WO1997044817A1

WO1997044817A1 - Hot ultra-pure water dewaxing process

Info

Publication number: WO1997044817A1
Application number: PCT/US1997/009049
Authority: WO
Inventors: Jay Wayne Harris
Original assignee: Seh America, Inc.
Priority date: 1996-05-24
Filing date: 1997-05-22
Publication date: 1997-11-27

Abstract

A process for removing waxes from polished single crystal semiconductor wafers is disclosed. The process uses heated recirculated ultra-pure water for wax liquefaction, and therefore eliminates methylene chloride from the process. An apparatus utilized to carry out the process of the present invention is also disclosed.

Description

HOT ULTRA-PURE WATER DRWΛXING PROCESS Background of the Invention Area of the Art

This invention relates to a dewaxing process, in particular, a hot ultra-pure water dewaxing of polished single crystal semiconductor wafers. Description of the Prior Art

After cutting a single crystal silicon ingot into individual wafers, many process steps are involved before the wafer is made into devices. These steps may include the main process steps of edge grinding, lapping, laser marking, chemical etching, wafer heat treatment, polysilicon deposition via low pressure chemical vapor deposition, silicon dioxide deposition via chemical vapor deposition, polishing, final cleaning and final inspection. During the fabrication processes, submicron particles, ionic and metallic impurities, and organic residues are the major surface contaminants which result in more than seventy-five percent of the yield loss in advanced device manufacturing. Contamination control involves both prevention of contaminant deposition on wafer surfaces and removal of deposited contaminants from wafer surfaces. Because complete prevention of contaminant deposition is unattainable, wafer cleaning is the most common operation performed during device fabrication.

For example, prior to polishing silicon wafers, the wafers are often mounted to a glass plate that had previously been sprayed with a controlled thickness of wax agent to position the wafers and hold them in place during the subsequent chemical and mechanical polishing steps. Once such steps are finished, the wafers are mechanically removed from the glass plates. Therefore, prior to any further cleaning processes, the polished wafers first are subjected to a wax removal process to remove all of the waxes and their traces from the wafers.

Typically, wax removal of polished wafers takes place in bath sequences. Polished wafers are first subjected to a bath containing methylene chloride for liquefying waxes and then transferred to other baths containing highly filtered water for further removal of the wax residuals. Then the polished wafers are subjected to a further cleaning process for removing the remaining organic contaminants and heavy metals. Figure 1 shows a flowchart of a typical prior art wax removing process followed by a chemical cleaning process. According to the process shown in Figure 1, silicon wafers are first subjected to bath 1 containing methylene chloride (CH₂C1₂) at room temperature for liquefying waxes deposited on the silicon wafers. Then the silicon wafers are sequentially subjected to baths 2 and 3 containing highly filtered water mixed with surfactant L-44 for further removing liquefied waxes away from the silicon wafers. Surfactant L-44 is added to maintain the surfaces of the silicon wafers hydrophilic. L-44 is a chemical trade name by the BASF A.G. and is a substance comprising of two monomers, namely poly-oxy-ethylene-glyco and poly-oxy- propylene-glycol, in their polymerized form which is known as ethylene-oxide- propylene-oxide co-polymer. After this wax removing process, the silicon wafers are subjected to a chemical cleaning process which is sequentially carried out in bath 4 to bath 10. Briefly, silicon wafers from bath 3 are sequentially subjected to baths 4 and 5 containing a sodium hydroxide solution at a temperature between 65° C and 70° C for removing remaining polishing slurry on the wafers. Ultrasonic (U.S.) waves may also be applied to baths 4 and 5 to facilitate the cleaning process. The silicon wafers are then subjected to bath 6 for rinsing by highly filtered water at room temperature. In bath 7, silicon wafers are subjected to a solution of ammonium hydroxide, hydrogen peroxide, and ultra-pure water. The H₃O₂ contained in bath 7 will oxidize all the remaining organic contaminants and the NH₄OH will help to remove heavy metals by forming complex-amine groups. Finally, silicon wafers are rinsed in baths 8 to 10 with highly filtered water at room temperature, and dried in dryer 1 1 in an atmosphere of isopropyl-alcohol (IPA).

