wet gel film, the processing atmosphere becomes substantially saturated in pore fluid. Unless the wafer is cooled at some point in a substantially saturated processing atmosphere, this method also naturally avoids problems with condensation, which should generally be avoided, particularly during high temperature processing.
In accordance with the present invention, a method is presented for post˜deposition processing (e.g. gelation, aging, and/or drying) of a wet gel thin film deposited on a semiconductor substrate. This method comprises the step of placing a substrate in a preferably substantially sealed chamber, where the substrate has a wet gel thin film, wetted by at least a first pore fluid, deposited thereon. The first pore fluid preferably principally comprises a polyol. The chamber is preferably not pressurized or uses at most a moderate overpressure such that there is little or no leakage, and atmospheric flow is generally avoided. This method further generally utilizes controlling the chamber at a temperature selected in the range of between 25� C. and 200� C. (and more preferably between 80� C. and 150� C.), with the temperature selected such that less than 5% (preferably less than 1%, and more preferably less than 0.5%) of the first pore fluid contained in the wet gel thin film is required to substantially saturate the atmosphere in the chamber.
In another aspect of the invention, an apparatus for processing a sol or wet gel thin film deposited on a substrate is disclosed. This apparatus comprises a body capable of substantially enclosing at least a first region of a substrate surface, where the first region has a sol or wet gel thin film deposited thereon. The body preferably has a chamber surface capable of being positioned substantially adjacent to the thin film without contacting the thin film, thus forming an extremely low˜volume atmospheric chamber adjacent the thin film. The apparatus also preferably comprises some means for controlling the temperature within the chamber.
The low˜volume chamber may be designed to have a volume relative to the volume of the wet gel film to be processed; e.g, generally less than or equal to 5000 times the volume of the thin film in the first region, or more preferably less than or equal to 1000 times greater, or still more preferably less than 500 times greater. Alternately, the chamber volume may be designed with a specific pore fluid, pore fluid volume and processing temperature in mind, such that less than a specified percentage of the pore fluid is required to substantially saturate the atmospheric chamber. Some specific designs have chamber surfaces which may be positioned to within 5 mm or even to within 1 mm or less of the thin film.
The term "thin film" as used herein refers to a film having an average thickness of less than 2 microns. It should be noted that the present invention is generally not applicable to bulk gel processing, where evaporation poses substantially different problems. For example, pore fluid evaporation during aging is generally not a problem in bulk gel processing, since the ratio of surface area to fluid volume may be four or five orders of magnitude less than the same ratio for a thin film. By the same token, however, high˜viscosity, low˜vapor pressure pore fluids, preferred for thin film applications, are extremely difficult to dry or solvent exchange from a bulk gel, and are generally not used in bulk gel applications.
FIG. 2 contains a graph of the change in gel times (without solvent evaporation) for bulk ethylene glycol˜based gels as a function of base catalyst;
FIG. 3 contains a graph of the variation of modulus with density for a non˜glycol˜based gel and an ethylene glycol˜based gel;
FIGS. 8A˜8B contain cross˜sections of a semiconductor substrate at several points during deposition of a thin film which may be fabricated using the present invention;
FIGS. 16A and 16B contain, respectively, a cross˜sectional and a plan view of a sol˜gel thin film processing apparatus according to the present invention;
FIG. 16C contains a cross˜sectional view of the same apparatus in contact with a substrate;
FIGS. 17A and 17B contain, respectively, cross˜sectional views of another apparatus according to the present invention, empty and enclosing a substrate;
FIGS. 18A and 18B contain, respectively, cross˜sectional views of yet another apparatus according to the present invention, empty and enclosing a substrate; and
FIGS. 19A, 19B and 19C contain cross˜sectional views of additional apparatus configurations which illustrate other aspects of the invention.
Several recent advances have resulted in more commercially viable aging processes for aerogel thin films. These include multi˜solvent sol precursors, low˜volatility polyol˜based sol precursors, and rapid aging techniques. For example, in one preferred system, all three of these improvements are combined. A multi˜solvent sol precursor comprises a first polyol solvent having a low volatility, which is used to control gel density and pore fluid evaporation, and a second solvent (e.g. ethanol) having a low viscosity and a relatively high volatility, which is used to control pre˜deposition reaction rate and to allow uniform coating. Preferably the low volatility solvent is one with a boiling point in the 175˜300� C. range and (for TEOS based gels) is miscible with both water and ethanol. Some suitable polyols are trihydric alcohols, such as glycerol, and dihydric alcohols (glycols), such as ethylene glycol, 1,4˜butylene glycol, and 1,5˜pentanediol. The low volatility solvent is preferably principally ("principally" as used herein means 50% or greater by volume) comprises the aging fluid in this system, such that a wet gel thin film may be aged at elevated temperature for a relatively short time period.
