2.4 S UMMARY
3.3.5 Trace Doping of MOS Capacitors
In Fig. 3-6, the GF filter was suggested to release more boron contaminants in an environment of HF vapor. Boron contaminants adsorbed onto the surface of the wafer will over-dope or count-dope the silicon surface and change its electrical parameters (such as the threshold voltage). These changes will become severe when the device is shrunk to nanometer dimensions. In the experiment, two filters were compared to investigate the effect of boron contaminants on device performance. The experimental procedure was similar to that described above, except in that the exposure time was 72 hrs and the devices were annealed at 900 ℃ for 30 min. after exposure. The silicon wafers were epitaxial wafers whose doping
were approximately 2×1015 /cm3. Figure 3-15 indicates that the surface concentrations of devices exposed in the CB with a GF filter varied over a wide range, whose maximum was about twice the minimum. In contrast, the devices exposed in the CB with a PTFE filter showed a uniform distribution of surface concentrations, which were almost equal to that of the unexposed samples. The results imply that the HF vapor corroded the glass fiber, releasing the boron contaminants, causing over-doping of the silicon surface. The results also suggested that HF vapor only slightly corroded the PTFE filter.
3.5SUMMARY
The qualitative and quantitative analyses of AMC in the CB with different filter module in HF vapor environment were investigated. The experimental results showed that the GF ULPA filter will release boron and organic contaminants in HF vapor environment, while the PTFE ULPA filter still maintained low concentration of contaminants. The effects of the materials from which air filters are made on device characteristics were also investigated. The glass-fiber ULPA filter released AMC when exposed in an HF vapor environment. These contaminants included organic compounds and boron trace dopants and degraded the device characteristics. In contrast, the HF vapor did not affect the PTFE ULPA filter. These results suggested that the PTFE fiber can be a good ULPA filter material for providing a very clean cleanroom environment.
TABLE 3-1
Summary of Air Sampling Experimental Conditions
Contamination Type
Sampling Equipments
Flow Rate Sampling Time
Filter module: 1. PTFE ULPA filter + Chemical filter 2. Glass-Fiber ULPA filter + Chemical filter
Summary of Wafer Sampling Experimental Conditions
Contamination
Analysis Method Elements Organic
Filter module: 1. PTFE ULPA filter + Chemical filter
TABLE 3-3
Air Sampling Results of Metals and Boron in HF Vapor Environment
Metals & Boron (µg/m3) CR Detection Limit CB_PTFE Detection Limit CB_GF Detection Limit
Inlet Filter - Chemical Filter Chemical Filter
Main Filter - PTFE Filter Glass-Fiber Filter
Na 0.25 0.002 <0.004 0.004 <0.002 0.002
Air Sampling Results of Organic Compounds in HF Vapor Environment
Organic Compounds (µg/m3) CR Detection Limit CB_PTFE Detection Limit CB_GF Detection Limit
Inlet Filter - Chemical Filter Chemical Filter
Main Filter - PTFE Filter Glass-Fiber Filter
D3:C6H18O3Si3 2.1 0.03 <0.03 0.03 *1 0.03
*1: Can not separate organic compounds peaks
TABLE 3-5
Wafer Sampling Results of Metals in HF Vapor Environment
Metals (1010 atoms/cm2) BLANK CB_PTFE CB_GF CR
Inlet Filter - Chemical Filter Chemical Filter -
Main Filter - PTFE Filter Glass-Fiber Filter -
Na - - - -
Mg - - - -
K 56.7 57.5065 - 317.7524
Ca - - - 468.67
Ti - - 4.319 43.2312
V - 4.719 2.96 4.926
Cr 4.172 4.456667 3.062 9.6664
Mn 4.952 4.695 2.55 9.1372
Fe - - 1.441 262.6284
Co - 1.943333 1.175 3.628667
Ni - - - -
Cu 3.014 2.276 - 4.5346
Zn - - 1.94 30.5904
Fig. 3-1. Schematic diagram of specially designed clean bench with the HF vapor.