Figure 2 is a block diagram of a typical prior art apparatus utilized in carrying out the above discussed process. For purposes of simplicity, the diagram only shows the first three bathes used in the process. Apparatus 40 comprises a quartz container 42 for bath 1, and quartz containers 44 and 46 for baths 2 and 3 respectively. Bath 1 contains pure methylene chloride, and baths 2 and 3 contain high filtered water mixed with surfactant such as L-44 for maintaining the surfaces of the silicon wafers hydrophilic. Apparatus 40 also comprises a control panel 50 for controlling the operation of each bath. As shown in Figure 3, control panel 50 has a suction indicator 52 and an on/off button 54. Suction hose is used to remove waxy substance or to replenish bath 1 with fresh methylene chloride. Methylene chloride waste is often stored in a big stainless steel tank (not shown). Apparatus 40 further comprises slots 48 for methylene chloride vapors that are directed to carbon absorption unit (CAU) scrubbers.

A variety of cleaning and rinsing combinations may be used for the cleaning process. However, methylene chloride is most commonly used as a wax liquefaction reagent. Unfortunately, methylene chloride is a toxic and environmentally adverse chemical. Therefore, the disposal of methylene chloride in accordance with most environmental statutes is very costly and environmentally unfriendly. A costly exhaust scrubber system for methylene chloride fumes must be used in the process.

In addition, the apparatus for carrying out the methylene chloride wax removal process uses a non-circulating system. The system tends to cause wax traces to cling to wafers and cassettes during transfer, and to be carried down the cleaning line thus contaminating the cleaning line. Furthermore, the process does not remove polishing slurry before being subjected to a chemical bath.

Summary of the Invention

It is an object of the present invention to provide a process and an apparatus for removing wax from single crystal semiconductor wafers without the above- mentioned problems, particularly for eliminating the use of methylene chloride in the wax removing process.

These and other objects and advantages are achieved by the process of the present invention. In accordance with the present invention, heated recirculated ultra- pure water is used to substitute methylene chloride for wax liquefaction. Such a process provides a number of advantages. A process of the present invention eliminates methylene chloride in polishing wax removal process. Because of this elimination, the process of the present invention is environmentally friendly and creates a safer work environment. The process of the present invention also reduces the chemical cost and waste disposal cost associated with the use of methylene chloride. In addition, the process of the present invention results in unexpected decreased levels of light point defects (LPD) and decreased levels of aluminum (Al) and sodium (Na) as indicated by vapor phase decomposition analysis (VPD).

The process of the present invention is well suited for use in connection with a dewaxing process for polished single crystal semiconductor wafers. The process is especially suited for contamination sensitive large diameter products (200 mm and above) due to the increasing specific demands of lower surface contaminations and particle levels. In this use of the process, single crystal semiconductor wafers are first subjected to hot circulated ultra-pure water for wax liquefaction. Then the dewaxed wafers are subjected to further chemical treatment to remove the remaining organic contaminants and heavy metals. Further benefits and advantages of the invention will become apparent from a consideration for the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention. Description of the Figures

The above-mentioned and other features of this invention and the manner of obtaining them will become more apparent, and will be best understood by reference to the following description, taken in conjunction with the accompanying drawings. These drawings depict only a typical embodiment of the invention and do not therefore limit its scope. They serve to add specificity and detail, in which:

FIGURE 1 is a flow chart of a prior art dewaxing process for single crystal semiconductor wafers.

FIGURE 2 is a simplified block diagram of a prior art apparatus for the dewax process of Figure 1.

FIGURE 3 is a block diagram of the control panel shown in Figure 2.

FIGURE 4 is a flow chart of the process of the present invention for removing waxes from semiconductor wafers.

FIGURE 5 is a simplified block diagram of an apparatus of the present iavention for the process of the present invention.

FIGURE 6 is a block diagram of the control panel of the apparatus of the present invention shown in FIGURE 5.

FIGURE 7 shows an illustrative diagram of the recirculating system of the apparatus of the present invention shown in FIGURE 5. FIGURE 8 shows "vapor phase decomposition analysis" (VPD) results of

Example 1.

FIGURE 9 shows "light point defects" (LPD) results of Example 1.

