WO1999022403A1 - Process and apparatus for preparation of epitaxial silicon layers free of grown-in defects - Google Patents

Abstract

The present invention relates to the preparation of epitaxial single crystal silicon wafers free of grown-in defects caused by the presence of particles on the surface of the single crystal silicon wafer substrate prior to epitaxial deposition. The process comprises cleaning a substrate to remove particles which are present on the substrate surface, the cleaning being performed under a substantially particle free atmosphere, and then depositing an epitaxial silicon layer free of grown-in defects on the substrate surface, the epitaxial deposition being performed in-line with the substrate cleaning such that the substrate remains under the substantially particle free atmosphere after cleaning and until epitaxial deposition is complete.

Description

PROCESS AND APPARATUS FOR PREPARING EPITAXIAL SILICON WAFERS FREE OF GROWN-IN DEFECTS

BACKGROUND OF THE INVENTION The process of the present invention relates generally to the preparation of epitaxial silicon wafers. More particularly, the present invention relates to the preparation of epitaxial silicon wafers free of grown-in defects caused by the presence of particles on the surface of the single crystal silicon wafer substrate. Epitaxial silicon growth typically involves a chemical vapor deposition process wherein a substrate wafer is heated while a gaseous silicon compound is passed over the wafer surface to effect pyrolysis or decomposition. When a single crystal silicon wafer is used as the substrate, the silicon is deposited in such a way as to continue the growth of the single crystal structure. As a result, defects present in the bulk of the wafer, i.e. crystallographic defects, or on the wafer surface directly impact the quality of the resulting epitaxial wafer. This is due to the fact that by continuing the growth of the single crystal structure crystallographic defects, if present, will continue to grow. Alternatively, if defects are present on the wafer surface, continued growth of the single crystal results in the formation of new crystal defects, i.e. grown-in defects. For example, epitaxial defects such as mounds, epi stacking faults and hillocks, having a maximum cross- sectional width ranging from the current detection limit of a laser-based auto-inspection device of about 0.1 microns to greater than about 100 microns, can result from the presence of particles on the surface of the wafer. This therefore means that even where the substrate wafer is defect-free, growth of a defect-free epitaxial layer can be prevented if particles are present on the wafer surface. As a result, defect-free epitaxial growth requires the elimination of these particles from the surface of the wafer before epitaxial deposition begins .

In the past, efforts have been focused on reducing the number and size of the grown-in defects present in the resulting epitaxial layer. Initially, the primary source of wafer surface particles was found to be the epitaxial reactor itself. Attempts to eliminate very large grown-in defects, i.e. defects having a maximum cross-sectional width ranging from about 25 microns to about 100 microns, or larger, as measured by conventional laser-based auto-inspection devices, in epitaxial silicon wafers were therefore focused on the develop of "cleaner" reactors and reactor equipment. Stated another way, attempts to eliminate these defects were focused on the development of reactors which were less likely to contaminate the wafer surface with particles. (See, e.g., Yamaguchi, U.S. Patent No. 5,439,523.) As a result of these efforts, epitaxial reactor designs have evolved from the older, barrel-type reactors which handled multiple wafers at one time to the present type of horizontal-flow, single wafer reactors which are designed to handle only a single wafer at a time.

Improvements in epitaxial reactor designs have generally been successful in removing a major source of particles from the epitaxial process. Today's single wafer reactors are now capable of producing epitaxial wafers which are generally free of the very large grown- in defects associated with these particles. However, the ever more stringent demands being imposed on epitaxial silicon wafer manufacturers by the integrated circuit industry have now caused efforts to be focused on the elimination of smaller grown-in defects, i.e. defects having maximum cross-sectional widths ranging from about 0.1 microns up to about 25 microns as measured by current laser-based auto- inspection devices.

To date, attempts to eliminate these smaller grown- in defects have been unsuccessful, in part due to the fact that there is potentially more than one cause for the formation of these defects. For example, one or more of the following problems could be responsible for the formation of these smaller grown-in defects: (i) crystallographic defects in the substrate wafer, resulting from either the crystal pulling process or wafer handling; (ii) growth conditions for the epitaxial layer which lead to slow diffusion rates of adsorbed silicon intermediates, resulting in the formation of polysilicon nodules and crystallographic defects; and, (iii) silicon dioxide islands or etch pits in the silicon substrate that are the result of an inefficient hydrogen gas pre-epitaxial bake process which is used to dissolved the silicon dioxide which is present, such as baking the substrate at a temperature which is below the standard temperature range of about 1000°C to about 1250°C.

