Wafer surface treatment method for reducing operation loss of DFG grinding wheel

Abstract

The invention discloses a wafer surface treatment method for reducing operation loss of a DFG grinding wheel, which comprises the following steps of: performing mixed acid advanced treatment before operation, performing ultrasonic cleaning, performing spin-drying by using a spin-drying machine, removing a natural oxide film / Poly and other film layers on the surface of a 12-inch wafer in advance, exposing pure silicon, keeping dry and entering DFG for operation, and finding the action according to tracking data, and the grinding wheel operation loss can be effectively reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor wafer regeneration, and particularly relates to a wafer surface treatment method for reducing the operation loss of a DFG grinding wheel. Background Art

[0002] Wear principle of DFG grinding wheel:

[0003] Abrasive wear: When the abrasive grains and the workpiece move relative to each other to generate friction, with the passage of grinding time, the abrasive grains gradually undergo mechanical wear, and the grooves on the worn surface are parallel to the cutting direction. If the workpiece structure is uneven and contains hard particles with higher hardness, it will exacerbate the mechanical wear of the abrasive grains.

[0004] Diffusion wear: When the grinding wheel grinds the workpiece, the elements on the surface of the grinding wheel and the workpiece surface will undergo element diffusion phenomenon when they come into contact at high temperature, weakening the surface layer of the abrasive grains and resulting in wear.

[0005] Thermal stress fracture wear: During the grinding process of the workpiece, the abrasive grains reach a high temperature instantaneously and are rapidly cooled under the action of the grinding fluid. Multiple intermittent cooling and heating increase the thermal stress of the abrasive grains themselves, resulting in cracking and splitting of the abrasive grain surface.

[0006] Diversification of wafer materials:

[0007] With the progress of semiconductor technology, in addition to traditional silicon (Si) wafers, the applications of compound semiconductors such as silicon carbide (SiC) are gradually increasing. These new materials have higher hardness and better performance, but they are also more difficult to process and have higher requirements for grinding equipment, and grinding equipment that can adapt to the characteristics of different materials is needed.

[0008] In the wafer regeneration process, there are often mechanical damage layers and residual oxide films on the surface of 12-inch Monitor Wafers. The traditional process uses an 8000-mesh DFG grinding wheel for direct grinding, but there are the following defects:

[0009] 1. The residual oxide film causes a sudden change in the friction coefficient during grinding, resulting in an over-limit voltage threshold (shutdown rate > 15%);

[0010] 2. The loss of the grinding wheel is as high as 0.12 mm / wafer, and the bearing life is shortened to 800 hours;

[0011] 3. Multiple rework polishings are required, and the production efficiency is reduced by 30%.

[0012] The prior art attempts to optimize by adjusting the grinding wheel parameters or improving the cleaning process, but neither has solved the problem of the interaction between high-mesh grinding wheels and surface film layers. Summary of the Invention

[0013] The purpose of the present invention is to provide a wafer surface treatment method for reducing the operation loss of DFG grinding wheels, so as to solve the problems raised in the above-mentioned background technology.

[0014] To achieve the above object, the present invention provides the following technical solution: A wafer surface treatment method for reducing the operation loss of DFG grinding wheels, comprising the following steps:

[0015] (1) Mixed acid treatment: Immerse the wafer in a mixed acid solution composed of nitric acid, hydrochloric acid and acetic acid, where the mass percentage concentration of nitric acid is 70 ± 2%, the mass percentage concentration of hydrochloric acid is 30 ± 2%, the mass percentage concentration of acetic acid is 98 ± 1%, and the volume ratio of the three is 1:6:7 ± 0.2;

[0016] (2) Ultrasonic cleaning: Perform ultrasonic cleaning treatment at a frequency of 26 ± 5 KHz for 30 - 45 seconds;

[0017] (3) Gradient centrifugal drying: Centrifuge and dry in four-stage gradient acceleration:

[0018] Stage 1: Centrifuge at a speed of 100 ± 10 rpm for 10 ± 1 second;

[0019] Stage 2: Centrifuge at a speed of 200 ± 15 rpm for 10 ± 1 second;

[0020] Stage 3: Centrifuge at a speed of 350 ± 20 rpm for 20 ± 2 seconds;

[0021] Stage 4: Centrifuge at a speed of 500 ± 25 rpm for 200 ± 5 seconds;

[0022] (4) DFG grinding wheel grinding: Grind using an 8000-mesh grinding wheel, and the parameter settings are: Spark Out: 5 ± 0.2 rev,

[0023] The feed rates of PX Cut and Escape Cut are both 0.3 ± 0.05 μm / s, and the rotational speeds are both 185 ± 5 rpm.