One attractive feature of using a polyol as a solvent is that at ambient temperature, the evaporation rate is sufficiently low so that several seconds at ambient conditions will not yield dramatic shrinkage for thin films. However, in addition to serving as a low vapor pressure and water˜miscible solvent, polyols may also participate in sol˜gel reactions. Although the exact reactions in this process have not been fully studied, some reactions can be predicted. If tetraethoxysilane (TEOS) is employed as a precursor with ethylene glycol solvent, the ethylene glycol can exchange with the ethoxy groups:
Si(OC2 H5)4 +x HOC2 H4 OH&#8417;Si(OC2 H5)4-x (OC2 H4 OH)x +x C2 H5 OH
Si(OC2 H5)4 +x [HOCH2 CH(OH)CH2 OH]&#8417;Si(OC2 H5)4-x [OC3 H5 (OH)2 ]x +x[C2 H5 OH]
Lower density, achievable without supercritical drying or pre˜drying surface modification
High optical clarity of bulk samples likely due to a narrower pore size distribution than conventional TEOS gels)
Low density˜˜With polyol solvent systems, it is possible to form dried gels at very low densities without pre˜drying surface modification or supercritical drying. These low densities can generally be down around 0.3 to 0.2 g/cm3 (non˜porous SiO2 has a density of 2.2 g/cm3), or with care, around 0.1 g/cm3. Stated in terms of porosity (porosity is the percentage of a structure which is hollow), this denotes porosities of about 86% and 91% (about 95% porosity with a density of 0.1 g/cm3). As shown in FIG. 7, these porosities correspond to dielectric constants of about 1.4 for the 86% porous, and 1.2 for 91% porous. The actual mechanism that allows these high porosities is not fully known. However, it may be because the gels have high mechanical strength, because the gels do not have as many surface OH (hydroxyl) groups, a combination of these, or some other factors. This method also obtains excellent uniformity across the wafer. FIG. 6 shows the refractive index (and thus generally the porosity) at several locations on a sample semiconductor substrate.
Density Prediction˜˜By varying the ratio of ethylene glycol (EG) to ethanol (EtOH) in the precursor (at a fixed silica content), the density after ethanol/water evaporation can be calculated. This is likely due to the low volatility of the second solvent. To the extent that further shrinkage is prevented during aging and drying, this allows prediction of the density (and thus porosity) of the dried gel. Although this density prediction had generally not been a large problem with bulk gels, thin film gels had typically needed excellent atmospheric controls to enable consistent density predictions. Table 1 shows the predicted and actual density for three different EG/EtOH ratios after substantial ethanol and water removal, but before drying (EG removal).
TABLE 1______________________________________Correlation between predicted andmeasured density of wet bulk gels afterethanol/water evaporation.                   Measured          Predicted                   Density (g/cm3)          Density  after drying @Stock Solution (g/cm3)                   80� C.______________________________________40%EtOH/60%EG  0.37     0.4051%EtOH/49%EG  0.43     0.4560%EtOH/40%EG  0.53     0.50______________________________________
To some degree, the glycerol˜based processes behave similarly to the ethylene glycol˜based processes. The glycerol˜based gels have dramatically lower evaporation and shrinkage rates during aging. This allows atmospheric control to be loosened during aging. We have fabricated acceptable glycerol˜based gels with no atmospheric controls during aging.
Shorter Gel Times˜˜In addition to enabling prediction of the density, the use of polyols may also change other properties of the sol˜gel process. FIG. 2 shows gel times for two different ethylene glycol˜based compositions as a function of the amount of ammonia catalyst used. These gel times are for bulk gels for which there is no evaporation of ethanol and/or water as there would be for thin films. Evaporation increases the silica content and thus decreases the gel time. Therefore, these gels times may be the upper limit for a given precursor/catalyst. The gel times reported in FIG. 2 are approximately an order of magnitude shorter than precursors without a polyol. Gel times are generally also a first order dependence on the concentration of ammonia catalyst. This implies that it may be possible to easily control the gel times. For thin films of these new polyol˜based gels, it is routine to obtain gelation within minutes, even without a gelation catalyst.