In view of the foregoing, a need continues to exist for a process wherein the formation of grown-in defects is prevented and the source of such defects removed prior to subjecting the wafer to an epitaxial deposition process.

SUMMARY OF THE INVENTION

Among the objects of the present invention, therefore, is the provision of a process for depositing an epitaxial layer on a single crystal silicon wafer substrate which is free of grown-in defects; the provision of such a process in which particles are removed from the wafer substrate surface prior to epitaxial deposition; the provision of such a process in which particles are removed and epitaxial deposition is performed under an atmosphere which is substantially particle free; the provision of such a process in which particles are removed and an epitaxial layer is formed without exposing the wafer to an ambient in which human operators are present; and, the provision of such a process in which particles are removed by cleaning the substrate wafer with aerosol crystals of a gaseous mixture .

Further among the several objects and features of the present invention may be noted the provision of an epitaxial cluster tool and reactor apparatus, for preparing an epitaxial wafer free of grown-in defect, to which is connected a cleaning unit for in-situ removal of particles from a surface of a single crystal silicon substrate wafer; the provision of such an apparatus in which a substantially particle free atmosphere is maintained throughout wafer handling, cleaning and epitaxial deposition; and, the provision of such an apparatus in which wafer handling, cleaning and epitaxial deposition occur without exposing the wafer to an ambient in which human operators are present .

Briefly, therefore, the present invention is directed to a process for preparing an epitaxial silicon wafer which is free of grown-in defects resulting from particles which are present on a surface of a single crystal silicon wafer substrate. The process comprises cleaning the substrate under a substantially particle free atmosphere to remove particles which are present on the substrate surface, and depositing an epitaxial silicon layer free of grown-in defects on the substrate surface. The epitaxial deposition is performed in-line with the substrate cleaning such that the substrate remains under a substantially particle free atmosphere after cleaning and until epitaxial deposition is complete. The present invention is further directed to an epitaxial deposition apparatus for preparing an epitaxial silicon wafer free of grown-in defects caused by particles present on a surface of a single crystal silicon wafer substrate. The apparatus comprises a centrally located epitaxial cluster tool comprising an inlet port, an outlet port, and a chamber containing a handling mechanism which is capable of maintaining a substantially particle free atmosphere. A cleaning unit is sealingly connected to the cluster tool and is capable of receiving a substrate from the handling mechanism and cleaning a surface of the substrate while maintaining the substantially particle free atmosphere. An epitaxial reactor is sealingly connected to the cluster tool and is capable of receiving the substrate from the handling mechanism and depositing a layer of silicon free of grown-in defects on the clean wafer surface while maintaining the atmosphere .

Other objects will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an image produced using scanning electron microscopy showing a human skin particle on the surface of a silicon wafer prior to epitaxial deposition. FIG. IB is an image produced using scanning electron microscopy showing grown-in defect, also known as a large area defect (LAD) or poly-epitaxial stacking fault, caused by the particle of Fig. 1A.

FIG. 2 is a graph showing how the number of grown-in defects, or large area defects (LADs) , vary with the wafer handling operator and cleanroom conditions.

FIG. 3 is a schematic, top plan view of a epitaxial cluster tool and epitaxial reactor combination, including inlet and outlet ports, cooling station and, in accordance with the present invention, a cryogenic cleaning unit.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the process of the present invention, it has been discovered that the formation of an epitaxial silicon wafer free of grown-in defects can be achieved by cleaning a surface of a single crystal silicon substrate, prepared in accordance with the Czochralski method, to remove particles prior to subjecting the surface to an epitaxial deposition process. As used herein, the phrase "free of grown-in defects" shall mean that grown-in defects caused by the presence of particles on the substrate surface cannot be detected on the surface of the epitaxial layer when analyzed by a conventional laser-based auto- inspection device, which currently has a grown-in defect detection limit about 0.1 microns (e.g., Tencor 6200 series laser scanners, commercially available from Tencor Inc. of Mountain View, California) . However, it is to be noted that the actual size of grown-in defects can be much larger than the measurements from conventional laser- based auto-inspection devices indicate, possibly two cr more orders of magnitude larger.

