Pretreatment method to improve resistivity performance of silicon wafer

Abstract

A pretreatment method for improving the resistivity performance of a silicon wafer includes step 1: grinding the surface of a silicon wafer with a grinder to make the surface of the silicon wafer flat; step 2: immersing the ground silicon wafer in a pre-prepared alkaline etching solution for alkaline washing; step 3: immersing the alkaline washed silicon wafer in a pre-prepared mixed acid solution for a first pickling; step 4: performing an oxidative heat treatment and rapid cooling on the pickled silicon wafer to remove thermal donors in the silicon wafer; step 5: performing a second pickling on the cooled silicon wafer with a mixed acid solution to obtain a silicon wafer for resistivity performance testing. This solution can improve the resistivity stability of the silicon wafer, and can restore the true resistivity of the silicon wafer by removing the thermal donors, thereby improving the accuracy of the resistivity detection of the silicon wafer.

Description

Technical Field

[0001] The invention relates to the technical field of semiconductor materials, and in particular to a pretreatment method for improving the resistivity performance of a silicon wafer. Background Art

[0002] The resistivity of silicon single crystals is closely related to semiconductor devices, especially the stability of resistivity. When the resistivity of silicon single crystals fluctuates greatly, it may cause the breakdown voltage of the device to decrease, thereby reducing the yield of the device and affecting the design and manufacturing of the device.

[0003] Before conventionally testing the resistivity of a single crystal, the silicon wafer is annealed and sandblasted. Annealing is due to the introduction of thermal donors at a low temperature of 450-550°C during the crystal pulling process, which causes the resistivity of the lightly doped sample to deviate from the target value and the resistivity accuracy is low. In order to eliminate the thermal donors and restore the true resistivity, the silicon wafer needs to be heat treated at 650°C. Because the silicon wafer has undergone a thermal oxidation process, its surface will be covered with a dense oxide film, which needs to be removed by sandblasting or grinding. However, although this method of sandblasting or grinding can remove the oxide layer to a certain extent, the sample used for the early evaluation of the resistivity is a truncated silicon wafer with a poor surface condition, and there are sawtooth marks, dirt, metal impurities, etc., so the measured resistivity is still affected by the surface state of the silicon wafer, the resistivity test fluctuates greatly, the stability is poor and the accuracy is low, which greatly limits the device manufacturing. Summary of the invention

[0004] In view of this, in view of the above shortcomings, it is necessary to propose a pretreatment method for improving the resistivity performance of silicon wafers to improve the resistivity stability and accuracy of silicon wafers.

[0005] The embodiment of the present invention provides a pretreatment method for improving the resistivity performance of a silicon wafer, comprising the following steps:

[0006] Step 1: Use a grinder to grind the surface of the silicon wafer to make the surface of the silicon wafer flat;

[0007] Step 2: Immerse the polished silicon wafer in a pre-prepared alkaline etching solution for alkaline washing;

[0008] Step 3: Immerse the alkali-washed silicon wafer in a pre-prepared mixed acid solution for the first acid wash;

[0009] Step 4: The pickled silicon wafer is subjected to oxidation heat treatment and rapid cooling to remove the heat donors in the silicon wafer;

[0010] Step 5: The cooled silicon wafer is pickled for a second time with the mixed acid solution to obtain a silicon wafer for resistivity performance testing.

[0011] Preferably, in step 1, the silicon wafer is polished using a grinding liquid under high-speed rotation of a turntable; wherein the grinding liquid is a mixed solution consisting of 1500 mesh grinding sand, alumina suspension and water, and the mixing ratio is 1500 mesh grinding sand: alumina suspension: water = 5-10kg: 2-5L: 10-15L.

[0012] Preferably, in step 1, the silicon wafer is polished using a diamond grinding head in the grinder.

[0013] Preferably, in step 1, the amount of silicon wafer removed by the grinder is 100 to 300 μm.

[0014] Preferably, the step 2 specifically comprises: placing the polished silicon wafer into the alkaline etching solution and ultrasonicating for 5 to 10 minutes.

