Specific Process Knowledge/Thermal Process/Dope with Phosphorus

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The Phosphorus Predep furnace (A4) can be used for phosphorus pre-deposition of silicon wafers, resulting in n-type doping. In the furnace, the silicon wafers are positioned vertically in a quarts boat.

Test of the Phosphorus Predep furnace

This page is written by DTU Nanolab internal

Purpose

To study the correlation between the temperature for the predeposition process and drive-in of the phosphorus doping in the Phosphorus Predep furnace (A4) at DTU Nanolab.

Experimental setup

20 boron doped device wafers (p-type) were used - Four wafers for each of the five different predeposition temperatures (see table below). In the furnace five dummy wafers were placed on each side of the device wafers. The dummy wafers nearest to the device wafers were changed in-between the runs to minimize doping from these dummy wafers.

Run #	Temperature	Process time with POCl3	Annealing time in O2	Wafer #
1	850 oC	15 minutes	20 minutes	1, 2, 3, 4
2	900 oC	15 minutes	20 minutes	5, 6, 7, 8
3	950 oC	15 minutes	20 minutes	9, 10, 11, 12
4	1000 oC	15 minutes	20 minuntes	13, 14, 15, 16
5	1050 oC	15 minutes	20 minutes	17, 18, 19, 20

After the predeposition two wafers from each run were taken out to be further processed. These wafers were: 1, 2, 5, 6, 9, 10, 13, 14, 17, 18. These wafers were dipped in BHF to remove the phosphorus glass layer before the drive-in process.

The drive-in process was made in the Phosphorus Drive-in furnace (A3) for all the mentioned wafers at same time. At the drive-in process a dummy wafer was placed in-between the wafers that have been predeposited at different temperatures, so doping from wafer to wafer was minimized. The phosphorus drive-in was done with the process "DRY1050" which is a dry oxidation at 1050 ^oC for 100 minutes and 20 minutes annealing. At the oxidation the O₂ flow was 5 SLM, and the N₂ flow for annealing was 3 SLM.

Results

Several measurements were done for the different device wafers. After the predeposition, the thickness of the grown phosphorus glass layer was measured, and the sheet resistance and slice resistivity were measured on the same wafer after a BHF etch.

Measurements after phosphorus predeposition

		Ellipsometer (center point only)		Four Point Probe
Wafer #	Temperature [C]	Oxide thickness [nm]	Refractive index	Sheet resistance [Ωsq]	Slice Resistivity [Ωcm]
3	850	27,4	1,4623	311	6,22*10-3
7	900	45,27	1,4622	138,5	4,85*10-3
11	950	61,36	1,4625	16,1	1,29*10-3
15	1000	80,45	1,4624	7,4	0,89*10-3
19	1050	119,37	1,4623	6,6	1,49*10-3

The oxide layer was etch in BHF before the drive-in of the doping with a dry oxidation at 1050 ^oC for 100 minutes and 20 minutes annealing.

Measurements after phosphorus drive-in

		Avg. five point on Filmtek		Four Point Probe
Wafer #	Temperature [oC]	Oxide thickness [nm]	Refractive index	Sheet resistance [Ωsq]	Slice Resistivity [Ωcm]
2	850	110,44	1,4654	189,7	3,79*10-3
6	900	116,28	1,4629	101,6	3,56*10-3
9	950	137,06	1,4604	10,0	0,8*10-3
14	1000	141,46	1,4651	4,72	0,57*10-3
18	1050	139,87	1,4659	3,23	0,73*10-3
Blank Si wafer		110,71	1,46286	-	-

Resistivity	Sheet resistance
SIMS Measurement After Pre-dep	SIMS Measurement After Drive-in Process at 1050 oC

Looking at the "SIMS Measurement After Drive-in Process at 1050 ^oC" graph, it can be seen that the two curves "Pre-dep at 950 ^oC" and "Pre-dep at 1000 ^oC" are crossing each other, but according to the theory they should not do that. Only one wafer has been measured so there is not that much statistical data to verify it with.

Test of the Phosphorus Predep furnace

This page is written by DTU Nanolab internal

Purpose

To find the result of a specific process using the Phosphorus Predep furnace (A4) followed by a drive in and oxidating the substrate in the Phosphorus drive in furnace (A3).