Without being held to any particular theory, in view of the improvements made in epitaxial reactor designs, it is now believed that the primary cause of grown-in defects in epitaxial silicon wafers prepared in current horizontal flow, single wafer-type reactors is particles which are present on the surface of the substrate prior to it entering the epitaxial reactor. It is believed that this problem has not previously been appreciated due to the failure to optimize the epitaxial process in order to eliminate other potential problems, previously noted above. For example, by improving single crystal silicon growth and subsequent wafer handling, the associated crystallographic defects in the substrate wafer can be eliminated. Likewise, by improving the hydrogen pre-bake and epitaxial deposition process conditions, the defects associated with these problems can be eliminated as well. Studies conducted on epitaxial wafers having grown- in defects in the epitaxial layer suggest that typical organic species, including carbon, oxygen, sodium, potassium, magnesium and calcium, are present under these sites. In addition, studies conducted wherein a substrate was seeded with particles of human skin prior to epitaxial deposition show that these particles also lead to the formation of grown-in defects. Referring now to Figs. 1A and IB, scanning electron microscopy has been used to support this observation by identifying a human skin particle on the surface of a silicon substrate prior to epitaxial deposition (Fig. 1A) , and then locating the grown-in defect in the corresponding position of the epitaxial wafer after deposition is complete (Fig. IB) . Such results support the conclusion that the presence of particles on the substrate surface prior to epitaxial deposition cause the formation of these grown- in defects. These results also suggest that the reactor is no longer the primary source of particle contamination on the substrate surface. Therefore, how the substrate is handled prior to entering the cluster tool, or the handling chamber, of the epitaxial reactor becomes critical to the elimination of grown-in defects. Referring now to Fig. 2, studies have shown that the skill level of the operator handling the substrate wafers, as well as the cleanliness of the cleanroom in which the substrates are handled, can significantly effect the outcome of the epitaxial process, in terms of the resulting concentration of grown-in defects. For example, operator 1 achieved a yield in excess of 95%; that is, the operator was able to produce acceptable epitaxial wafers in excess of 95% of the time. Similarly, operator 2 achieved a yield in excess of 90%. However, operator 3 achieved a yield of only about 65%, in a cleanroom comparable to those of operators 1 and 2. Referring now to Fig. 3, a standard epitaxial 'cluster tool and reactor combination, such as an EPI CENTURA^® cluster tool and reactor (commercially available from Applied Materials) , are schematically shown. Typically, such a combination comprises a centrally located, six sided cluster tool 10, or handling chamber, to which is connected a load lock inlet 12 and outlet 14, at least one epitaxial deposition reactor 16, and a post- epitaxial deposition cooling station 18. Commonly, a second epitaxial reactor will also be connected to the cluster tool in order to increase the rate at which the substrate wafers can be processed. Each of these units, or chambers, is sealingly connected to the cluster tool 10, by means which are standard in the art for maintaining a vacuum. Each unit is sealingly connected to the central cluster tool in order to ensure that the atmosphere within each unit, as well as the integrated system as a whole, is controlled. In this way, a substantially particle free environment can be maintained throughout the entire epitaxial process.

For a conventional process, prior to epitaxial deposition a polished single crystal silicon wafer substrate, which is substantially free of crystallographic defects, is subjected to a post- polishing final cleaning step. This is typically done as close to the epitaxial process as possible so as to limit the potential for contamination resulting from wafer handling and transport. Once the surface is prepared, the substrate is loaded into a cassette which is capable of holding a number of substrate wafers. The cassette (not shown) enters the cluster tool 10 through inlet 12. One at a time, each substrate is passed via a handling mechanism (not shown) to one of the epitaxial reactors 16 where it is subjected to a pre-bake, typically at a temperature ranging from about 1000°C to about 1250°C for a duration of about 10 seconds to about 90 seconds, in a pure hydrogen atmosphere. This hydrogen pre-bake is performed in order to remove the native silicon oxide layer which formed as after the post-polishing, final cleaning step. After the hydrogen pre-bake is complete, the wafer is subjected to an epitaxial deposition process. The wafer is then removed from the reactor and transferred, again via the handling mechanism, to the cooling station 18 before being placed in a cassette and removed from the cluster tool through outlet 14.