Higher Strength˜˜The properties of the polyol˜based samples appear to be quite different from regular gels as evidenced by both a low degree of drying shrinkage and differences in qualitative handling of the wet and dry gels. Thus, upon physical inspection, both the glycerol˜based and ethylene glycol˜based dried gels seem to have improved mechanical properties as compared to conventional dried gels. FIG. 3 shows the bulk modulus measured during isostatic compaction measurements of one sample prepared using one ethylene glycol˜based and one conventional ethanol˜based dried bulk gel (both have the same initial density). After initial changes attributed to buckling of the structure, both samples exhibit power law dependence of modulus with density. This power law dependence is usually observed in dried gels. However, what is surprising is the strength of the ethylene glycol˜based dried gel. At a given density (and thus, dielectric constant), the modulus of the ethylene glycol based gel is an order of magnitude higher than the conventional precursor gel. The glycerol˜based gels also seem to have a high strength; generally, the strength is at least as good as the ethylene glycol˜based gels. The reasons for this strength increase are unclear but correspondingly may be related to the very high surface area of these dried gels (>1,000 m2 /g) and the seemingly narrow pore size distribution.
High surface area˜˜We measured the surface areas of some dried bulk gels. These surface areas were on the order of 1,000 m2 /g, as compared to our typical dried gels which have surface area in the 600˜800 m2 / g range. These higher surface areas imply smaller pore size which may lead to improved mechanical properties. It is unclear at this time why these higher surface areas are obtained with the polyol˜based dried gels.
Pore size distribution˜˜The optical clarity of these dried bulk gels was greater than any dried gels at this density that we have previously made. It is possible that this excellent optical clarity is due to a very narrow pore size distribution. However, it is unclear why the polyols have this effect. It is still not clear whether the apparently narrow pore size distribution is a result of a different microstructure at the gelation stage or differences in aging. Preliminary measurements on a bulk gel sample (with a density of about 0.22 g/cm3) showed that the mean pore diameter was 16.8 nm.
As shown above, some properties of the polyol˜based gels apply to both bulk gels and thin films. However, some advantages are most evident when applied to thin films, such as nanoporous dielectric films on semiconductor wafers. One important advantage is that this new method allows high quality nanoporous films to be processed with no atmospheric controls during deposition or gelation.
As an example, we have found that with some polyol˜based gels, such as the ethylene glycol˜ and glycerol˜based gels, a satisfactory aging time at room temperature is on the order of a day. However, Table 2 shows that, by using higher temperatures, we can age with times on the order of minutes. Thus, when these times and temperatures are combined with the evaporation rates of FIG. 1, FIG. 5, and FIG. 11, they give the approximate thickness loss during aging as shown in Table 3. These estimated thickness losses need to be compared with acceptable thickness losses. While no firm guidelines for acceptable thickness loss exist, one proposed guideline, for some microcircuit applications such as nanoporous dielectrics, is that the thickness losses should be less than 2% of the film thickness. For a hypothetical nominal film thickness of 1 μm (actual film thicknesses may typically vary from significantly less than 0.5 μm to several μm thick), this gives an allowable thickness loss of 20 nm. As shown in Table 3, the glycerol˜based gels (and other polyol˜based gels with low vapor pressures) can achieve this preliminary goal without atmospheric control at room temperature. However, other solvent systems and/or higher temperature aging require at least some degree of atmospheric control.
TABLE 2______________________________________Approximate Aging Time as a Function of Temperature For Some Polyol-Based GelsAging Temperature        Aging Time For Polyol-Based Gels(Degrees C.) (Order of Magnitude Approximations)______________________________________25           1 day100          5 minutes140          1 minute______________________________________
TABLE 3______________________________________Approximate Thickness Loss During Aging vs. Saturation Ratio.Aging Thickness Loss During AgingTime/                           Glycerol-Tem-  Ethanol-Based Gel              EG-Based Gel Based Gelpera- % Saturation % Saturation % Saturationture  0%     50%    99%  0%   50%  99%  0%   50%  99%______________________________________1 day/ 8      7      86   17   7    172  13   5    125� C. mm     mm     &#956;m                    &#956;m                         &#956;m                              nm   nm   nm   nm300   --     --     --   3    1.2  90   600  420  9sec/                     &#956;m                         &#956;m                              nm   nm   nm   nm100� C.60    --     --     --   --   --   --   6    3    60sec/                                    &#956;m                                        &#956;m                                             nm140� C.______________________________________
One disadvantage of polyols, especially trihydric alcohols and other higher viscosity polyols, are their relatively high viscosities which could cause problems with gap˜filling and/or planarization. As described in copending US patent application serial #TBD (Attorney's Docket TI˜21623), titled Aerogel Thin Film Formation From Multi˜Solvent Systems, by Smith et al., a low viscosity, high volatility solvent can be used to lower the viscosity. FIG. 4 shows the calculated viscosity of some ethylene glycol/alcohol and glycerol/alcohol mixtures at room temperature. As the figure shows, a small quantity of alcohol significantly reduces the viscosity of these mixtures. Also, if the viscosity using ethanol in the stock solution is higher than desired, further improvement can be realized by employing methanol and tetramethoxysilane in the precursor solution. The viscosities reported in FIG. 4 are for pure fluid mixtures only. In fact, depending upon the film precursor solution, the precursor solution might contain glycerol, alcohol, water, acid and partially reacted metal alkoxides. After refluxing, but before catalysis, the measured viscosity as a function of ethylene glycol content is shown in Table 4. As predicted, the use of methanol significantly lowers the viscosity. Of course, the viscosity can be increased before deposition by catalyzing the condensation reaction and hence, the values reported in Table 4 represent lower bounds.