FIGURE 10 shows VPD results of Example 2. Detailed Description of the Invention

The embodiment of the present invention described herein provides a process for removing waxes from single crystal semiconductor wafers. The process comprises the steps of providing ultra-pure water, heating said ultra-pure water, recirculating said heated ultra-pure water, and exposing said single crystal semiconductor wafers to said recirculated heated ultra-pure water for liquefying said waxes. The process of the present invention greatly reduces the chemical cost and waste disposal cost associated with the use of methylene chloride for wax liquefaction. It is environmentally friendly and safer to use. In addition, the process of the present invention unexpectedly results in decreased levels of light point defects (LPDs) and decreased levels of aluminum (Al) and sodium (Na) as indicated by vapor phase decomposition analysis (VPD).

Preferably, Ultra-pure water of the present invention is the water with a specific resistivity around 18 MegaOhmcm or higher (MΩcm). It includes, but is not limited to water which is highly filtered, deionized, and ultra-violet irradiated and cleaned through reverse osmosis (RO) process. Depending on the source of origin, other processes known in the art for purifying water of specific resistivity may also be used. The water is heated to a temperature range of approximately 70° - 95°C. Preferably, the temperature of the water is heated to about 90°C. The water is recirculated at a rate and for a time that is sufficient to liquefy waxes deposited on the single crystal semiconductor waiters. In a preferred embodiment, the water is circulated at a rate of approximately 2 to 5 L/Min. for about 3-5 min per time cycle.

In a preferred embodiment, the ultra-pure water is filtered during recirculation. A filter having a pore size that can stop the passing through of liquefied wax, but allow the passing through of ultra-pure water is preferred. In a preferred embodiment, the pore size of the filter is approximately equal to or less than 0.1 μm. The pore size of the filter can be smaller than 0.1 μm as long as the water can be filtered through.

Preferably, a surfactant detergent, such as but not limited to L-44 detergent, is added to the ultra-pure water for keeping the surfaces of the single crystal semiconductor wafers hydrophilic. In a preferred embodiment, the concentration of the L-44 detergent is about 0.4%- 0.9% by volume of the ultra-pure water. L-44 is an industrially available detergent for maintaining hydrophilic surfaces.

In accordance with the present invention, the single crystal semiconductor wafers are exposed to the heated recirculated ultra-pure water in an amount that is sufficient to liquefy wax deposited on the wafers. The amount of ultra-pure water used is determined in accordance with the number of single crystal semiconductor wafers needed to be cleaned, the bath volume, the recirculating flow rate, and the time cycle per bath. One skilled in the art can readily determine the amount of ultra-pure water used without undue experimentation. In a preferred embodiment, single crystal wafers may be exposed to multiple baths containing sufficient amount of heated recirculated ultra-pure water for achieving better wax liquefaction.

In a preferred embodiment, the process of the present invention also comprises additional steps for removing organic contaminations and heavy metals contained on the surface of the single crystal semiconductor wafers. Preferably, after treated by the heated recirculated ultra-pure water, the single crystal semiconductor wafers are further subjected to a solution containing hydrogen peroxide (H₂O₂) and ammonium hydroxide (NH₄OH) in water, which will oxidize all the remaining organic contaminants and remove heavy metals by forming complex-amine groups. This solution is also referred to in the industry as Super Cleaning 1 type solution (SCI ) or RCA since RCA company developed such solution. Apparently, other solutions which perform substantially the same functions as RCA may also be used. For example, Super Cleaning 2 type solution (SC2) may also be used. SC2 is a solution containing hydrochloride acid, citric acid, and hydrogen peroxide chemicals mixed with water.