[0015] Preferably, the alkaline etching solution is a mixed solution composed of 49% by mass of sodium hydroxide, deionized water and a surfactant in a volume ratio of 35-40:221:0.1-0.5.

[0016] Preferably, the pickling conditions for the first pickling in step 3 and the second pickling in step 5 are both: the silicon wafer is completely immersed in the mixed acid solution and reacts for 2-3 minutes.

[0017] Preferably, the mixed acid solution is a mixed solution composed of 49% by mass hydrofluoric acid, 70% by mass nitric acid, 99% by mass acetic acid and deionized water in a volume ratio of 1:3-4:2-5:5-8.

[0018] Preferably, the step 4 specifically includes:

[0019] Before oxidation, preheat the oxidation annealing furnace at a preheating temperature of 600-800°C for 20 minutes;

[0020] Placing the silicon wafer after the first pickling in a quartz boat, and transferring the quartz boat to the oxidation annealing furnace for oxidation annealing for 30 minutes to 120 minutes;

[0021] After annealing, take out the quartz boat and use a fan to quickly cool the silicon wafer. Turn off the fan after cooling for 30 to 50 minutes.

[0022] Preferably, after the silicon wafer is acid-washed or alkaline-washed, the silicon wafer is rinsed with overflowing deionized water.

[0023] It can be seen from the above technical scheme that the pretreatment method for improving the resistivity performance of silicon wafers provided by the embodiment of the present invention includes the processes of grinding, alkali washing, first acid washing, heat treatment and second acid washing. The grinding method can make the surface of the silicon wafer flat, the alkali washing can remove the grease and dirt on the surface of the silicon wafer, and the mixed acid removes the metal impurities and the damaged layer, so that the stress in the silicon wafer is released, so that the surface of the silicon wafer can reach a more ideal state, reducing the influence of the surface state of the silicon wafer on the resistivity. Furthermore, by heat treatment and removing the oxide layer again with mixed acid, the thermal donor can be eliminated and the real resistivity can be restored. It can be seen that the detection accuracy of the silicon wafer can be improved through this scheme, and since the influence of the surface state of the silicon wafer on the resistivity stability is reduced, the resistivity stability can be greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A flow chart of a pretreatment method for improving the resistivity performance of a silicon wafer provided in an embodiment of the present invention.

[0025] Figure 2 This is a surface image of a silicon wafer before polishing.

[0026] Figure 3 This is a surface image of a silicon wafer after polishing. DETAILED DESCRIPTION

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0028] See also Figure 1 The embodiment of the present invention provides a pretreatment method for improving the resistivity performance of a silicon wafer, comprising the following steps:

[0029] Step 1: Use a grinder to grind the surface of the silicon wafer to make the surface of the silicon wafer flat;

[0030] Step 2: Immerse the polished silicon wafer in a pre-prepared alkaline etching solution for alkaline washing;

[0031] Step 3: Immerse the alkali-washed silicon wafer in a pre-prepared mixed acid solution for the first acid wash;

[0032] Step 4: The pickled silicon wafer is subjected to oxidation heat treatment and rapid cooling to remove the heat donors in the silicon wafer;

[0033] Step 5: The cooled silicon wafer is pickled for the second time with a mixed acid solution to obtain a silicon wafer for resistivity performance testing.

[0034] The present invention aims to solve the problems of unstable resistivity test, low accuracy, uneven radial resistivity distribution, etc. Compared with the conventional pre-treatment method of resistivity detection using heat treatment and sandblasting, the pre-treatment method used in this patent includes five steps: grinding, alkali washing, mixed acid polishing, heat treatment, and mixed acid polishing. In order to make the resistivity test of the sample stable and not affected by the surface state, this embodiment uses grinding to make the sample surface flat, then removes surface grease and dirt through alkali washing, removes metal impurities and damaged layers through mixed acid, releases the internal stress of the silicon wafer, and other steps to make the sample surface reach a more ideal state, and finally restores the true resistivity through heat treatment, and then removes the oxide layer with mixed acid, so as to obtain a stable resistivity value.

