Page 95 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

76 STANDARD MICROELECTRONIC TECHNOLOGIES

Table 4.4 Some wet etchants used in processing wafers for semiconductor devices
Material to etch Composition of etchant Etch rate Temperature
(nm/min) (°C)
Thermal SiO2 Buffered oxide etch 4: 1 to 80-120 20-30
7:1 NH4F/HF(49%)
Deposited SiO2 3:3:2 NF 4F/acetic 180-220 20-30
acid/water
Polycrystalline silicon 1:50:20 HF/HNO 3/water or 350-500 20-30
KOH
Aluminum 50:10:2:3 Phosphoric 200-600 20-40
acid/acetic acid/nitric
acid/water
Silicon nitride Phosphoric acid 5-7.5 160-175

Table 4.5 Source materials for doping silicon substrates

Element \/D at Solid Compound State Use
1100°C solubility at name
(um 1150°C

n-type:
Antimony 0.110 7xl0 1 9 a Antimony trioxide Solid Subcollector
Arsenic 0.090 1.8 x 10 21 Arsenic trioxide Solid Closed tube or source
furnace;
subcollector
Arsine Gas Subcollector/emitter
Phosphorus 0.329 1.4X10 21 Phosphoric Solid Emitters
pentoxide
Phosphoric Liquid Emitters
oxychloride
Phosphine Gas Emitters
Phosphoric Liquid Emitters
oxychloride
Silicon Solid Wafer source
pyrophosphate
p-type: Silicon Solid Wafer source
pyrophosphate
Boron 0.329 5 x lO 20a Boron trioxide Solid Base/isolation
Boron tribromide Liquid Base/isolation
Diborane Gas Base/isolation
Boron nitride Solid Wafer source
a
At 1250°C
t, C(x, t), is determined by the following equation:

C(x, t) = C s erfc (4.8)

where D is the diffusion coefficient.
Table 4.5 shows the different sources used to dope semiconductors. After predeposition,
there is a drive-in step in which the existing dopant is driven into the silicon and a