TABLE 4______________________________________Measured Viscosity and Density of Glycol-Based Stock Solutions BeforeActivation.              Viscosity @                         Viscosity @              25� C.                         40� C.Stock Solution "Solvent"              (cp)       (cp)______________________________________100% EtOH          1.5        --40% Ethylene glycol/60% EtOH              3.1        --49% Ethylene glycol/51% EtOH              4.0        --60% Ethylene glycol/40% EtOH              5.4        --40% Ethylene glycol/60% Methanol              1.6        --100% Ethylene glycol              11.0       7.840% Glycerol/60% EtOH              5.8        --50% Glycerol/50% EtOH              9.0        --60% Glycerol/40% EtOH              15.5       --100% Glycerol      1000.      7.8______________________________________
This multi˜solvent approach may be combined with or replaced with an alternative approach. This alternate approach uses elevated temperatures to reduce the sol viscosity during application. For example, the measured viscosity of the TEOS/ethylene glycol/water/nitric acid precursor described in the second preferred embodiment is 11 centipoise (cp) at 25 degrees C., but only 7.8 cp at 40 degrees C. Thus by heating and/or diluting the precursor during deposition, (such as by heating the transfer line and deposition nozzle of a wafer spin station) the viscosity of the precursor sol can be lowered to nearly any given viscosity. Not only does this preheat lower the sol viscosity, it may also speed gel times and accelerate the evaporation of any high volatility solvents. It may also be desirable to preheat the wafer. This wafer preheat should improve process control and may improve gap fill, particularly for the more viscous precursors. However, for many applications, wafer preheat is not required, thus simplifying process flows. When using a spin˜on application method with this no wafer preheat approach, the spin station would not require a temperature controlled spinner.
The present invention allows high boiling point solvent gel processing at elevated temperatures, with acceptable shrinkage, by substantially enclosing the gel thin film in a relatively small, substantially closed chamber, at least during high˜temperature aging. In operation, whatever evaporation does occur from the wafer raises the solvent saturation ratio of the atmosphere inside the closed chamber. At any given temperature, this evaporation continues until the partial pressure of the vapor increases enough to equal the vapor pressure of the liquid. Thus, solvent/temperature combinations with lower vapor pressure solvent will not allow as much liquid solvent to evaporate as a higher vapor pressure solvent combination allows. FIG. 12 shows how vapor pressure varies with temperature for several solvents. If the chamber size is known, the amount of evaporation can be calculated. FIG. 13 shows an estimate of how thick of a layer of solvent could potentially be evaporated from a 70% porous gel placed in a closed chamber with a 5 mm high airspace above the wafer. FIG. 14 shows a similar estimate for a chamber with a 1 mm high airspace above the wafer. These figures show that, with a 5 mm high airspace, the 20 nm preliminary goal is feasible up to 50� C. for ethylene glycol˜based gels and up to 120� C. for glycerol˜based gels. With the 1 mm airspace, the 20 nm goal is feasible up to 80� C. for the ethylene glycol˜based gels and 150� C. for the glycerol˜based gels. Of course, lower temperature processing allows less evaporation. Volumetric evaporation control using the 1 mm chambers allows correspondingly less than 1 nm of thickness loss for both ethylene glycol˜based and glycerol˜based gels at 20� C.
