RCA and IMEC/SC2 Clean: Metallic Immunity and Gate Oxide Integrity
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Surface and oxide metal contamination was measured by VPD-DSE-TXRF. Bulk Fe contamination was measured by SPV. Particles, larger than 0.2 Vtm, were also measured. The experimental splits are shown in Figure 1. New prime test wafers(p-type, (100) CZ) are added to each of the steps, i.e. pre-gate clean, gate oxidation and polysilicon deposition in both process A and B. This wafers are used to check particle counts, Fe contamination, as well as oxide and poly film thickness.
Figure 1. Experimental splits of RCA and IMEC/SC2 clean comparison study. Results and Discussion Test Wafer results Test wafer results provide direct and quick comparison of the results of each individual process step on different recipes for both process A and process B. Correlation of test wafer results and GOI data provides the baseline for in-line process control and recipe development. Therefore it is very crucial to understand the test wafer results. (1) Chemical oxide and gate oxide thickness 226
It is well known that after RCA clean, the silicon wafer will have about 10A chemical oxide on the surface. Table 1 indicates the chemical oxide and gate oxide thicknesses for the various cleans. For wafers cleaned using the IMEC/SC2 clean, residual chemical oxide of -7A was obtained. (2) Particle adders Both cleans in process A and B have less than 20 particle adders(>0.2 jim), which is the baseline. The test for the particle adders was performed along with product wafers. Although some of the clean processes ended up with large amount of particles, no direct correlation was found to the GOI data. Table 1: Chemical oxide and gate oxide thickness measured by ellipsometer.
Process Process Process Process
A, RCA A, IMECISC2 B, RCA B, IMECISC2
Chemical Oxide Tox(A) STD(A) 11.45 0.13 7.74 0.36 12.45 0.30 8.90 0.44
1
Tox( 80.7 79.9 85 84.7
Gate Oxide STD(A) 0.816 0.77 1.33 1.24
)
Table 2: Larger than 0.2 jim particle added in clean processes. Particle adders Pre Post Process A, RCA 59 179
Delta 120
Process A, IMEClSC2
3
2210
2207
Process B, RCA Process B, IMEC/SC2
40
996 1384
956 1383
1
(3) Metallic contamination
As mentioned in the section 2, the metallic contamination from wet benches and furnaces was measured using VPD-DSE-TXRF and SPV. VPD-DSE-TXRF is a surface sensitive technique, which measures almost all metallic elements on the surface and in the oxide. It has an unit of atoms per cm 2. SPV utilizes surface voltage technique to measure the electron diffusion length in p-type material and calculates the Fe contamination level in the silicon bulk(in atoms per cm'). To detect the surface Fe contamination, SPV requires a thermal process to drive the surface Fe into the bulk and a reference control wafer to detect contamination from furnace. The control wafer is a new prime wafer which was exposed only in the gate oxide step. SPV results on the control wafers provide the Fe contamination levels from the starting wafers and the furnaces. The TXRF control wafers are also new prime test wafers. Table 3 shows Fe and Ca contaminatio
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