In the past, it has been believed that the harsh conditions associated with the hydrogen pre-bake effectively reduced any particles present on the surface of the substrate wafer before epitaxial deposition occurred. For example, it was thought that organic particles would be reduced in this environment to gaseous hydrocarbons, such as methane and water. However, it has now been discovered that, while the hydrogen pre-bake may effectively consume small oxide particles, it is generally not an effective means by which to remove particles from the surface of the substrate wafer. Any particle containing carbon, for example, may react to form silicon carbide and will remain on the surface after the pre-bake is complete. As a result of the inadequacy of the hydrogen pre- bake, these particles will still be present on the wafer surface when the epitaxial deposition occurs, and this will ultimately lead to the formation of grown-in defects. Accordingly, the present invention affords a means by which to prepare epitaxial silicon wafers, wherein grown-in defects are eliminated, by removing these particles from the wafer surface via a cleaning step prior to epitaxial deposition. By removing particles at this stage of the epitaxial process, grown- in defects commonly causes by such particles are not formed.

In a preferred embodiment, cleaning will occur in- situ, or in-line, with epitaxial deposition. This is achieved by integrating a cleaning unit 20 with the epitaxial reactor cluster tool 10. Given that typically only five of the six sides of the cluster tool are occupied, the cleaning unit may be connected without disrupting the standard epitaxial process, thus allowing for the removal of particles prior to the substrate wafer entering the epitaxial reactor.

The present invention addresses the problems associated with existing epitaxial processes. As the results discussed above indicate, wafer handling and cleanroom conditions are currently a primary source of particle contamination, mainly due to the presence of human operators. In-situ cleaning addresses these problems by removing these particles prior to epitaxial deposition. The cleaning unit is preferably sealingly connected to the cluster tool by means which are standard in the art, thus ensuring the substrate wafers remain under a controlled atmosphere which is substantially particle free after cleaning is completed.

It is to be noted in this regard that, as used herein, the terms "in-situ" and "in-line" refer to a process and apparatus in which cleaning of the substrate wafer surface and subsequent epitaxial deposition occur under a controlled, substantially particle free atmosphere. Stated another way, during the cleaning step and after the particles have been removed from the wafer surface, the wafer is isolated from an ambient in which human operators are present. After cleaning, the wafer is not exposed to an ambient in which human operators are present before being placed in the epitaxial reactor. Handling the wafer in this way prevents the surface from being re-contaminated with particles before epitaxial deposition occurs.

Generally, any cleaning process which can effectively remove particles having a maximum cross- sectional width of at least about 0.1 microns, as measured by conventional laser-based auto-inspection devices, can be used provided it is capable of being performed in an atmosphere which is substantially free of particles. However, currently most particle removal processes involve wetting the substrate wafer surface in some way, which acts to complicate how the wafer will subsequently be passed via the wafer handling chamber to the reactor. For example, the wafer handling chamber must remain free of residual water and oxygen. As a result, if a wet cleaning process were employed, an efficient drying step would be needed in order to ensure the handling chamber is not contaminated by the wet cleaning process .

In view of the complicating aspects associated with a wet cleaning process, the preferred method by which to remove particles in-situ with epitaxial deposition is by using a dry process wherein solid particles of argon or a argon/nitrogen mixture are used to contact the surface of the substrate and dislodge particles which are present. This cryogenic aerosol cleaning system will most preferably be integrated with the epitaxial cluster tool.

A cryogenic aerosol cleaning system, such as the ARIES™ CryoKinetic™ Cleaning System (commercially available from FSI International) , avoids the limitations created by wet cleaning processes because liquid solutions are not employed. The cryogenic cleaning process is performed accordingly to process conditions known in the art. (See, e.g., U.S. Patent Nos . 5,062,898 and 5,209,028, incorporated herein by reference.) Generally speaking, the cryogenic cleaning method employs an aerosol, generated by initially passing argon or a mixture of argon and nitrogen gasses through a liquid nitrogen heat exchanger, which acts to decrease the temperature of the gaseous mixture to cryogenic levels. After pre-cooling the gaseous mixture, it is then circulated to an array of jets, or nozzles, which are directed at a surface of the substrate wafer. As the gaseous mixture exits the jets it expands within the cleaning chamber, which is under vacuum. The expansion of the gaseous mixture causes further cooling to occur. As a result, solid aerosol crystals are formed as the temperature and pressure within the chamber ultimately fall below the triple point of the mixture, (i.e. the thermodynamic state at which the three phases of a substance exist in equilibrium) . Without being held to any particular theory, it is generally believed that as the aerosol crystals strike the surface of the wafer, they first thermally shock the particles which are present to help detach them from the substrate, while the kinetic energy of the aerosol crystals dislodge the particles. The dislodged particles are carried away, along with the aerosol crystals, from the wafer surface in the gas stream and are vented to remove them from the cleaning apparatus .