[0024] Preferably, in the mixed acid treatment step, the concentration ratio of nitric acid to hydrochloric acid is controlled to be (70 ± 2%):(30 ± 2%), and the addition amount of acetic acid reduces the corrosion rate of the mixed acid system by 30 - 35% compared with the case without acetic acid.

[0025] Preferably, the frequency range of the ultrasonic cleaning is determined through stress damage testing. When the frequency exceeds 31 KHz, visible microcracks appear on the wafer surface, and when it is lower than 21 KHz, the residual amount of the oxide film increases by more than 40%.

[0026] Preferably, the acceleration change rate of the gradient centrifugal drying is controlled within 15 - 20 rpm / s 2 , and the rotational speed error of each stage does not exceed 5% of the nominal value.

[0027] Preferably, in the DFG grinding wheel parameter settings, the relationship between the Spark Out parameter and the grinding wheel wear amount satisfies the following empirical formula:

[0028] Reduction rate of wear amount (%) = [1.8×(5 - actual spark out value) + 25] ± 2%,

[0029] When the spark out value deviates from 5 rev, the grinding wheel wear amount increases by 18 - 22% for every 1 rev increase.

[0030] Preferably, the feed speed combination of the PX Cut and Escape Cut enables the surface roughness Ra value to be controlled within the range of 0.02 - 0.05 μm, and the single - time grinding removal amount is reduced to 60 - 70% of the conventional process.

[0031] Preferably, in the mixed - acid treatment step, the solution temperature is maintained at 25 ± 1°C, and the treatment time is dynamically adjusted according to the oxide film thickness. The adjustment formula is:

[0032] Treatment time (min) = 0.8×film thickness (nm) / 100 ± 0.5 min.

[0033] Preferably, the total duration of the gradient centrifugal drying step is shortened by 15 - 20% compared with the conventional single - stage drying, and the surface residual water film thickness ≤ 2 nm.

[0034] Preferably, after the wafer surface is treated by the method, the oxygen content on the surface is reduced to less than 10% of the untreated sample, and the surface micro - crack density ≤ 3 pieces / cm 2 .

[0035] Compared with the prior art, the beneficial effects of the present invention are:

[0036] 1. Reduction of grinding wheel wear:

[0037] The mixed - acid pretreatment reduces the surface oxygen content from 12 at% to 1.2 at% (XPS test), and the stability of the friction coefficient is improved by 70%;

[0038] The grinding wheel wear amount is reduced from 0.12 mm / slice to 0.07 mm / slice (a decrease of 41.7%).

[0039] 2. Improvement of equipment reliability:

[0040] The downtime rate is reduced from 15% to 3%, and the bearing life is extended from 800 hours to 2400 hours;

[0041] By controlling Spark Out = 5 rev, the grinding pressure fluctuation range is narrowed from ±8 N to ±2 N.

[0042] 3. Surface quality optimization:

[0043] The surface roughness Ra is improved from 0.15 μm to 0.03 μm (AFM test);

[0044] The microcrack density ≤ 3 pieces / cm 2 (SEM observation), meeting the 12-inch wafer regeneration standard.

[0045] 4. Process efficiency improvement:

[0046] The single grinding time is shortened by 20%, and the rework rate is reduced from 25% to 5%;

[0047] The gradient spin-drying time is reduced by 18% compared with the traditional process (250 s → 205 s). Description of the drawings

[0048] Figure 1 is the process flow chart of the present invention;

[0049] Figure 2 is the relationship diagram between the gradient spin-drying parameters and the residual water film thickness;

[0050] Figure 3 is the comparison diagram of the corrosion rate with different mixed acid ratios;

[0051] Figure 4 : The relationship diagram between the gradient spin-drying parameters and the residual water film thickness;

[0052] Figure 5 : The comparison diagram of the loss amount of the grinding wheel before and after the optimization of the grinding wheel parameters. Detailed implementation manners

[0053] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0054] Please refer to Figures 1-5 , the present invention provides a technical solution: a wafer surface treatment method for reducing the operation loss amount of a DFG grinding wheel, including the following steps:

[0055] (1) Mixed acid treatment: Immerse the wafer in a mixed acid solution composed of nitric acid, hydrochloric acid and acetic acid, where the mass percentage concentration of nitric acid is 70 ± 2%, the mass percentage concentration of hydrochloric acid is 30 ± 2%, the mass percentage concentration of acetic acid is 98 ± 1%, and the volume ratio of the three is 1:6:7 ± 0.2;

[0056] (2) Ultrasonic cleaning: Perform ultrasonic cleaning treatment at a frequency of 26 ± 5 KHz for 30 - 45 seconds;

[0057] (3) Gradient centrifugal drying: Dry by gradient acceleration in four stages:

[0058] Stage 1: Centrifuge at a speed of 100 ± 10 rpm for 10 ± 1 second;

[0059] Stage 2: Centrifuge at a speed of 200 ± 15 rpm for 10 ± 1 second;

[0060] Stage 3: Centrifuge at a speed of 350 ± 20 rpm for 20 ± 2 seconds;

[0061] Stage 4: Centrifuge at a speed of 500 ± 25 rpm for 200 ± 5 seconds;

[0062] (4) DFG grinding wheel grinding: Grind with an 8000 - mesh grinding wheel, and the parameter settings are: Spark Out: 5 ± 0.2 rev,

[0063] The feed rates of PX Cut and Escape Cut are both 0.3 ± 0.05 μm / s, and the rotational speeds are both 185 ± 5 rpm.

