碳化硅基板抛光整体解决方案

Present silicon carbide substrate polishing process including lapping, bulk polishing, and fine polishing.  We are now providing the total solutions for bulk polishing, fine polishing, and cleaning. For bulk polishing slurries, it uses potassium permanganate as oxidant and alumina as abrasive which can provide Ra = 0.7 ~  0.9 Å with high throughput. For fine polishing slurries, it uses colloidal silica as abrasive and hydrogen  peroxide as oxidant, and it can further improve the Ra to 0.5 ~ 0.7Å. To remove the potassium  permanganate and its water insoluble byproduct - manganese dioxide; we provide an acidic solution – GC13020 (pH = 1 ~ 2), it can quickly remove these manganese compounds from tool, pad and wafers.  GC13020 can also remove the alumina particles which adsorbed on the surface of silicon carbide wafer,  and its cleaning efficiency is better than 5% hydrogen peroxide solution.

Introduction 

Silicon carbide is a compound composed of  silicon and carbon. Its dielectric breakdown field  strength is 10 times that of Si, and the energy gap  width is 3 times that of Si. Based on these  advantages, SiC are widely investigated as a  power device material that exceeds the limit of Si. The use of SiC can realize a high voltage diode  with a voltage of more than 600V through the SBD  (Schottky Diode) structure, which is a device with  high speed characteristics. Therefore, replacing  FRD (Fast Recovery Diode) with SiC SBD can  significantly reduce reverse recovery losses. The  drift layer resistance of SiC devices is lower than  that of Si devices, so high withstand voltage and  low resistance can be achieved at the same time  through MOSFETs with high-speed device. SiC  power device equipped with SiC-MOSFETs and  SiC-SBDs can significantly reduce switching  losses caused by tail currents of IGBTs and  reverse recovery currents of FRDs.

As showed in figure 1, general SiC polishing  process including ingot slicing; lapping; bulk polishing and fine polishing. For lapping process, it  needs rapidly reduce the thickness of SiC  substrate to about 350 μm ~ 600 μm. Thus,  diamond as abrasive based slurry is widely used in  this process. Depended on different lapping  process, it will cause different depth of scratches,  and its range is about 1 ~ 2 μm. To remove the  scratches caused by lapping process in bulk  polishing step, it needs a slurry with high SiC  removal rate and good roughness. Thus, a slurry which uses alumina as abrasive and potassium  permanganate as oxidant is widely applied in bulk  polishing step. Although the bulk polishing slurry  can remove the scratches caused by lapping  process, but it still causes some tiny scratches  during bulk polishing step. Thus, fine polishing slurry is used to remove these tiny scratches. In  general, fine polishing slurry uses colloidal silica as  abrasive and hydrogen peroxide as oxidant, but  the hardness of colloidal silica is smaller than  alumina and the oxidative ability of hydrogen is  weaker than potassium permanganate. Thus, fine  polishing removal rate is much lower than bulk  polishing removal rate.

Figure 1

SiC removal mechanism for both bulk polishing  slurry and fine polishing slurry are showing in figure  2. Metal ion is inserted into silicon carbon bond  and then weakens the silicon carbon bond. Finally,  -OH group replaces the Si – M – C bond to form  Si-OH bond and C-OH bond. According to the oxidant type, C-OH may be further oxidized to  COOH. 

Figure 2

The solutions for bulk polishing 

In general, bulk polishing process can be divided  into two groups. One is single substrate polishing,  and substrate size is over than 8 inches. The  requirement for this group is high SiC removal rate being at least over than 8.0 um/h; the other is  multiple substrate polishing, and its removal rate is  about 1.0 um ~ 3.0 um/h which depends on their  polishing recipe.

We are now providing SN12500 series bulk  polishing slurry which can cover the requirements  of two groups. As showed in table 1, SN12500 can  provide 1.8 um/h SiC removal rate under lower  mechanical force, and its roughness is about 0.7 ~  0.9 Å with few tiny scratches. And SN12503 can  provide higher throughput than SN12500 with  similar roughness. It is also possible to further  boost SiC removal rate over than 8.0 um/h under higher mechanical force.

Table 1

During the bulk polishing process, slurry is always  used as “recycle use”. Thus, its pot life is very important because of cost concern. According to  experiments, the SiC removal rate of SN12500  was dropped to 1.2 um/h after 5 hours passed.  There are two root causes for SiC removal rate  dropping during the polishing process. The first is  the consumption of KMnO4. According to the  analysis, KMnO4 dropped to 33% of original  dosage after 5 hours passed, and this consumption  didn’t only include the oxidation of SiC but also the  oxidation of byproduct, and its mechanism is shown in equation 1 & 2 (equation is not  balanced): SiC + KMnO4 → Mn2+ + S-OH + C-OH (1) KMnO4 + Mn2+ → MnO2 (2)

The second root cause is the coating of MnO2 on  the alumina surface. Due to water insoluble of  MnO2, it tends to coat on alumina surface during  the polishing process which leads to SiC removal  rate dropping. To demonstrate these assumptions,  we added fresh KMnO4 into used SN12500 to its original dosage but SiC removal rate didn’t turn  back to original level. We also prepared the MnO2 coated alumina, and we found that the more MnO2 coated, the more removal rate drop.

