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 polySi, a deposition has been done using the standard recipe "POLYPH3" with the standard temperature, pressure and gas flow settings and with three oxidized 4" Si test wafers distributed over the quartz boat in the furnace.
Process
Recipe: POLYPH3
Deposition parameters:
- Temperature: 525 °C (zone 1), 518 °C (zone 2), 525 °C (zone 3)
- Pressure: 200 mtorr
- Silane (SiH4) flow: 80 sccm
- Phosphine (PH3)/N2 flow: 7 sccm
Deposition time: 2 hours
Annealing: No annealing afterwards
Wafers
- Test wafers:
- w6 (towards furnace), w15 (center), w24 (towards door)
- Oxide thickness: 472 nm
- The wafer flat was pointing upwards in the furnace boat
- Dummy wafers: w1 - w5, w7 - w14, w16 - w23, w25 - w26
Results
An image of one 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 VASE.
| 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 for the phosphorous doped polySi | |||
|---|---|---|---|
| Test wafer number | w6 | w15 | w24 |
| Deposition rate | 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 wafer 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 on the three test wafers. The measurements have been done in three point on each wafer: On the top (near the wafer flat), in the center point and in the bottom.
in the center point of the three test wafers. The measured has been done using the 4-point probe Jandel.
| Average sheet resistance for the phosphorous doped polySi | |||
|---|---|---|---|
| Test wafer number | w6 | w15 | w24 |
| Sheet resistance | 1.42 Ohm/sq | 1.47 Ohm/sq | 1.52 Ohm/sq |
| 4 point probe measurements of the the sheet resistance | ||||
|---|---|---|---|---|
| Test wafer number | w6 | w15 | w24 | |
| 4 point probe measurement points | ||||
| Wafer top
Wafer flat |
1.38 kOhm/sq | 1.45 kOhm/sq | 1.48 kOhm/s | |
| Wafer center | 1.48 kOhm/sq | 1.51 kOhm/sq | 1.51 kOhm/sq | |
| Wafer bottom | 1.51 kOhm/sq | 1.54 kOhm/sq | 1.52 kOhm/sq | |
| Average thickness | 1.44 kOhm/sq | 1.50 kOhm/sq | 1.52 kOhm/sq | |