[0064] In the mixed - acid treatment step, the concentration ratio of nitric acid to hydrochloric acid is controlled at (70 ± 2%):(30 ± 2%), and the addition amount of acetic acid reduces the corrosion rate of the mixed - acid system by 30 - 35% compared with that without acetic acid.

[0065] The frequency range of ultrasonic cleaning is determined by stress - damage testing. When the frequency exceeds 31 KHz, visible micro - cracks appear on the wafer surface, and when it is lower than 21 KHz, the residual amount of the oxide film increases by more than 40%.

[0066] The acceleration change rate of gradient centrifugal drying is controlled within 15 - 20 rpm / s 2 , and the rotational - speed error of each stage does not exceed 5% of the nominal value.

[0067] In the DFG grinding - wheel parameter settings, the relationship between the Spark Out parameter and the grinding - wheel loss amount satisfies the following empirical formula:

[0068] Loss - amount reduction rate (%) = [1.8×(5 - actual spark out value)+25] ± 2%.

[0069] The combined feed rates of the PX Cut and Escape Cut control the surface roughness Ra value within the range of 0.02 - 0.05 μm, and the single - time grinding removal amount is reduced to 60 - 70% of the conventional process.

[0070] In the mixed acid treatment step, the solution temperature is maintained at 25 ± 1°C, and the treatment time is dynamically adjusted according to the thickness of the oxide film. The adjustment formula is as follows:

[0071] Treatment time (min) = 0.8 × film thickness (nm) / 100 ± 0.5 min.

[0072] The total duration of the gradient centrifugal drying step is 15 - 20% shorter than that of the conventional single-stage drying, and the thickness of the surface residual water film ≤ 2 nm.

[0073] After the method treatment, the oxygen content on the surface of the wafer is reduced to less than 10% of the untreated sample, and the surface microcrack density ≤ 3 pieces / cm 2 .

[0074] The following is a comparison table of the loss amounts before and after the optimization of the grinding wheel parameters:

[0075]

[0076] The loss of the optimized grinding wheel is reduced by 41.7%.

[0077] Example 1 (optimal parameter combination)

[0078] Immerse a 12-inch recycled wafer into a mixed acid bath (HNO3:HCl:CH3COOH = 1:6:7, 25°C) and treat for 8 minutes;

[0079] Transfer to a 26 KHz ultrasonic bath and clean for 35 seconds;

[0080] Four-stage drying (100 rpm / 10 s → 200 rpm / 10 s → 350 rpm / 20 s → 500 rpm / 200 s);

[0081] DFG grinding (8000-mesh grinding wheel, Spark Out = 5 rev, PX / Escape Cut = 0.3 μm / s).

[0082] Result: The loss amount of the grinding wheel is 0.068 mm / slice, the surface Ra = 0.028 μm, and no downtime occurs.

[0083] Example 2 (comparison of mixed acid ratio)

[0084] Immerse a 12-inch recycled wafer into a mixed acid bath (HNO3:HCl:CH3COOH = 1:7:6, 25°C) and treat for 8 minutes. The subsequent steps are the same as those in Example 1.

[0085] The corrosion rate is increased by 40%, but a flocculent matte surface appears (Ra = 0.12 μm), and the loss of the grinding wheel rebounds to 0.11 mm / slice.

[0086] Example 3 (optimization of ultrasonic frequency)

[0087] The frequency is increased to 35 KHz (the others are the same as in Example 1). After cleaning, the surface oxygen content increases to 2.8 at%, and the microcrack density reaches 8 cracks / cm 2 .

[0088] Example 4 (Adjustment of spin-drying parameters)

[0089] Single-stage spin-drying at 500 rpm is adopted (the others are the same as in Example 1). The thickness of the residual water film reaches 5.2 nm. Water vapor interference occurs during grinding, and the grinding wheel loss increases to 0.09 mm per wafer.

[0090] Example 5 (Comparison of grinding wheel parameters)

[0091] Set Spark Out = 8 rev (the others are the same as in Example 1). The grinding wheel loss increases to 0.13 mm per wafer, and the bearing temperature rises by 12 °C.

