Specific Process Knowledge/Thin film deposition/Furnace LPCVD PolySilicon/Phosphorous doped poly-Si
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Phosphorous doped poly silicon by using B4 4" LPCVD polysilicon furnace
Result from Niccolò Bottauscio, DTU Nanolab, November 2025
In order to check the deposition rate, uniformity and sheet resistance for phosphorous doped polysilicon, a deposition has been done using the standard recipe "POLYPH3" with the standard temperature, pressure and gas flows and with three 4" test wafers distributed over the quartz boat in the furnace.
Process parameters
- Recipe: POLYPH3
- Deposition time: 2 hours
- Temperature: 525 °C (zone 1), 518 °C (zone 2), 525 °C (zone 3)
- Pressure: 200 mtorr
- Silane (SiH4)/N2 flow: 80 sccm
- Phosphine (PH3)/N2 flow: 7 sccm
Annealing: No annealing afterwards
Wafer positions:
- Test wafers: w6 (towards furnace), w15 (center), w24 (towards door)
- Dummy wafers: w1 - w5, w7 - w14, w16 - w23, w25 - w26
Wafer flat:
- The wafer flat was pointing upwards in the furnace boat
Results
An image of on of the test wafers after the deposition can be seen here:
From the image it can be seen that the color and thus the phosphorous doping is not very uniform over the entire wafer surface. However, if the edge of the wafer is ignored, a good uniformity is actually obtained.
The thickness of the deposited phosphorous doped polySi layer has been measured in five point over each of the three test wafers using the Ellipsometer.
| Ellipsometer measurements of the phosphorous doped polySi thickness | ||||
|---|---|---|---|---|
| Test wafer number | w6 | w15 | w24 | |
| Ellipsometer measurement
points (x [cm], y [cm]) and thickness | ||||
| (0,0)
Center point |
102.5 nm | 98.6 nm | 96.2 nm | |
| (-4,0) | 111.6 nm | 105.0 nm | 104.0 nm | |
| (0,4)
Wafer flat |
115.9 nm | 106.7 nm | 103.1 nm | |
| (-4,0) | 110.9 nm | 105.2 nm | 105.6 nm | |
| (-4,0) | 102.5 nm | 100.5 nm | 101.3 nm | |
| Average thickness | 108.7 nm | 103.2 nm | 102.0 nm | |
From the average thickness, the average deposition rate for each of the three test wafers can be calculated.
| Deposition rate | |||
|---|---|---|---|
| Test wafer number | w6 | w15 | w24 |
| 0.91 nm/min | 0.86 nm/min | 0.85 nm/min | |
The deposition rate is much slower than the deposition rate for undoped polySi. The reason for this is that the phosphorous passivates the surface and thus slows down the deposition. This also explains the non-uniformity of the deposited layer. Furthermore, phosphorous doped polySi becomes poly-crystalline at lower temperatures than undoped polySi.
The sheet resistance has also been measured for the three test wafers.
| Average sheet resistance | |||
|---|---|---|---|
| Test wafer number | w6 | w15 | w24 |
| Ohm/sq | Ohm/sq | Ohm/sq | |
Boron-doped amorphous silicon by using 4" LPCVD polysilicon furnace (B4)
Result from Trine Holm Christensen, @Space, Feb. 2015
Process parameters
The test was made on a n-type silicon wafer (100) with a 110 nm grown thermal oxide)
Pressure: 250 mtorr
Temperature: 580 °C
Silane (SiH4) flow: 80 sccm
Diborane (B2H6) flow: 7 sccm
Time: 75 min
Anneal: @950°C for 60 min in N2
Test wafer position: Center of the boat
A better uniformity of the resistivity across the wafer was achieved by subsequently annealing in N2, the annealing was done in Anneal Oxide Furnace (C1). The measured film thickness was 370 nm and the sheet resistance was measured to 200 Ω/sq +/- 15 Ω/sq.