Pre Spin Coating

1. Particulate/dust contaminating liquid SOG.

2. Explanation: If disruptions to the surface tension of the liquid SOG are seen before spinning, then some type of particulate is in the SOG and this warrants restarting the test. There are three possible causes of this…

   1. Dust landed onto the wafer after the SOG was applied

   2. Dust already existed on the wafer from improper cleaning

   3. Some SOG has hardened inside the bottle creating fine flakes of SiO2. These flakes then get sucked up by the pipette and deposited onto the wafer with the liquid SOG (Example picture above). This warrants preparing a new bottle of SOG based on this document.

Post Spin Coating

1. Particulate/dust contaminating wafer surface.

   1. Often, after spin coating disruptions to the SOG evenness/thickness can be seen in localized spots on the wafer. There are possible causes of this…

   2. Some SOG has hardened inside the bottle creating fine flakes of SiO2. These flakes then get sucked up by the pipette and deposited onto the wafer with the liquid SOG. These flakes are clear and sometimes not visible pre spinning, and are then only visible after spinning.

2. Particulate Landed on the wafer during spinning. Sometimes the spin coater can create turbulent air and kick up dust from inside the spin coater which then lands onto the wafer.

3. The wafer had a preexisting scratch or mark big enough to cause surface tension issues which result in the liquid SOG “un-wetting” in that spot and exposing the substrate. These scratches are often caused by scratches that occur during cleaving, slipping across the wafer when using hard tip tweezers, or dropping the wafer.

1. Radial variation in thickness/diffraction color

   1. Sometimes after spin coating, there may be a radial pattern of changing color on the wafer. The varying colors indicate variation in thickness since SiO2 is translucent so thin films diffract and appear different colors based on thickness. These variations are usually on the order of a few hundred Angstroms (A) (film thickness ~2000 A) and may not warrant restarting.

During Spin Coating

1. Wafer flies off the chuck while spinning

   1. Sometimes either due to bad two sided tape adhesion (regular spin coater) or improper sealing onto the vacuum chuck (vacuum spin coater). The chip may fly off the chuck due to inertial forces. This usually warrants restarting since when the chip flies off it hits the side causing uneven spreading of the SOG or lands onto the SOG.

Post-Anneal

Identifying “defects” in the deposited SOG

Pinholes

Possible Causes

1. Scratches or dust particles on the wafer Cause the liquid SOG to “un-wett” in some spots. This essentially creates a dip in the SOG layer that extends to the wafer surface below the SOG.

Particulate resting on top of the SOG

Possible Causes:

1. Dust coming into the fume hood and landing onto the wafer after the SOG layer has solidified.

Process Parameters

Purpose

Three types of "spin on glass" are currently used in the Hacker Fab. 700B, P504, and B154, all three sourced from Filmtronics. Each of these begin as liquids, which are spun onto the surface of the chip, then annealed to form solid thin films. This is known as a sol-gel process.

700B becomes an undoped thin film of SiO2, but with lower density than a thermally grown SiO2. After high temperature exposure (~1100C) this thin film densifies to the density of a thermally grown oxide.

P504 becomes a thin film of SiO2, with a small amount of Phosphorous in it, also with lower density than a thermally grown SiO2. This is also referred to as a "spin on diffusant." After high temperature diffusion (~1100C) this thin film densifies to the density of a thermally grown oxide.

B154 does not have SiO2 precursors like 700B and P504, but becomes a solid thin film with a small amount of Boron in it. The composition of this solid film after relatively low temperature exposure (~200) is not clear, since the formula is proprietary, but it is believed to be some sort of polymer. Upon diffusion (exposure around ~1100C) the B154 thin film becomes SiO2 of thermally grown density.

These spin on glass films are often etched with a dilute HF solution, or BOE (HF + NH4F) due to the high selectivity with Si. However, the etch rate of spin on glass annealed around 400C is much faster than that of spin on glass that has been densified at around 1100C. Additionally, the B154 film that has been annealed at 200C is not effectively etched with HF, but is effectively etched with HF after diffusion at 1100C.

As indicated above, the purpose of the P504 and B154 deposition is to create a dopant source at the surface of the silicon, which can be diffused into the silicon substrate. 700B is used as a diffusion barrier, or a dielectric layer to fabricate metal interconnects on top of.

Defects

The main concern with spin on glass, is its tendency to crack (extent of cracking varies, but full shatter can occur leading to flake off), have variation in uniformity, and have pinholes. Cracks and pinholes can lead to uneven doping across the surface, failure of 700B to act as a diffusion barrier, or metal interconnects shorting to the Si below. Factors like humidity, annealing temperature profile, particulate contamination, and shelf life of the spin on glass solution.

However, the CMU Hacker Fab has found particulate contamination to be the number one cause of defects, and this issue has been mitigated greatly by using filtered syringe tips during deposition, as seen below in the procedure.

Tools

Materials

1. Filmtronics 700B, P504, or B154.
2. Acetone

3. Isopropanol

Procedure

Wafer Cleaning

1. Check that the working volume of spin on glass is labeled with the date it was poured. For 700B and P504, ensure that the solution has not been out for more than a month (main bottle is stored at 5C).