[0092] Example 6 (Optimization of comprehensive parameters)

[0093] Based on Example 1, the mixed acid temperature is increased to 30 °C, the corrosion time is shortened to 6 minutes, the surface oxygen content is 1.5 at%, and the grinding wheel loss remains 0.07 mm per wafer.

[0094] The present invention has been applied to a 12-inch wafer re-production line. A total of 100,000 wafers have been processed. The grinding wheel replacement cycle is extended from 7 days to 12 days, and the annual maintenance cost is reduced by 2.8 million yuan

[0095] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made in these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A wafer surface treatment method for reducing the operation loss of a DFG grinding wheel, characterized in that It includes the following steps: (1) Mixed acid treatment: Immerse the wafer in a mixed acid solution composed of nitric acid, hydrochloric acid, and acetic acid, where the mass percentage concentration of nitric acid is 70 ± 2%, the mass percentage concentration of hydrochloric acid is 30 ± 2%, the mass percentage concentration of acetic acid is 98 ± 1%, and the volume ratio of the three is 1:6:7 ± 0.2; (2) Ultrasonic cleaning: Perform ultrasonic cleaning treatment at a frequency of 26 ± 5 KHz for 30 - 45 seconds; (3) Gradient centrifugal drying: Dry by four-stage gradient acceleration: Stage 1: Centrifuge at a speed of 100 ± 10 rpm for 10 ± 1 second; Stage 2: Centrifuge at a speed of 200 ± 15 rpm for 10 ± 1 second; Stage 3: Centrifuge at a speed of 350 ± 20 rpm for 20 ± 2 seconds; Stage 4: Centrifuge at a speed of 500 ± 25 rpm for 200 ± 5 seconds; (4) DFG grinding wheel grinding: Grind with an 8000-mesh grinding wheel, and the parameter settings are: Spark Out: 5 ± 0.2 rev, The feed rates of PX Cut and Escape Cut are both 0.3 ± 0.05 μm / s, and the rotation speeds are both 185 ± 5 rpm.

2. A wafer surface treatment method for reducing the operating loss of a DFG grinding wheel according to claim 1, characterized in that: In the mixed acid treatment step, the concentration ratio of nitric acid to hydrochloric acid is controlled to be (70 ± 2%):(30 ± 2%), and the addition amount of acetic acid reduces the corrosion rate of the mixed acid system by 30 - 35% compared to when there is no acetic acid.

3. A wafer surface treatment method for reducing the operation loss of a DFG grinding wheel according to claim 1, characterized in that: The frequency range of the ultrasonic cleaning is determined by stress damage testing. When the frequency exceeds 31 KHz, visible microcracks appear on the wafer surface, and when it is lower than 21 KHz, the residual amount of the oxide film increases by more than 40%.

4. A wafer surface treatment method for reducing the operation loss of a DFG grinding wheel according to claim 1, characterized in that: The acceleration change rate of the gradient centrifugal drying is controlled within 15 - 20 rpm / s2, and the speed error of each stage does not exceed 5% of the nominal value.

5. A wafer surface treatment method for reducing the operation loss of a DFG grinding wheel according to claim 1, characterized in that: In the DFG grinding wheel parameter settings, the relationship between the Spark Out parameter and the grinding wheel loss amount satisfies the following empirical formula: Loss reduction rate (%) = [1.8×(5 - actual spark out value) + 25] ± 2%, When the spark out value deviates from 5 rev, the grinding wheel loss amount increases by 18 - 22% for each 1 rev increase.

6. A wafer surface treatment method for reducing the operation loss of a DFG grinding wheel according to claim 1, characterized in that: The combination of the feed rates of PX Cut and Escape Cut controls the surface roughness Ra value within the range of 0.02 - 0.05 μm, and the single-pass grinding removal amount is reduced to 60 - 70% of the conventional process.

7. A wafer surface treatment method for reducing the operation loss of a DFG grinding wheel according to claim 1, characterized in that: In the mixed acid treatment step, the solution temperature is maintained at 25 ± 1°C, and the treatment time is dynamically adjusted according to the oxide film thickness. The adjustment formula is: Treatment time (min) = 0.8×film thickness (nm) / 100 ± 0.5 min.

8. A wafer surface treatment method for reducing the operation loss of a DFG grinding wheel according to claim 1, characterized in that: The total duration of the gradient centrifugal drying step is shortened by 15 - 20% compared to the conventional single-stage drying, and the thickness of the surface residual water film ≤ 2 nm.

9. A wafer surface treatment method for reducing the operation loss of a DFG grinding wheel according to claim 1, characterized in that: After the wafer is treated by the method, the oxygen content on the wafer surface is reduced to less than 10% of the untreated sample, and the surface microcrack density ≤ 3 pieces / cm2.

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