One embodiment of the present invention is illustrated in FIGS. 16A, 16B and 16C. In this embodiment, a processing apparatus comprises a body 20, having a substantially planar plate 22 with a resilient seal 24 attached thereto. Plate 22 need only be planar to the extent necessary to provide clearance with a thin film during operation, and may be constructed of any material compatible with semiconductor fabrication, although materials with high thermal conductivity, such as stainless steel, glass, or aluminum are preferred. Resilient seal 24 should preferably be designed to withstand wet gel processing temperatures and pore fluids; many suitable TEFLON˜ or neoprene˜based materials are known to those with ordinary skill in the art. Depending on the nature of temperature control used in the apparatus, it may be preferable to have seal 24 be either substantially thermally insulating or thermally conductive.
In operation, body 20 may simply be rested on a substrate 26, as shown in FIG. 6C. In this embodiment, seal 24 functions both as an atmospheric seal and as a spacer which sets the volume of chamber 32 formed by substrate surface 28, chamber surface 30 and seal 24. For example, seal 24 may be designed to compress to a thickness of about 1 mm under the weight of plate 22, thus creating chamber 32 with a 1 mm height when body 20 is placed on substrate 26. For many thin film applications, chamber 32 need only be substantially sealed, as some small degree of vapor leakage over the course of processing substrate 26 will not appreciably affect the final film properties.
Also in accordance with the present invention, several preferred embodiments for thin film nanoporous dielectric deposition methods are presented herein. Referring now to FIG. 8A, a semiconductor substrate 26 (typically in wafer form) is shown. Common substrates include silicon, germanium, and gallium arsenide, and the substrate may include active devices, lower level wiring and insulation layers, and many other common structures not shown but known to those skilled in the art. Several patterned conductors 25 (e.g., of an Al˜0.5%Cu composition) are shown on substrate 26. Conductors 25 typically run parallel for at least part of their length, such that they are separated by gaps 27 of a predetermined width (typically a fraction of a micron). Both the conductors and gaps may have height˜to˜width ratios much greater than shown, with larger ratios typically found in devices with smaller feature sizes.
In accordance with a first nanoporous dielectric method, 61.0 mL tetraethylorthosilicate (TEOS), 61.0 mL glycerol, 4.87 mL water, and 0.2 mL 1M HNO3 are mixed and refluxed for 1.5 hours at ˜60� C. After a cooling period, the solution may be diluted down with ethanol to a composition of 80% (by volume) original stock solution and 20% (by volume) ethanol, thus reducing the viscosity. This is mixed vigorously and typically stored in a refrigerator at ˜7� C. to maintain stability until use. The solution is warmed to room temperature prior to film deposition. 3˜5 mL of this precursor sol may be dispensed at room temperature onto substrate 26, which is then spun at 1500 to 5000 rpm (depending on desired film thickness) for about 5˜10 seconds to form sol thin film 29. The deposition can be performed in an atmosphere that has no special control of solvent saturation (e.g., in a cleanroom with standard humidity controls). During and after this deposition and spinning, the ethanol/water azeotropic mixture is evaporating from film 29, but due to glycerol's low volatility, no substantial evaporation of the glycerol is occurring. This evaporation shrinks thin film 29 and concentrates the silica content of the sol forming reduced thickness film 33. FIG. 8B shows a reduced thickness sol film 18 obtained after substantially all (about 95% or more) of the ethanol has been removed. This concentrated sol typically gels within minutes or seconds.
Film 33 has an approximately known volume ratio of silica to pore fluid at the gel point. This ratio is approximately equal to the ratio of TEOS to glycerol in the as˜deposited sol (with minor changes due to remaining water, continued reactions and incidental evaporation). To the extent that the gel is prevented from collapsing, this ratio will determine the density of the aerogel film that will be produced from the sol thin film.
After gelation, the thin film wet gel 33 comprises a porous solid and a pore fluid. The pore fluid may preferably be left in place, although it may be diluted or replaced by a different fluid (e.g. replace glycerol and water mixture with glycerol). Whether this fluid is identical to the as˜gelled fluid or not, the pore fluid that is present during aging is sometimes referred to as "aging fluid". Aging is most preferably carried out in one of the limited volume chambers of the present invention, e.g. for about 1 minute at 130˜150� C., although temperatures in the range of 25� C. to 200� C., as well as aging times as short as several seconds or as long as one day are also comprehended. It should be noted that the pore fluid changes somewhat during processing. These changes may be due to continued reactions, evaporation, condensation, or chemical additions to the thin film.