Semi-insulating SiC substrate is wildly  investigated in recent years, and SN12500 series can also provide good throughput. Its data are  showing in table 2.

Table 2

SiC substrate containing two face: silicon atom  rich face (Si-face) and carbon atom rich face  (C-face). C- face is more easily removed than Si-face and its data are showing in table 3. C-face  removal rate is much higher than Si-face.

To use KMnO4 based slurry, polyurethane type  hard pad is recommended to prevent the corrosion  of KMnO4. On the contrary, poromeric fluff type soft  pad is easily corroded by KMnO4. Besides, the  glue used in back side or sub pad also needs anti-corrosive ability against KMnO4; otherwise the  pad will be peeled off during the polishing process  which leads to SiC substrate broken.

Table 3

The solution for pad/tool/substrate cleaning 

Because KMnO4 and its byproduct (MnO2) is  hard to be removed by water or hydrogen peroxide  once it dried on tool, tank, or pad. Besides, waste  KMnO4 pollution is another key issue in  environmental concern. Thus, we have developed  a cleaner - GC13020 which can rapidly convert  KMnO4 and MnO2 to water soluble Mn2+ ion (as  showed in figure 3), even it was already dried on  the tool, tank, or pad.

Figure 3

GC13020 can also remove alumina particle  residual after SN12500 polishing.

Figure 4

As showed in figure 4, we use 5% hydrogen  peroxide solution as contrast. Immerse SiC  substrate into SN12500 for 10 minutes, and then  clean the SiC substrate by diluted GC13020 or 5%  hydrogen peroxide solution under ultrasonic  vibration condition, and finally dry with nitrogen gas.  AFM image are showing in figure 5:

Figure 5

In contrast to GC13020 cleaning, 5% hydrogen  peroxide solution cleaning still have some alumina  residual and roughness is much higher than  GC13020 cleaning.

The solutions for fine polishing 

Fine polishing process is always adopted to  further improve scratch defect and roughness. To  achieves these targets, colloidal silica has been  used as abrasive, and oxidant is depended on  catalyst type, in general, hydrogen peroxide has  been adopted as oxidant.

We have two solutions for fine polishing process depended on “single pass use” or “recycle use”.  SN12002 was designed for single pass use, and it  uses hydrogen peroxide as oxidant.  

Figure 6

SN12002 polishing data are showing in figure 6: its SiC removal rate is about 100 nm/h ~ 150 nm/h  depended on hydrogen peroxide dosage (40  mL/Kg means add 40 mL hydrogen peroxide per  kilogram of SN12002 and H2O2 is 31% w/w solution). Flow rate is 30 mL/min for 32” platen size.  Larger platen size needs higher flow rate to make  sure the slurry that can spread all over the platen. It  needs to note that slurry isn’t stirred during the  polishing; otherwise hydrogen peroxide will  decompose very fast. According to experimental  data, SiC removal rate will drop to 30 nm/h after five hours if keep stirring the slurry. Roughness  can be improved to 0.5 Å ~ 0.7 Å after SN12002  polishing and tiny scratches caused by SN12500  can also be removed.

SN12001 also uses hydrogen peroxide as oxidant,  and it was designed for recycle use, which means  circulate the slurry between polisher and slurry  tank during the polishing process with high flow  rate (such as 3 L/min). It needs to proceed filtration  between slurry out and polisher to remove the  debris caused by polishing.

Figure 7

We have tested the recycle runs of SN12001,  polishing time is 3 hours for each run, and its average removal rate for each run are showing in  figure 7. SN12001 can provide stable removal rate  within 4 runs polishing (12 hours totally). Because  recycle use adopts high flow rate, it will cause  stirring effect while SN12001 back to the slurry  tank. The effect leads to the increasing  decomposition rate of hydrogen peroxide. Thus, it  needs to add hydrogen peroxide every 3 hours.  SN12001 also provides good roughness (0.5 ~ 0.7  Å).

Conclusion 

SN12500 series provide high throughput with  good roughness. SN12001 & SN12002 can further  improve roughness to 0.5 Å ~ 0.7 Å and fix the post  polishing scratches. GC13020 can remove KMnO4 and MnO2 rapidly even these compounds already  dried on the tool, tank, or pad. GC13020 can be  used as post cleaner to remove alumina residual and its cleaning efficiency is better than 5% H2O2 solution. Our total solutions can satisfy the  requirements of silicon carbide substrate CMP  process.