pwell lithography & diffusiontuttle.merc.iastate.edu/ee432/lab/reports/report...

2017

PWELL Lithography & Diffusion

GROUP 1

RICHARD JILES – GROUP LEADER CHRIS LEGNER CHANG SUN AMIN GORJI-BANDPY

Group 1 – Richard Jiles EE 432 20th Feb. 2017

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PWELL Lithography and Diffusion of Boron Lab

Overview In this lab there were two different stages required for correct operation. Firstly, in the previous week

the wafers were patterned via lithography of the photo resist material and created designs on the wafer.

These designs were implemented by using a mask and blasting the wafer with high intensity UV light

through the mask. This then weakened the areas of photo resist that we wanted to remove. Then the

wafers were put in the BOE bath to remove unneeded sections. This was only done after the photoresist

hardened using the post bake procedure

After performing the lithography, the next step is to deposit the dopant via putting wafers of boron next

to the silicon wafers in the oven. Before this was possible, verification is needed of the post bake results

which hardened the lithography designs. This was to ensure that boron was deposited in the correct

areas. We took pictures of our wafers to show how accurate they were. Then we did a standard clean

and put the wafers in the furnace with the boron wafer deposit pads and ran the cycle. This was for the

initial deposit of boron. After depositing the boron, we then did the DRIVE step which consisted some

wet oxide growth and a set temperature and time to complete the process.

Photolithography

When performing the lithography on the wafers the objective is to place a pattern on the wafers that

will determine where our dopants go. In this case we are preparing for a boron diffusion of the PWELL in

our CyMos process. First off, we power up the mask and UV machine due to the fact it takes a while to

warm up. The first actual step in the lithography process is applying the photo resist to the wafers

themselves. We prime the wafer using the adhesive promotor HMDS and then spin the wafer. Directly

after spinning HDMS we then apply the photo-resist material. We used AZ5214E-IR photoresist material

and span it at 4000 rpm for 25 seconds.


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After applying the photoresist material, we then have to do the prebake on the wafers. This hardens the

material so that it is ready for the photolithography and mask machine. This consisted of us baking the

wafers at 84C for 25 minutes. After the post bake we put the wafers into the lithography machine and

used the first mask for PWELLs. We aligned the wafers, but this was easy because it was our first mask.

After aligning the wafers with the mask we exposed the wafers to 1 minute and 30 seconds of UV time.

Directly after exposing the wafers we put them in a developing solution called MIF-300 for 60 – 90

seconds. Rinsing the wafers after the developing solution with the cascade rinse tub. The timing for this

state is important as over developing can lead to too much material being removed.

We then performed the post bake of the wafers which was done in the oven at 125C for 25 minutes.

This was done to further stabilize and harden the photoresist material. We are now ready to etch the

material. This etching is done so that we remove the unwanted oxide while leaving the wanted oxide.

We used a test wafer to check that we had removed the oxide completely by checking the wafer

periodically to see if it looked wet. Only after the oxide is completely removed could move on. We

needed 4:45 seconds to etch our wafers. For references to oxide thickness of our wafers refer to page 8

on the previous report (Field Oxide Lab). After all the correct oxide has been removed, we can get rid of

the photo resist by using acetone and methanol baths, followed by a cascade rinse.

Boron Deposition and Drive

We start off by performing a standard clean on the wafers. This is a necessary step as before we do any

high temperature operations we must perform the standard clean. The purpose of the standard clean is

to prepare the wafers for boron deposition. We clean the surface in such a way to get rid of all

contaminants so that the boron deposits evenly and without any defects. Specifically, the standard clean

is needed whenever we are using high temperature furnace operations on the wafers. When performing

the standard clean we must always use clean solutions as this is implied when you are trying to clean


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something. We put the wafers into the transporting container that can be dipped into various solutions

and resist the chemicals (Fig. 1).

We start with de-ionized water and then mix in ammonium hydroxide (NH4OH) into one tub and

hydrochloric acid (HCl) into a different tub. We then raise the temperature of these solutions to ensure

that we have a consistent and known rate of reaction. Once the solutions are up to temperature you

start the wafers by putting them in the left tub and timing for 15 minutes. After the chemicals are done

cleaning the wafer we put them into a water and nitrogen bubbles bath to safely rinse the chemicals off.