   1. If you are pouring a new working volume of P504 or 700B, let it come to room temperature before using it (ideally, wait 24 hours).

3. In the fume hood, hold the wafer with tweezers over the sink.

4. Rinse the polished side of the wafer thoroughly with acetone, then isopropyl alcohol.

   1. The acetone leaves a residue that must be removed by the isopropyl alcohol rinse.

5. Blow the wafer dry with the nitrogen gun by pressing the wafer against a cleanroom wipe on the table to ensure it does not fly away. Get a good grip on it with your tweezers.

   1. Even when the wafer appears dry, there may still be moisture on the edges, so dry both sides liberally for ~20 seconds

   2. if there is visible acetone residue after drying, repeat steps 2-3

Prebake

1. Place the wafer in the center of the hotplate for 20 seconds

   1. Be sure to handle the wafer with tweezers that can handle high temperatures (metal tweezers)

   2. No need to turn the hotplate off since annealing will require 100°C initially as well

Spin Coat

2. Open the SOG container while keeping the bottom resting on the table

   1. Never open containers up in the air or outside of the fume hood

3. Pipette 1-2 drop of SOG using Luer Lock filtered syringe:

   1. Take a clean syringe and draw it up ~½ to create an air pocket in the tube

   2. Suck up SOG about halfway up the syringe. This is a lot more than you need for 1 drop, because the filter needs to be wetted by the excess solution before drops are released.

   3. Twist on a syringe filter

   4. Release 2 drops back into the SOG container

   5. Apply 1 or 2 drops to your chip, make sure the chip is completely coated in solution but do not use more than 2 drops

   6. Dump any remaining SOG in the syringe back into the SOG container

   7. Dispose of the syringe and the filter in the waste bucket

1. Spin coat the wafer (Remember to switch on the vacuum!)

   1. After spin coating, the SOG application should appear even. (See pictures below)

2. Immediately move onto annealing

Hot Plate Annealing.

1. The hot plate should already be at 100C

2. 1. Be sure to handle the wafer with tweezers that can handle high temperatures

3. Set the hotplate to the desired annealing temperature (200C for P504 or B154, 400C for 700B)

4. After the desired anneal time has passed (10-15 minutes for P504 and B154, 20-30 minutes for 700B), use metal tweezers to remove the chip. Be careful not to burn yourself.

Tube Furnace Anneal/Diffusion P504

Inspection

Safety

Be sure to work under fume hood when working with SOG. The SOG can give off toxic vapors (especially during annealing)

DO NOT touch the hot plate during operation. A 400 C hot plate will cause severe burns. (Reminder: 400 C = 752 F). The same is true of the glass cover, which is why it is important to ramp down before touching.

Purpose

The main purpose of this SOP is to describe a method that we can use to approximate the thickness of SoG. This is especially important in certain plasma etching processes, where removal of SiO2 is needed.

Tools

• Spectrometer

• Spectrogryph opened on computer connected to microscope view

• (should end with .sgd, can only be opened in the Spectrogryph app)

Picture of Spectrometer Below:

Materials

• Silicon Chip with 700B/P5O4 SOG

Steps

Setting up the software + obtaining spectrometer data

First, ensure that the spectrometer is connected to the computer.

In addition, place SoG on silicon chip sample on the slide under the microscope.

1. Open Spectrogryph software on Computer

2. Under Plot/Views, select “New Acquisition View”

After which, you should see something that looks like:

1. In the upper left corner, select “Device Type” and select ASEQ. Then press connect.

2. Change exposure to 5000 ms (or more, if you want less noisy data)

3. Select single shot or continuous

   1. Use Single Shot if you want to get one graph that is taken after 5 seconds of exposure

   2. Use Continuous if you want a graph that is constantly being recorded (hence changing)

4. Pressing “Acquire” would start the readings. Before pressing “Acquire”, ensure that the probe is connected to the microscope (will need to press the probe against the microscope to get good readings, as shown in picture below).

7. Once a graph of Count against Wavelength is obtained, we need to normalise the counts. This is done by clicking on "Process" and "Normalise (Peak)".

1. Once normalized, we can now save this spectral plot in file (save it in the .sgd format, so that you can re-open it in spectrogryph later)

Determining Thickness of SoG

1. Create another New Acquisition View

You should see this:

The color of the graphs correspond to the actual observed colors of the glass. This can serve as a good benchmark as to what the thickness of your glass might be.

As seen, the SoG with the lowest thickness (190 nm) had the maximum peak at a wavelength of ~590nm. As the glass thickness increases from 190 nm to 255 nm, the wavelength at maximum peak increased accordingly. Following that, from 255 nm to 276 nm, it can be seen that a second peak is starting to form (at a wavelength of around 480 nm). Finally, from 276 nm to 314 nm, the peak at 590 nm disappears, and now the peak at 480 nm becomes more distinct.

1. Lastly, drag the .sgd file that you previously saved in Step 7 into the plot above.

2. Determine which shape your graph most closely resembles to determine SoG thickness.

Also note that the color of your glass is a pretty good indicator of what thickness it will be. So for instance, if your chip looks pink, just by looking at the calibration graphs, we can determine that thickness is around 270 nm.

Useful Links