Experimental setup

Substrate: p-type Si(100,)

Resistivity: 1-20 Ωcm

Recipe: POCL900

Temperature: 900 °C

Time: 15 min

Anneal: 20 min in N₂

After removing the oxide in BHF the wafers were oxidized in Phosphor Drive in Furnace (A3) using DRY1050 at 1050°C for 70 min without further annealing in N₂. Results are listed in the table below:

Results

Measurements after phosphorus predeposition

	Afer predeposition	After Oxidation
Test date	Sheet resistance [Ω/sq]	Oxide thickness [nm]	Sheet resistance [Ω/sq
Aug2012	47.0	93.1	41.8
Jan2013	50.5	91.3	55.6
Aug2013	58.0	91.8	41.8
Jan2014	54.1	93.3	33.8
Jan2015	50.8	94.2	39.4

One wafer from Jan2015 was analyzed with SIMS and the profile is shown on the graph below:

SIMS Measurement After Predeposition, before and after oxidation respectively

Result from GreenBelt, @nanolab,Jan 2015

Test of the Phosphorus Predep furnace

Purpose

To find the doping profile after a specific process flow. The test was made by only performing the high-temperature process step after the pre-deposition step done in the Phosphorus Predep furnace (A4). The subsequently high temperature processes was done by using the Phosphorous drive in furnace (A3) and the Oxide Anneal Furnace (C3).

Experimental setup

Substrate: p-type Si(100,)

Resistivity: 1-20 Ωcm

The total thermal budget for the wafers:

Predeposition in Furnace (A4): POCL1000, 15 min, no further anneal

Wet Oxidation in Furnace (A3): WET1050, 30 min, no further anneal

Anneal in Furnace (C1): ANN1000, 30 min in N₂

Wet oxidation in Furnace (C1): WET1050, 30 min, no further anneal

Results

The wafer was analyzed with SIMS and the profile is shown on the graph below:

SIMS Measurement after Phosphorous Predeposition and several oxidations and annealings

Result from Trine Holm Christensen, @Space, Feb. 2015

Phosphorous doping test after hardware and software upgrades

This page is written by DTU Nanolab internal

In 2022, the POCl₃ line on the furnace was modified to make the setup more safe. At the same time some software changes were also made, and the recipes on the furnace were updated.

Afterwards, a phosphorus doping test was made to see, if the modification had affected the phosphorus doping process. The test was done with a test wafer and four dummy wafers (two dummy wafers on each side of the test wafers) in the furnace.

Process:

• RCA cleaning of the test wafer (p-type, 1-20 Ωcm) and the dummy wafers
• Phosphorus pre-deposition in the Phosphorus Pre-dep furnace (A4).
  • Recipe: "POCL900"
  • Temperature: 900 °C
  • Time: 15 min phosphorus pre-dep and 20 min annealing/N₂ post purge
• BHF dip in the RCA cleaning bench to remove phosphorus glass from the test wafer
• Measure sheet resistance using the 4-point proble

Results:

After the process describe above, the sheet resistance on the test wafer was measured.

The sheet resistance was 54 Ωsq in the center of the wafer and 45-48 Ωsq in the edge of the wafer. This correspond quite good to the results that previously had been obtained for the furnace as can be seen on this LabAdviser page.

Notes about the POCL recipes

A new recipe for POCL doping called was made, after the POCl₃ line was modified. The N₂ high flow (3 SLM) is on all the time through the recipe. There is a 2 min pre-purge with first N2 low (150 sccm) through the POCl3 bubbler, i.e. with the valves v4a and v4b closed, and then with O2 (0.50 SLM), before the pre-deposition time starts. After the pre-deposition, an N2 low post-purge (500 sccm) is done for 2 minutes with v4a and v4b closed.

For safety reasons it is important that the POCl₃ is completely removed from the furnace, before it is opened to load or unload wafers.

With 150 sccm N2 low through 25°C POCl₃ liquid + 3 slm N₂ high + 0.5 slm O₂, 3300 ppm Cl₂ is created in the tube. These 3300 ppm Cl₂ must be diluted to less than 0.5 ppm (the TLV value).

The tube diameter is 16 cm and the length without ball valve is about 125 cm, and thus the volume is about 25 L. After the phosphorus pre-deposition step, the N₂ high flow (3 SLM) is on during the annealing/N₂ post purge, the cooling and a waiting step. The time for the waiting step is 30 minutes, so even if the annealing time is set to 0 minutes, and the furnace does not have to cool down, there will still be an N₂ high flow for 30 minutes to remove the POCl₃ from the furnace. The waiting time is the same in the abort recipe that is activated, if the POCL doping recipe is aborted.

The recipe can be modified with other temperatures and names.