[0035] In one embodiment, step 1 can be performed to polish the silicon wafer in two ways:

[0036] Method 1: Grinding the silicon wafer with a grinding liquid under high-speed rotation of a turntable; wherein the grinding liquid is a mixed solution composed of 1500 mesh grinding sand, alumina suspension and water, and the mixing ratio is 1500 mesh grinding sand: alumina suspension: water = 5~10kg: 2~5L: 10~15L.

[0037] Method 2: Use a diamond grinding head in a grinder to grind the silicon wafer.

[0038] In this embodiment, method 1 uses a grinding liquid to grind the silicon wafer, and method 2 uses a diamond grinding head to grind the silicon wafer. Method 1 can grind multiple silicon wafers at the same time and has a higher grinding efficiency. Therefore, in actual mass production, method 1 has more advantages. For a small number of silicon wafers used for testing, method 2 can be used for grinding. Method 2 does not use grinding liquid, can meet the grinding needs of a small number of test silicon wafers, and can save the cost of grinding liquid.

[0039] In addition, since the surface condition of the silicon wafer after truncation is poor, with unevenness and a difference of 100-300 μm, it is considered to control the amount of silicon wafer removed by the grinder within 100-300 μm according to the specific surface condition of the silicon wafer.

[0040] like Figure 2 and Figure 3 The figure shows the surface of the silicon wafer after being polished by the diamond grinding head in this solution. The flatness of the silicon wafer has been significantly improved after polishing. Moreover, the thickness deviation of the two measurements of the front and back centers of the silicon wafer before polishing is 4.5%, while the thickness deviation of the two measurements of the front and back centers of the silicon wafer after polishing is 0.06%.

[0041] Furthermore, in one embodiment, in step 2, the polished silicon wafer can be placed in an alkaline etching solution for ultrasonic treatment for 5 to 10 minutes during alkaline washing. The alkaline etching solution is a mixed solution composed of 49% sodium hydroxide by mass, deionized water and a surfactant in a volume ratio of 35 to 40:221:0.1 to 0.5.

[0042] In this embodiment, the ground silicon wafer is completely immersed in an alkaline etching solution to remove grease and other foreign particles on the surface. The alkaline etching solution is a mixed solution composed of sodium hydroxide, surfactant TSC-1 and deionized water. The volume ratio is NaOH (49%): TSC-1: H2O = 35-40: 221: 0.1-0.5. The silicon wafer is taken out after ultrasonic alkaline washing for 5-10 minutes.

[0043] It is easy to understand that after ultrasonic alkaline cleaning, the silicon wafer should be cleaned with overflowing deionized water to remove the alkaline solution remaining on the surface of the silicon wafer.

[0044] Further, in one embodiment, the pickling conditions of the first pickling in step 3 and the second pickling in step 5 are: the silicon wafer is completely immersed in the mixed acid solution for 2-3 minutes. The mixed acid solution is a mixed solution composed of 49% by mass hydrofluoric acid, 70% by mass nitric acid, 99% by mass acetic acid and deionized water in a volume ratio of 1:3-4:2-5:5-8.

[0045] In this embodiment, the overflowing deionized water is used to clean the silicon wafer, and after removing the alkali solution remaining on the surface of the silicon wafer, it is then placed in a mixed acid solution for a full reaction of 2-3 minutes. The mixed acid solution is a mixed solution composed of hydrofluoric acid, nitric acid, acetic acid and deionized water, and the volume ratio is HF (49%): HNO3 (70%): CH3COOH (99%): H2O = 1: 3-4: 2-5: 5-8. Among them, nitric acid has an oxidizing effect and will react with the silicon wafer, and hydrofluoric acid will remove the oxide. Through repeated corrosion reactions, the silicon wafer removes the damaged layer and metal impurities to achieve the purpose of polishing. Among them, the reaction equation of pickling is as follows: Si + 4HNO3 + 6HF = H2SiF6 + 4NO2 + 4H2O. After the reaction is completed, the overflowing deionized water is used to rinse off the residual liquid on the surface.