Figure 4 shows a flow chart of one preferred embodiment of the process of the present invention. According to the process shown in Figure 4, single crystal semiconductor wafers are first subjected to bath 1 containing heated recirculated ultra- pure water for liquefying waxes mounted on the semiconductor wafers. Preferably, bath 1 contains about 25 to 35 liters of ultra-pure water, and approximately 180 to 220 ml of L-44 detergent is added to the water. The water is heated to a temperature range between 80°C and 93 °C, and is recirculated at a constant flow rate of about 2 to 5 liter/min. The single crystal semiconductor wafer is exposed to bath 1 for approximately 3 to 5 minutes. During water recirculation, a filter having a pore size of equal or less than O.lμm is used for filtering the recirculated water. Then the semiconductor wafers are sequencially subjected to baths 2 and 3 containing heated recirculated ultra-pure water for further wax liquefaction and polishing slurry removal. The parameters of the heated recirculated ultra-pure water contained in baths 2 and 3 is similar to that of bath 1 except that the temperature of the water contained in baths 2 and 3 may be in a range of 70° to 93 °C. After bath 3, the semiconductor wafers are subjected to a chemical cleaning process which is sequentially carried out in bath 4 to bath 10. Alternatively, any other chemical cleaning processes which are known in the art for cleaning single crystal semiconductor wafers after wax liquefaction by methylene chloride may also be used. Briefly, semiconductor wafers from bath 3 are sequentially subjected to baths 4 and 5 containing approximately 360 to 440 ml of TMAH (tetra-methylene-ammoniurn- hydroxide) mixed with proper amount of ultra-pure water at a temperature between 65° C and 70° C for about 3 to 5 minutes respectively. Then the semiconductor wafers are subjected to bath 6 for rinsing by highly filtered water at room temperature. In bath 7, semiconductor wafers are subjected to a solution of ammonium hydroxide, hydrogen peroxide, and ultra-pure water. The H₂O₂ contained in bath 7 will oxidize all the remaining organic contaminants and the NH₄OH will help to remove heavy metals by forming complex-amine groups. Finally, semiconductor wafers are rinsed in baths 8 to 10 by highly filtered water at room temperature, and dried in dryer 1 1.

According to Figure 4, TMAH is used to substitute sodium hydroxide in baths 4 and 5. The prior art process uses sodium hydroxide for removing remaining polishing slurry on the wafers. However, at the same time, sodium hydroxide will remove about 300 Angstrom (0.03 micrometer) per bath of the backside CVD

(chemical vapor deposition) layer and thus will cause problems for the layer thickness control. In addition, sodium ions may be harmful for devices used in the process. The process in accordance with the present invention, however, removes remaining polishing slurry very effectively so that NaOH bath steps may be eliminated. Therefore, the process of the present invention allows a better control over the thickness of the CVD film on the surface of wafers, and reduces the risk associated with the use of sodium hydroxide. In a preferred embodiment, TMAH may be used to remove a very thin layer (approximately 10 Angstrom per bath) of a CVD film and its contaminants. Alternatively, TMAH baths may also be eliminated and replaced by additional ultra-pure water baths.

Figure 5 is an illustrated block diagram of a typical apparatus utilized in carrying out the process of the present invention. The diagram, for purposes of simplification, only shows the first three baths used in the process. Apparatus 140 comprises a container 142 as bath 1, and containers 144 and 146 as baths 2 and 3 respectively. Preferably, the containers are quartz containers. Baths 1 , 2 and 3 contain ultra-pure water which is heated and recirculated by individual independent re-circulation systems coupled to quartz containers 1 to 3 respectively. A control panel 150 is used for controlling the operation of each bath. Steam generated from the heated ultra-pure water is routed by vent exhaust lines through slots 148 directly to the atmosphere.

Figure 6 shows a preferred embodiment of a control panel for controlling the operation of each bath. The control panel 150 has switches, buttons and/or indicators depicted in Figure 6 including: temperature control 152 for setting the temperature parameter of the ultra-pure water, over temperature or error indicator 154, heater/on button 156 for turning the heater on, out of temperature indicator 158, recirculation button 160 for starting the recirculation of the ultra-pure water, a heater off button 162 for turning the heater off, low/level indicator 164 to indicate that the level of ultra- pure water level in a bath is low, fill button 166 for refill of the ultra-pure water to a process level, heater leak indicator 168, and drain button 170 for draining the ultra- pure water out of the bath.