The intensity of the aerosol stream, as well as the energy and size of the aerosol crystals, can be controlled by adjusting various process parameters, including (i) the total gas mixture flow rate, (ii) the ratio of argon to nitrogen in the mixture (which typically may range from about 10% argon up to about 100% argon) , (iii) the temperature of the gaseous mixture, and 13

(iv) the pressure of the chamber. Furthermore, the process may be optimized for a given application by varying the composition of the gaseous mixture, the angle at which the surface is struck by the particles, and the duration of the treatment, among other things.

The process and apparatus of the present invention affords a means by which to grow epitaxial wafers free of grown-in defects caused by particles which are present on the wafer surface prior to epitaxial deposition. In particular, the present invention removes particles from the wafer surface and prevents the re-contamination of this surface prior to epitaxial deposition by maintaining the clean wafer surface in a substantially particle free atmosphere, i.e. during and after cleaning the wafer is not exposed to an ambient in which human operators are present prior to epitaxial deposition.

As the following Example illustrates, cryogenic cleaning may be employed to clean the substrate wafer surface of particles. Used as part of the epitaxial cluster tool and in conjunction with epitaxial deposition, it is believed that the present process will enable the production of superior epitaxial wafers. It is to be noted, however, that alternative dry cleaning processes, such as laser-assisted cleaning or carbon dioxide aerosol crystals, could also be similarly employed as part of the epitaxial process to achieve comparable results .

The following Example sets forth only one set of conditions and, therefore, should not be interpreted in a limiting sense.

EXAMPLE In accordance with the present process, about 23 single crystal silicon substrate wafers, after being intentionally contaminated with particles, were analyzed using a Tencor 6200 laser scanner in order to determine the number of particles present on the surface of each (denoted "Pre-proc") ranging in sizes ranging from about 0.15 microns up to about 5 microns (with particles having a size of about 10 microns denoted by the "Area Cnt" column) . Immediately following the analysis of each wafer, the wafer was subjected to a cryogenic cleaning process under conditions common in the art. More specifically, each wafer was cleaned using similar gas compositions, treatment times and flow rates. However, wafers 14, 15 and 16 were cleaned using a nozzle angle of about 60° (relative to the wafer surface) while all of the other wafers were cleaned using an angle of about 45°. Immediately following the cleaning process, each wafer was subjected to the surface analysis once again (denoted "Post-proc . " ) . The total number of defects detected within the range of 0.15 microns to 5 microns before and after cleaning are provided in the column denoted "LPD." The results of the analyses, as well as the difference between the post-process and pre-process results (denoted "Delta") are presented in Table I, below.

It may be observed from the results that, with the exception of wafers 10 and 14, the process was generally successful in reducing the total number of particles, of all size ranges, present on the wafer surface. As can be noted from the post-process area count ("Area Cnt"), the process is generally more efficient at removing larger particles; that is, after cleaning the number of large defects present is generally lower than the number of defects present having a size of less than about 5 microns.

In comparing the results of wafers 15 and 16 to the results of wafers 1-13 and 17-23, it is to be noted that the total number of particles remaining after the cleaning process were higher for these wafer. Without being held to any particular theory, these results suggest that the nozzle angle does have some effect on the efficiency of the cleaning process. Accordingly, the nozzle angle may be varied for each application, in addition to other process conditions, in order to optimize the present cleaning process.

It is to be noted that substrate wafers employed in an epitaxial process preferably have a particle count of less than about 20, for particles in excess of about 0.12 microns. By comparison, the wafers subjected to the present cryogenic cleaning process, with few exceptions, were more heavily contaminated both before and after cleaning. However, the experimental data indicates that the present process is successful in removing particles of all sizes from the substrate wafer surface. In addition, it may be observed from the results of wafer 9 that the present process is capable of producing a wafer which meets the preferred particle count of less than about 20, wafer 9 having a post-process LPD count of 8 and an area count of 3. It is to be further noted that while the present cleaning process is generally successful in removing particles from the surface when several hundred particles are present, it is also success in reducing the number of particles when the initial numbers are much lowers.