After rinsing the first set of chemicals of we then transfer the wafers into a hydrofluoric (HF) acid bath.

After this HF bath we again rinse the wafers with water. After rinsing the wafers, then transfer them to

the SC-2 tub for a further 15 minutes. This is the final chemical process before rinsing them with water

and running them through the wafer cleaner machine. The wafers are now sufficiently clean to begin

furnace work. For this lab the furnace work involves boron deposition for the PWELL.

Cascade Rinse

Fig. 1. Chemical bath layout in lab

HF Bath

SC-1

SC-2


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Boron Deposition For the Boron deposition we take an already cooled wafer boat and put boron wafers, which are white,

with some of our silicon wafers which have the oxide patterned on them. We align our wafers such that

the shiny side (i.e. the patterned side) is facing the boron wafers. This allows the Boron from the source

wafer to flow to the silicon wafer. This can be seen in the aligned photo below. From here it’s quite

simple. You put the wafers in the oven using the temperatures and times found with the excel

spreadsheet. For our specific applications we chose to do 850C for 45 minutes for the first stage Boron

deposition. For this lab a two-step diffusion consisting of a deposit step and a drive step was performed.

The excel document can do this math. Alternatively, the exact steps are shown in the appendix for the

two step diffusion process.

Our first stage consisted of an 850C Furnace temperature for 45 minutes, followed by a second stage

drive step at 1100C for a total of 1 hour. The excel doc above shows this, as well as the calculations in

the appendix.

Figure 2 – Excel Calculations for our Dose and time and temp


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So then we loaded up the wafers and put them in the oven at the desired 850C temperature. The wafers

were aligned as in the following figure 3 photo.

According to the source wafer Boron data sheet we used the BN-975 type wafer. As our temperature for

the deposit step was 850C, this falls within the tolerable limits.

Figure 3 – Boron wafers with silicon wafers

Figure 4 – Specs for sheet resistance of source wafers

Figure 5 – Gas rates for deposition process


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In figure 5 the documentation shows the general procedure for using their wafers and depositing the

dopant. Taking into account these suggested gas rates, the CyMos process is conducted using the

following steps. In the furnace tube take 2 slpm of nitrogen while pushing the wafer boat into the center

of the tube (a 5-minute process, 1 inch every 12 seconds). Then once the boat is in the middle of the

furnace set the gas flow so that oxygen is 1 slpm and nitrogen is 1 slpm. Continue this gas flow for 20

minutes. The two above steps are the “Recovery” section. The next step is the source step. The

hydrogen then flowed at 40 sccm for 2 minutes, after which it was shut off. This does not 100% align

with the datasheet, variations have been made for our specific applications and hardware.

We then have our soak process, whereby we have nitrogen flowing at 2 slpm into the furnace. This is the

step that we calculated the temperature and time in the previous section. The excel doc indicates these

values. Then after this step, according to the CyMos process we actually pull out the wafers, setting the

furnace to IDLE mode after we have extracted the wafers. We ramp down the furnace to 400C and 0.3

slpm of nitrogen (As on page 2-21 of the SOP). Once the wafers have cooled, we perform the BOE bath

on them to deglaze them and finish that off with a cascade rinse.

Boron Drive

This step was performed by the TA/Dr Tuttle. It consisted of taking the wafers that we had finish the

deposition step, and putting them in the furnace overnight. In order to do so the standard clean should

have been performed on the wafers. We then ramp up the furnace to 800C and put in 1 slpm of

nitrogen. Start the bubbler with DI water. We vent the furnace with steam. The nitrogen into the

bubbler is set to 200 sccm. It will take 15 minutes for the bubbler to come up to 98C for our process. It is

at this point we can load the wafers into the machine. We then push in the wafers at 1 inch per 12

seconds.


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We are now performing low temperature oxidation for 30 minutes. We then stop the wet oxidation

flow. Removing the wafer slowly and performing the BOE for 30 seconds. Finishing with a cascade rinse.

Now the wafers are ready for the drive step. This is going to consist of putting the wafer back in the

furnace at 1 inch per 12 seconds. Ramping up the furnace to 1135 degrees C. We will now be steaming

both water and nitrogen into the tube. We perform oxidation at around 13.5 minutes at 1135C. Then

turn off the bubbler and shutdown the nitrogen after the time is up. Setting the nitrogen to 1 slpm for

the drive step. In our case we ramped down the furnace to 600C and left the wafer in the furnace

overnight for 16 hours with nitrogen flow of 0.3 slpm. We can then remove the wafers slowly and put

the furnace in the IDLE state.