After aging, wet gel film 33 may be dried without substantial densification by one of several methods, including supercritical fluid extraction. However, with polyol˜based gels, one alternative is to use a solvent exchange to replace the aging fluid with a drying fluid and then air dry the film 33 from this drying fluid. This drying method uses a solvent exchange to dilute the aging fluid or replace it with a different fluid (e.g. use a large volume of acetone to dilute the glycerol and water mixture, thus forming a mixture dominated by acetone). Whether this fluid is identical to the aging fluid or not, the pore fluid that is present during drying is sometimes referred to as "drying fluid". If used, the solvent exchange replaces the aging fluid that is dominated by the glycerol and its associated high surface tension with a drying fluid that has a lower surface tension. This solvent exchange may preferably be carried out by dispensing approximately 5˜8 mL of acetone at room temperature onto aged thin film 18, then spinning the wafer between approximately 250 and 500 rpm for about 5˜10 seconds. In this solvent exchange method, it is preferred to remove nearly all the glycerol before drying. The drying fluid (acetone in this case) is finally allowed to evaporate from the wet gel 18, forming a dry porous dielectric (dried gel).
An alternate method may be used to age and dry, e.g. glycerol˜based, films without solvent exchange using a limited volume chamber 32. An unaged wafer is placed in a temperature˜controlled limited volume chamber, preferably at room temperature and ambient pressure. The chamber remains substantially sealed as the temperature is ramped up, aging the film. After the chamber reaches a temperature at which the glycerol surface tension is low enough such that the aged film is sufficiently strong to withstand capillary drying pressures, a process is begun that removes glycerol from the chamber atmosphere. Note that the preferred drying temperature, in many applications, is greater than the boiling point of glycerol, in which case the chamber should be pressurized before the boiling point is reached. Also, care should be taken that the glycerol in the chamber atmosphere is, especially at first, slowly removed. The glycerol in the chamber atmosphere may, e.g., be removed by bleeding off the pressure, by vacuum pumping, by sweeping the glycerol off with an annealing gas (e.g. forming gas), or by forcing condensation on chamber wall 30 (see, e.g. the configuration of FIG. 19C). The chamber temperature may be held constant or it may continue to be raised while the glycerol is being removed (the chamber may be ramped on up to annealing temperature while sweeping the glycerol off with the annealing gas). While some glycerol can be introduced during initial heating to minimize evaporation from the film (until a temperature has been reached where the surface tension of the fluid is sufficiently reduced), preferably the chamber volume is low enough that evaporation does not significantly reduce film thickness even without the introduction of glycerol during heating.
In accordance with a second, ethylene glycol˜containing sol aerogel process, mix tetraethylorthosilicate (TEOS), ethylene glycol, ethanol, water, and acid (1M HNO3) in a molar ratio of 1:2.4:1.5:1:0.042 and reflux for 1.5 hours at ˜60� C. After the mixture is allowed to cool, the solution is diluted down with ethanol to a composition of 70% (by volume) original stock solution and 30% (by volume) ethanol. This is mixed vigorously and typically stored in a refrigerator at ˜7� C. to maintain stability until use. The solution is warmed to room temperature prior to film deposition. A mixture of stock solution and 0.25M NH4 OH catalyst (10:1 volume ratio) is combined and mixed. This sol may be deposited on substrate 26 in the manner described in conjunction with the glycerol solvent embodiment.