Figure 7 is a schematic flow diagram which shows a preferred embodiment of a recirculation system of the present invention. According to Figure 7, ultra-pure water is filled into container 142 through line 184 to a process level 186. When the level of ultra-pure water reaches a low level 188, more ultra-pure water will be filled in for keeping the water at a process level 186. During the recirculation cycle, the ultra-pure water is recirculated by the recirculation system 200 from container 142 to I 10 the pump/heater box 220. Preferably, a containing 182 is coupled to container 142 and in flow communication with 142 such that the ultra-pure water is recirculated through container 182 to the pump/heater box 220. The ultra-pure water is first filtered by the front end filter 222 for removing any liquefied waxes and particles. Then it is heated by heater 226. Preferably, the heater is an electric quartz light heater. Before the ultra-pure water is circulated back to container 182, the water is further filtered by back end filter 230. A pump 224 is utilized in the pump/heater system 220 for recirculating the ultra-pure water in the system. Preferably, the heated recirculated water is recirculated back to container 142 through a dispersion plate 190 which is utilized to provide a vertical flow of the water within container 142. A heater level protection means 228 may also be used to control the operation of heater 226. At the end of the cleaning, water contained in container 142 will be drained out through valve 192. The water in the recirculation system will be drained out through three way valve 210. Containers 144 and 146 also use independently the same recirculation system as shown in Figure 7. It is preferred that each individual bath is supplied with an individual recirculation system to avoid cross contaminations. In a preferred embodiment, the first bath removes about 80% of all contaminants on the wafers, the second one removes a majority of the remaining 20% of contaminants, and the third bath ensures the specified cleanliness.

Apparently, any apparatus which is known in the art for carrying out semiconductor wafer dewaxing process utilizing methylene chloride may be used to perform the process of the present invention by including a recirculation system of the present invention in the apparatus. In addition, alternative designs for the recirculation system may also be used for carrying out the process of the present invention without departing from the spirit of the present invention.

The following examples are intended to illustrate but not to limit the invention. The advantages of the present inventions over a methylene chloride dewaxing process are demonstrated in the examples.

EXAMPLE 1 VPD results and LPD results of silicon wafers treated hv the process of the present invention The process of the present invention as shown in Figure 4 was compared with the prior art process as shown in Figure 1. Single crystal silicon wafers treated respectively by process of Figure 4 and by process of Figure 1 were analyzed by their "Light Point Defect" (LPD) counts, and "Vapor Phase Decomposition Analysis" (VPD) parameters.

Light point defects are microscopical, particle irregularities on highly polished silicon wafer surfaces. In the case of non-etching ambient, such as cleanlines LPD measurements reveal surface particles adsorbed on the surface of specimens after bath sequences have been applied. LPD data for cleaning evaluations are used to quantify the level of generated or unremoved particles on silicon wafer surfaces. Any methods and instruments for measuring LPD which are known in the art may be used to measure LPD counts. For example, the commercially available "ESTEK™ CR 80" instrument may be used. Briefly, the laser surface scan technique applied in the

"ESTEK™ CR 80" will detect small angle diffractions of reflected laser light caused by surface irregularities and determine through software evaluation the corresponding size of such surface irregularities.

Vapor phase decomposition analysis (VPD) is a technique being used to determine metallic trace elements on wafer surfaces. The VPD teclinique is also known in the art (Fucsko et al, J. Electrochem. Soc, 140:4, 1994. pp. 1 105-1 109). Briefly, the wafer is subjected to hydrogen fluoride vapor to decompose and release metal contaminants from a surface oxide. The trace metals are then collected by scanning a small drop of dilute acid solution throughout the wafer surface. The solution is then analyzed by either "inductively Coupled Mass Spectrometry" (ICPMS) or "Graphite Furnace Atomic Absorption " (GFAA).

The VPD results for silicon wafers treated respectively by process of Figure 1 and process of Figure 4 are summarized in table 1 , which is depicted in Figure 8. The numbers in parenthesis are standard deviations or sigmas. Table 1. VPD results of the methylene chloride dewaxing process and the process of the present invention

Figure 8 shows that, when compared to the methylene chloride dewaxing process, silicon wafers treated by the process of the present invention have a surface decrease of aluminum (Al) from 2.8 x 10'7cm² (CH₂CL₂-2 cleanline) to 1.9 x 10¹⁰ /cm² and a surface decrease of sodium (Na) from 6.5 x 10^g /Cm² (CH₂CL₂-2 cleanline) to 6.2 x IO⁸ /cm². The other readings, particularly with regard to iron (Fe) and zinc (Zn) are subjected to large sigmas and may not be conclusive. The LPD results for silicon wafers treated respectively by process of Figure 1 and process of Figure 4 are summarized in table 2, which is depicted in Figure 9.