Referring specifically to the results of wafers 20, 23 and 24, it may be observed that the process was successful in reducing the number of particles present even when the initial count was less than one hundred. However, it may also be observed that as the initial number of particles present decreases, the efficiency of the process generally decreases. In fact, the results of wafers 10 and 14 show that the cleaning process caused the number of particles to increase in some cases. Without being held to any particular theory, it is generally believed the tests results suggest that as the number of particles present approaches some minimum, the cleaning process can reach a maximum in terms of the number of particles it is able to remove. Stated another way, it is believed that for these wafers the process reached a limit in terms of the final number count due to a residual, or background, level of particle contamination within the systems itself. Accordingly, while the cleaning process may successfully be employed to remove particles and yield a preferred substrate wafer (see, e.g., wafer 9), further refinements by means common in the art may be necessary in order to consistently exhaust or remove residual particles and lower the particle background, thus enabling a total particle count of less than about 20 to be consistently attained.

In view of the above, it will be seen that the several objects of the invention are achieved.

As various changes could be made in the above compositions and processes without departing from the scope of the invention, it is intended that all matter contained in the above description be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. A process for preparing an epitaxial silicon wafer which is free of grown-in defects resulting from particles which are present on a surface of a single crystal silicon wafer substrate, the process comprising: cleaning the substrate to remove particles which are present on the substrate surface, said cleaning being performed under a substantially particle free atmosphere; and, depositing an epitaxial silicon layer free of grown- in defects on the substrate surface, said epitaxial deposition being performed in-line with the substrate cleaning such that the substrate remains under a substantially particle free atmosphere after cleaning and until epitaxial deposition is complete.

2. The process as set forth in claim 1 wherein the substrate surface is cleaned using aerosol crystals of a gaseous mixture, the crystals formed by decreasing the temperature and pressure of the mixture below the triple point .

3. The process as set forth in claim 2 wherein the gaseous mixture is comprised of argon and nitrogen.

4. The process as set forth in claim 2 wherein the aerosol crystals contact the substrate surface at an angle of about 45┬░.

5. The process as set forth in claim 1 wherein the substrate is baked in a hydrogen atmosphere after cleaning and prior to epitaxial deposition.

6. A process for preparing an epitaxial silicon wafer free of grown-in defects by removing particles present on a surface of a single crystal silicon wafer substrate, prepared by the Czochralski method, the process- comprising: placing the substrate via an inlet port into an epitaxial cluster tool; transferring the substrate via a handling mechanism to an in-line cleaning unit; cleaning a surface of the substrate by contacting the surface with aerosol crystals of a gaseous mixture, the crystals formed by decreasing the temperature and pressure of the mixture below the triple point, in order to dislodge particles which are present on the substrate surface; transferring the substrate via the handling mechanism to an in-line epitaxial deposition reactor and baking the substrate in a hydrogen atmosphere; and depositing an epitaxial silicon layer which is free of grown-in defects on the surface of the substrate.

7. The process as set forth in claim 6 wherein the gaseous mixture is comprised of argon and nitrogen.

8. The process as set forth in claim 6 wherein the aerosol crystals contact the substrate surface at an angle of about 45┬░.

9. An epitaxial deposition apparatus for preparing epitaxial silicon wafers free of grown-in defects caused by particles present on the surface of a single crystal silicon wafer substrate, the apparatus comprising: a centrally located epitaxial cluster tool comprising an inlet port, an outlet port, and a chamber containing a handling mechanism, said chamber being capable of maintaining a substantially particle free atmosphere therein; a cleaning unit sealingly connected to the cluster tool which is capable of receiving a substrate from the handling mechanism and cleaning a surface of the substrate while maintaining the atmosphere; and an epitaxial reactor sealingly connected to the cluster tool which is capable of receiving a substrate from the handling mechanism and depositing a layer of silicon free of grown-in defects on the clean surface while maintaining the atmosphere.

10. The apparatus as set forth in claim 9 wherein the cleaning unit forms aerosol crystals of a gaseous mixture by decreasing the temperature and pressure of the mixture below the triple point, the aerosol crystals contacting the surface of the substrate to remove particles therefrom.