Results

The results of our photolithograph can be seen as follows. We took pictures before performing the

boron deposition and found the following patterns on our wafers.

For the most part all of our patterns are perfect and any of the residue around the edges will be

removed with the standard clean process before furnace work.

Figure 6 – Wafer results with good patterns for alignment purposes


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But not everything is perfect, we were able to find a defect on one of our wafers which can be seen on

once of the rectangles:

Figure 7 – Macro picture for NMOS layouts

Figure 8 – Imperfection during the lithography stage of CyMos process


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From the pictures above we can conclude that it was successful, but during one step the DOE took away

a little too much material. Finally, the details of the Oxide growth from the Boron Drive can be seen as

follows:

TW1 7883 Å 7855 Å 7902 Å 7848 Å

TW2 8513 Å 8515 Å 8523 Å 8544 Å

TW3 8434 Å 8432 Å 8438 Å 8453 Å

TW4 8431 Å 8474 Å 8493 Å 8427 Å

We took 4 different data points to ensure the wafer had uniformly grown oxide and wasn’t lop sided.

However, in this situation we found the oxide growth was much higher than calculated. This isn’t too big

of a deal however as we just needed to etch for longer in the future lab. Specifically, we think that the

bubbler tubing was harboring some condensed water. This water stayed in the tube and continued to

boil even after the bubbler was set to vent. This growth of additional oxide was fixed with a BOE etch

time of around 12 – 13 minutes.

Conclusion

This lab successfully performed the PWELL photolithography as well as diffusion steps outlined in the

process traveler, weekly instructions and instructions manual. Photographs were able to confirm the

patterning on the wafers and the mask machine and chemical process works well. Even though a little

too much oxide was grown, this was easily coped with by increasing the BOE etch time. The wafers are

ready for PMOS lithography and diffusion steps.


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Appendix


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𝐷1 = 𝐷0 ∗ 𝑒−𝐸𝐴𝑘𝑇 = (1.0

𝑐𝑚2

𝑠) ∗ 𝑒

(−3.5𝑒𝑉

8.617∗10−5𝑒𝑉𝐾

∗1123𝐾)

= 1.95956 ∗ 10−16 𝑐𝑚2

𝑠

𝐷𝑡1 = 1.95956 ∗ 10−16 𝑐𝑚2

𝑠∗ 2700𝑠 = 5.29081 ∗ 10−13 𝑐𝑚2

𝑄 =2𝑁𝑠

√𝜋∗ √𝐷𝑡1 =

2 ∗ 9.5 ∗ 1019𝑐𝑚−3

√𝜋∗ √3.83401 ∗ 10−10 𝑐𝑚2 = 7.79722 ∗ 1013𝑐𝑚−2

𝐷2 = 𝐷0 ∗ 𝑒−𝐸𝐴𝑘𝑇 = (1.0

𝑐𝑚2

𝑠) ∗ 𝑒

(−3.5𝑒𝑉

8.617∗10−5𝑒𝑉𝐾

∗1373𝐾)

= 1.42001 ∗ 10−13 𝑐𝑚2

𝑠

𝐷𝑡2 = 1.42001 ∗ 10−13 𝑐𝑚2

𝑠∗ 3600𝑠 = 5.11202 ∗ 10−10 𝑐𝑚2

𝑁(0) =𝑄

√𝜋𝐷𝑡2

=7.79722 ∗ 1013𝑐𝑚−2

√𝜋 ∗ 5.11202 ∗ 10−10 𝑐𝑚2= 1.94567 ∗ 1018𝑐𝑚−3


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𝑥𝑗 = √4 ∗ 𝐷𝑡2 ∗ ln (𝑁(0)

𝑁𝐵) = √4 ∗ 5.11202 ∗ 10−10 𝑐𝑚2 ∗ ln (

1.94567 ∗ 1018𝑐𝑚−3

3.0 ∗ 1015𝑐𝑚−3)

= 1.1506 ∗ 10−4 𝑐𝑚

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