Table 2. LPD results of the methylene chloride dewaxing process and the process of the present invention

Figure 9 shows a clear difference between the methylene chloride process as compared to the heated recirculated water process of the present invention. The light point defects were significantly reduced from 225 counts per wafer (CH₂CL₂-2 cleanline) to 59 counts per wafer for particle sizes between 0.13 μm and 0.16 μm, from 57 counts to 24 counts per wafer for particle sizes between 0.16 μm and 30 μm, and from 18 counts to 1 count per wafer for particle sizes larger than 0.30 μm.

EXAMPLE 2

To further compare the process of the present invention with the process utilizing methylene chloride, "at point of use" test is carried out. In this test, two processes were used to treat silicon wafers: a methylene chloride process utilizing steps of baths 1-3 and 9-10 of Figure 1, and a hot water process utilizing steps of baths 1 -3 and 9-11 of Figure 4. In other words, NaOH and SCI bath steps were left out such that the comparison was made directly between the cleaning effect of methylene chloride and hot ultra-pure water. The VPD analysis of the two processes is summarized in Figure 10.

Figure 10 clearly indicates that silicon wafers treated by the process of the present invention, when compared with the methylene chloride process, have a surface decrease of aluminum (Al), calcium (Ca), copper (Cu), iron (Fe), sodium (Na), nickel (Ni), and zinc (Zn). A process and an apparatus in accordance with the present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiment is to be considered in all respects only as illustrative and not as restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of the equivalence of the claims are to be embraced within their scope.

Claims

I claim:

1. A process for removing wax from the surface of crystal wafers comprising the steps of: a. providing ultra-pure water; b. heating said ultra-pure water; c. recirculating said heated ultra-pure water; and d. exposing said crystal wafers to said heated recirculated ultra-pure water for liquefying said wax.

2. The process of claim 1, wherein said crystal wafers are crystal semiconductor wafers.

3. The process of claim 2, wherein said crystal semiconductor wafers are single crystal semiconductor wafers.

4. The process of claim 3, wherein said single crystal semiconductor wafers are single crystal silicon wafers.

5. The process of claim 1 further comprising a step of filtering said ultra-pure water during said recirculating step.

6. The process of claim 5, wherein said ultra-pure water is filtered by a filter having a pore size of equal to or less than 0.1 μm

7. The process of claim 6, wherein said ultra-pure water is filtered by a filter having a pore size of 0.1 μm

8. The process of claim 1 further comprising additional steps repeating steps a through d for further liquefying said wax.

9. The process of claim 1 , wherein said ultra-pure water is heated to a temperature in excess of 70°C.

10. The process of claim 9, wherein said ultra-pure water is heated to a temperature of approximately 90°C.

11. The process of claim 1 , wherein said ultra-pure water contains a surfactant in an amount sufficient to maintain a hydrophilic surface of said crystal wafers.

12. The process of claim 11, wherein said surfactant is L-44.

13. The process of claim 1, wherein said crystal wafers contain organic contaminants and heavy metals, and wherein said process comprises a further step of removing said organic contaminants and heavy metals.

14. The process of claim 13, wherein said removing step is carried out by exposing said single crystal silicon wafers to TMAH-added water.

15. The process of claim 13, wherein said removing step is carried out by exposing said single crystal semiconductor wafers to RCA.

16. An apparatus for removing wax from the surface of crystal wafers comprises: a container containing ultra-pure water; means for heating said ultra-pure water; means for recirculating said heated ultra-pure water to said container to expose said crystal wafers to said recirculated heated ultra-pure water for liquefying said wax.

17. The apparatus of claim 16, wherein said heating means includes a heater.

18. The apparatus of claim 17, wherein said heater is an electric quartz light heater.

19. The apparatus of claim 16, wherein said recirculating means includes a pump.

20. The apparatus of claim 16, wherein said recirculating means further comprises at least one filter for filtering said ultra-pure water.

21. The apparatus of claim 20, wherein said filter has a pore size of equal to or less than 0.1 μm.

22. The apparatus of claim 16, wherein said container comprises a dispersion plate to provide a vertical flow of said recirculated heated ultra-pure water inside said container.