Fig. 3-2. Air sampling equipments used for the evaluation of metals and organic contaminations.
LOCOS Isolation
Sacrificial Oxidation
Remove SC_oxide
RCA Clean
Gate Oxidation
Aluminum
Deposition and Patterning SC_Oxide
Exposure
Expose in CB
@ 1ppm HF Vapor
Without Exposure
Fig. 3-3. Process flow of MOS capacitor exposure experiment used to evaluate the electrical characteristics.
LOCOS Isolation
Sacrificial Oxidation
Remove SC_oxide
RCA Clean
Gate Oxidation
Aluminum
Deposition and Patterning SC_Oxide
Exposure
Expose in CB
@ 1ppm HF Vapor
Without Exposure
Annealing
Fig. 3-4. Process flow of MOS capacitor exposure experiment used to evaluate the surface doping concentration.
NEUROFINE
PTFE Fiber Glass Fiber
Pre-corroded
Post-corroded
Pre-corroded
Post-corroded
(a) (b)
Fig. 3-5. The SEM photographs of (a) PTFE-fiber and (b) Glass-fiber structures.
CR CB_PTFE CB_GF
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Con centrati on (
µg/ m
3) Na K
Ca Mg Al Fe B
Fe B
Al CaMg NaK FeB
Mg Al KCa Na B Fe MgAl Ca K Na
Fig. 3-6. Air sampling results of metals and boron in HF vapor environment.
CR CB_PTFE CB_GF
0.0 0.5 1.0 1.5 2.0 2.5
Con ce n tr ati on (
µg/m
3)
D3 D4 D5 D6 DEP DBP TEP TBP BHT
Fig. 3-7. Air sampling results of organic compounds in HF vapor environment.
Na Mg K Ca Ti V Cr Mn Fe Co Ni Cu Zn 100
101 102 103
Metal Den sity (1 0
10Ato m s/c m
2)
UnexposedCB_PTFE CB_GF CR
Fig. 3-8. Wafer sampling results of metals in HF vapor environment.
00:00:00 00:28:48 00:57:36 01:26:24 01:55:12
Fig. 3-9. TDS-APIMS analysis for unexposed sample: (a) Time depend ion
00:00:00 00:28:48 00:57:36 01:26:24 01:55:12
Fig. 3-10. TDS-APIMS analysis for CB_PTFE filter sample: (a) Time depend ion intensity plot, (b) Mass spectrum at temperature = 400℃.
00:00:00 00:28:48 00:57:36 01:26:24 01:55:12
Fig. 3-11. TDS-APIMS analysis for CB_GF filter sample: (a) Time depend ion
00:00:00 00:28:48 00:57:36 01:26:24 01:55:12
Fig. 3-12. TDS-APIMS analysis for CR sample: (a) Time depend ion intensity plot, (b) Mass spectrum at temperature = 400℃.
0 20 40 60 80 100
10-11 10-10 10-9 10-8 10-7
Unexposed CB_PTFE CB_GF tox = 5 nm
Area = 200 x 200 µm2
@ 4 MV/cm
Leakage Current Density (A/cm2)
Cumulative Probability (%)
Fig. 3-13. The leakage current density distribution of MOS capacitors under different exposure environments.
-4 -3 -2 -1 0 1 2 3
10-3 10-2 10-1 100 101
Charge-to-Breakdown, Qbd (C/cm2) Unexposed
CB_PTFE CB_GF tox = 5 nm
Area = 200 x 200 µm2
ln(-ln(1-F))
Fig. 3-14. Weibull plots of charge-to-breakdown (Qbd) characteristics of MOS capacitors under different exposure environments.
0 5 10 15 20 1x1015
2x1015 3x1015 4x1015 5x1015
Unexposed CB_PTFE CB_GF tox = 5 nm
Area = 200 x 200 µm2
Surface Concentration (1/cm3 )
Numbers
Fig. 3-15. Silicon surface doping concentration of MOS capacitors under different exposure environments.