[0046] In one embodiment, when removing the thermal donors in the silicon wafer, step 4 can be implemented in the following manner:

[0047] Before oxidation, preheat the oxidation annealing furnace at a preheating temperature of 600-800°C for 20 minutes;

[0048] The silicon wafer after the first pickling is placed in a quartz boat, and the quartz boat is transferred to an oxidation annealing furnace for oxidation annealing for 30 minutes to 120 minutes;

[0049] After annealing, take out the quartz boat and use a fan to quickly cool the silicon wafer. Turn off the fan after cooling for 30 to 50 minutes.

[0050] In this embodiment, in order to remove the thermal donors in the silicon wafer and restore the true resistivity, the silicon wafer needs to be subjected to oxidation heat treatment and rapidly cooled through a temperature range of 450-550°C. Before oxidation, the oxidation annealing furnace is preheated to a temperature of 600-800°C. After preheating for 20 minutes, the temperature in the furnace is kept stable. The quartz boat with the silicon wafer is transferred to the annealing furnace for oxidation annealing for 30 minutes to 120 minutes. After the annealing is completed, the quartz boat is quickly taken out and the silicon wafer is rapidly cooled with a fan. The fan is turned off after cooling for 30 minutes to 50 minutes. Since the thermal donors are introduced during the low-temperature heat treatment of 450-550°C during the crystal pulling process, and through this embodiment, the silicon wafer is quickly passed through a temperature range of 450-550°C, the thermal donors can be eliminated and the true resistivity of the silicon wafer can be restored.

[0051] In the above embodiments, the quartz boat used is made of high temperature resistant polytetrafluoroethylene material.

[0052] Finally, the cooled silicon wafer is immersed in the mixed acid solution for a second pickling. The mixed acid solution is the same as that used above. The oxide layer on the surface of the silicon wafer is removed by mixed acid, and then the silicon wafer is cleaned and dried with overflowing deionized water. Before testing, the silicon wafer is placed in an environment of 20-25℃ for 120min-180min to stabilize the resistivity of the silicon wafer. The thickness of the silicon wafer is measured by a thickness meter, and then the resistivity value of the center of the silicon wafer is measured by a four-probe resistivity tester.

[0053] The following is an explanation with reference to specific comparative tests.

[0054] Option 1: Alkaline washing - strong acid washing - removal of heat donors - grinding;

[0055] Option 2: Alkaline washing - strong acid washing - heat removal - sandblasting;

[0056] Option 3: Alkaline washing - CP - removal of heat donors - grinding;

[0057] Option 4: grinding - alkali washing - CP - removal of heat donor - sandblasting;

[0058] Option 5: grinding - alkali washing - CP - removal of heat donors - CP;

[0059] Among them, CP in the above schemes is the pickling process provided by the embodiment of the present invention, and grinding, alkali washing and heat removal are the corresponding processes provided by the present invention; the strong pickling process is consistent with the CP process, but the pickling solution ratio is different. The pickling solution in the strong pickling process is a mixture of 49% HF, 70% HNO3, 99% CH3COOH and water by mass fraction, wherein the volume fraction of HF in the mixture is 10-20%, HNO3 is 30-40%, CH3COOH is 15-25%, and water is 25-35%. The sandblasting process is: using a sandblasting machine to spray white corundum powder on the surface of the silicon wafer under the pressure of compressed air, and removing the uneven places on the surface of the silicon wafer by this impact method. The sandblasting is carried out in a cross-cross manner, wherein the main component of the white corundum powder is aluminum oxide, about 200 mesh size.

[0060] Three silicon wafers were obtained through the above schemes, and the central resistivity of the three silicon wafers was measured on the same day, the seventh day, and the fourteenth day. The measured results are as follows:

[0061]

[0062] From the above experimental results, it can be seen that the test deviations of schemes 1, 2, and 4 at different time points are large, so they are not adopted; although the test value of scheme 3 is relatively stable at present, this scheme has a processing flow in which customers complain about low resistivity in actual application, and the resistivity will decrease significantly after one month, so it is not adopted; the test results of scheme 5 are stable and consistent with the laboratory test values. It can be seen that the embodiment of scheme 5 provided by the present invention is a better scheme.