TABLE 5______________________________________Substance SummaryRef  Specific  Functional#    Example   Description                    Preferred Alternates______________________________________10   Silicon   Semi-     Ge, GaAs, active devices,          conductor lower level layers          Substrate12   Al-0.5%Cu Patterned Al, Cu, other metals,          Conductors                    polysiliconTEOS      Precursor Other silicon-based metal          Sol       alkoxides (TMOS, MTEOS, BTMSE,          Reactant  etc.), alkoxides of other                    metals, particulate metal                    oxides, organic precursors,                    and combinations thereofGlycerol  Precursor Other polyols, combinations          Sol First of glycerol, Ethylene glycol,          Solvent   1,4-butylene glycol,          (Low      1,5-pentanediol, and/or          volatility)                    other polyols.Nitric Acid          Precursor Other acids(HNO3)          Sol          StabilizerEthanol   Precursor Methanol, other alcohols          Sol          Second          Solvent          (High          volatility)Ethanol   Viscosity Methanol, other alcohols          Reduction          SolventTMCS      Surface   Hexamethyldisilazane (HMDS),          Modification                    trimethylmethoxysilane,          Agent     dimethyldimethoxysilane,                    phenyl compounds and fluorocarbon                    compounds.Ammonium  Gelation  Ammonia, volatile amineHydroxide Catalyst  species, volatile fluorine(NH4 OH)       species, and other compounds                    that will raise the pH                    of the deposited sol.                    Nitric acid and other                    compounds that will                    lower the pH.As-Gelled Aging Fluid                    Glycerol, ethylene glycol,Pore Fluid          other polyols, water,                    ethanol, other alcohols,                    combinations thereof.Acetone   Drying Fluid                    Aging fluid, heated aging                    fluid, heptane, isopropanol,                    ethanol, methanol,                    2-ethylbutyl alcohol,                    alcohol/water mixtures,                    ehtylene glycol, other                    liquids that are miscible with                    the aging fluid, yet have                    lower surface tension                    than the aging fluid,                    combinations thereof.______________________________________
Although this invention has been described in terms of several embodiments, many steps may be modified or combined within the scope of the invention, and other steps can be included to enhance the overall process. For example, the initial thin film may be deposited by other common methods, such as dip˜coating or spray˜coating instead of spin˜coating. Likewise, the solvent exchange may use dip coating, spray coating, or immersion in a liquid or vaporous solvent instead of spin˜coating. When using a high vapor pressure solvent, the wafer may be cooled to a temperature lower than the atmosphere, thus promoting condensation on the wafer. While water might be considered a solvent in such a process, for discussion purposes in this application, water is not considered a solvent.
By modifying the mix ratios of polyol and alcohol in the sol˜gel process, the gel's properties can be changed. One such change is the gel time. Table 6 below shows the results of varying the ethanol to ethylene glycol ratios in the precursor sol of some sample bulk gels with catalysts. These gels generally used the same sol mixture as the ethylene glycol embodiment except for the ethanol to ethylene glycol ratio. Also, in the non˜polyol˜based mix, the catalyst concentration is different. This non˜polyol˜based gel used 0.5 M NH4 OH catalyst in a volume ratio of 1:10, instead of the 0.25 M NH4 OH used in the others.
TABLE 6______________________________________Effect of Varying the Ethylene Glycol Content of the Precursor Sol       Ethanol   Ethylene Glycol       Content   Content      Gel TimeBulk Example #       (mL)      (mL)         (minutes)______________________________________1           61        0             7 to 10(Non-Polyol-Based)2           36.6      24.4         5 to 73           30.5      30.5         2 to 34           24.4      36.6         1 to 25           0         61           1 to 2______________________________________
Another example of modification to the basic method is that, before drying (and typically, but not necessarily, after aging), the thin film wet gel 29 may have its pore surfaces modified with a surface modification agent. This surface modification step replaces a substantial number of the molecules on the pore walls with those of another species. If a surface modifier is applied, it is preferable to remove the water from the wet gel 29 before the surface modifier is added. The water can be removed by immersing the wafer in pure ethanol, preferably by a low speed spin coating as described in the solvent exchange in the first process example. This water removal could be beneficial, because water will react with many surface modification agents, such as TMCS; however, it is not necessary. With a polyol˜based method, surface modification need not be performed to help lessen pore collapse, but it can be used to impart other desirable properties to the dried gel. Some examples of potentially desirable properties are hydrophobicity, reduced dielectric constant, increased resistance to certain chemicals, and improved temperature stability. Some potential surface modifiers that may impart desirable properties include hexamethyldisilazane (HMDS), the alkyl chlorosilanes (trimethylchlorosilane (TMCS), dimethyldichlorosilane, etc.), the alkylalkoxysilanes (trimethylmethoxysilane, dimethyldimethoxysilane, etc.), phenyl compounds and fluorocarbon compounds. The useful phenyl compounds will typically follow the basic formula, Phx Ay SiB.sub.(4-x-y), where, Ph is a phenolic group, A is a reactive group such as Cl or OCH3, and B are the remaining ligands which, if there are two, can be the same group or different. Some examples of these phenyl surface modification agents include compounds with 1 phenolic group such as phenyltrichlorosilane, phenyltrifluorosilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenylmethylchlorosilane, phenylethyldichlorosilane, phenyldimethylethoxysilane, phenyldimethylchlorosilane, phenyldichlorosilane, phenyl(3˜chloropropyl)dichlorosilane, phenylmethylvinylchlorosilane, phenethyldimethylchlorosilane, phenyltrichlorosilane, phenyltrimethoxysilane, phenyltris(trimethylsiloxy)silane, and phenylallyldichlorosilane. Other examples of these phenyl surface modification agents include compounds with 2 phenolic groups such as diphenyldichlorosilane, diphenylchlorosilane, diphenylfluorosilane, diphenylmethylchlorosilane, diphenylethylchlorosilane, diphenyldimethoxysilane, diphenylmethoxysilane, diphenylethoxysilane, diphenylmethylmethoxysilane, diphenyl˜methylethoxysilane and diphenyldiethoxysilane. These phenyl surface modification agents also include compounds with 3 phenolic groups such as triphenylchlorosilane, triphenylflourosilane, and triphenylethoxysilane. Another important phenyl compound, 1,3˜diphenyltetramethyldisilazane, is an exception to this basic formula. These lists are not exhaustive, but do convey the basic nature of the group. The useful fluorocarbon based surface modification agents include (3,3,3˜trifluoropropyl)trimethoxysilane), (tridecafluoro˜1,1,2,2˜tetrahydrooctyl)˜1dimethylchlorsilane, and other fluorocarbon groups that have a reactive group, such as Cl or OCH3, that will form covalent bonds with a hydroxyl group.