[0063] In summary, the silicon wafer obtained by the grinding, alkali washing, acid washing, heat donor removal, and acid washing again provided in this solution can significantly improve the resistivity stability of the silicon wafer. Moreover, the heat donor removal treatment can make the resistivity detection value of the silicon wafer more accurate.

[0064] The modules or units in the device of the embodiment of the present invention can be combined, divided and deleted according to actual needs. The above disclosure is only the preferred embodiment of the present invention, and of course it cannot be used to limit the scope of the rights of the present invention. Those skilled in the art can understand that all or part of the processes of the above embodiment are implemented, and the equivalent changes made according to the claims of the present invention still fall within the scope of the invention.

Claims

1. A pretreatment method for improving the resistivity performance of a silicon wafer, characterized in that: The following steps are involved: Step 1: Use a grinder to grind the surface of the silicon wafer to make the surface of the silicon wafer flat; Step 2: Immerse the polished silicon wafer in a pre-prepared alkaline etching solution for alkaline washing; Step 3: Immerse the alkali-washed silicon wafer in a pre-prepared mixed acid solution for the first acid wash; Step 4: The pickled silicon wafer is subjected to oxidation heat treatment and rapid cooling to remove the heat donors in the silicon wafer; Step 5: The cooled silicon wafer is pickled for a second time with the mixed acid solution to obtain a silicon wafer for resistivity performance testing; Wherein, the pickling conditions of the first pickling in step 3 and the second pickling in step 5 are both: the silicon wafer is completely immersed in the mixed acid solution and reacts for 2-3 minutes; The mixed acid solution is a mixed solution composed of 49% by mass hydrofluoric acid, 70% by mass nitric acid, 99% by mass acetic acid and deionized water in a volume ratio of 1:3-4:2-5:5-8.

2. The pretreatment method for improving the resistivity performance of silicon wafer according to claim 1, characterized in that: In step 1, the silicon wafer is polished with a grinding liquid under high-speed rotation of a turntable; wherein the grinding liquid is a mixed solution composed of 1500 mesh grinding sand, alumina suspension and water, and the mixing ratio is 1500 mesh grinding sand: alumina suspension: water = 5-10kg: 2-5L: 10-15L.

3. The pretreatment method for improving the resistivity performance of silicon wafer according to claim 1, characterized in that: In the step 1, the silicon wafer is polished using a diamond grinding head in the grinding machine.

4. The pretreatment method for improving the resistivity performance of a silicon wafer according to claim 2 or 3, characterized in that: In the step 1, the grinding machine removes 100 to 300 μm of silicon wafer.

5. The pretreatment method for improving the resistivity performance of silicon wafer according to claim 1, characterized in that: The step 2 specifically includes: placing the polished silicon wafer into the alkaline etching solution and ultrasonicating for 5 to 10 minutes.

6. The pretreatment method for improving the resistivity performance of silicon wafers according to claim 5, characterized in that: The alkaline etching solution is a mixed solution composed of 49% by mass of sodium hydroxide, deionized water and a surfactant in a volume ratio of 35-40:221:0.1-0.

5.

7. The pretreatment method for improving the resistivity performance of silicon wafers according to claim 1, characterized in that: The step 4 specifically includes: Before oxidation, preheat the oxidation annealing furnace at a preheating temperature of 600-800°C for 20 minutes; Placing the silicon wafer after the first pickling in a quartz boat, and transferring the quartz boat to the oxidation annealing furnace for oxidation annealing for 30 minutes to 120 minutes; After annealing, take out the quartz boat and use a fan to quickly cool the silicon wafer. Turn off the fan after cooling for 30 to 50 minutes.

8. The pretreatment method for improving the resistivity performance of silicon wafers according to claim 1, characterized in that: After the silicon wafer is acid-washed or alkaline-washed, the silicon wafer is rinsed with overflowing deionized water.

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