This invention also comprises using gelation catalysts with the glycerol˜based and other polyol˜based sols, not just the ethylene glycol˜based sols. This also includes the allowance of other gelation catalysts in place of the ammonium hydroxide and/or for the gelation catalyst to be added after deposition. Typically, these alternate catalysts modify the pH of the sol. It is preferable to use catalysts that raise the pH, although acid catalysts can be used. Typically, acid catalysis results in longer processing times and a denser dielectric than a base catalyzed process. Some examples of other preferred gelation catalysts include ammonia, the volatile amine species (low molecular weight amines) and volatile fluorine species. When the catalyst is added after deposition, it is preferable to add the catalyst as a vapor, mist, or other vaporish form.
Thus, this invention can allow production of nanoporous dielectrics at room temperature and atmospheric pressure, without a separate surface modification step. Although not required to prevent substantial densification, this new method does not exclude the use of supercritical drying or surface modification steps prior to drying. To the extent that the freezing rates are fast enough to prevent large .(e.g. 50 nm) crystals, it is also compatible with freeze drying. In general, this new method is compatible with most prior art aerogel techniques. Although this new method allows fabrication of aerogels without substantial pore collapse during drying, there may be some permanent shrinkage during aging and/or drying. This shrinkage mechanism is not well understood; however, it behaves in a manner similar to syneresis.
Although TEOS has been used as a representative example of a reactant, other metal alkoxides may be used either alone or in combination with TEOS or each other to form a silica network. These metal alkoxides include tetramethylorthosilicate (TMOS), methyl˜triethoxysilane (MTEOS), 1,2˜Bis(trimethoxysilyl)ethane (BTMSE), combinations thereof, and other silicon˜based metal alkoxides known in the art. A sol may also be formed from alkoxides of other metals known in the art such as aluminum and titanium. Some other precursor sols known in the art include particulate metal oxides and organic precursors. Two representative particulate metal oxides are pyrogenic (fumed) silica and colloidal silica. Some representative organic precursors are melamine, phenol furfural, and resorcinol. In addition to alternate reactants, alternate solvents may also be used. Some examples of preferred alternates for ethanol are methanol and the other higher alcohols. Other acids may be used as a precursor sol stabilizer in place of the nitric acid.
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Properties of Porous Dielectric Materials* Cited by examinerClassifications U.S. Classification438/787, 257/E21.273, 438/909, 438/790, 438/778International ClassificationH01L21/316Cooperative ClassificationY10S438/909, H01L21/02126, H01L21/02216, H01L21/02282, H01L21/02343, H01L21/31695, H01L21/02203, H01L21/02337European ClassificationH01L21/02K2T8J, H01L21/02K2E3L, H01L21/02K2C1L1, H01L21/02K2C5, H01L21/02K2C7C4B, H01L21/02K2T8H, H01L21/316PLegal EventsDateCodeEventDescriptionFeb 24, 1997ASAssignmentOwner name: TEXAS INSTRUMENTS INCORPORATED, TEXASFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:MASKARA, ALOK;RAMOS, TERESA;SMITH, DOUGLAS M.;REEL/FRAME:008463/0297Effective date: 19961112Jul 31, 2001CCCertificate of correctionAug 28, 2003FPAYFee paymentYear of fee payment: 4Aug 20, 2007FPAYFee paymentYear of fee payment: 8Aug 24, 2011FPAYFee paymentYear of fee payment: 12RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - 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