Overview

Samples and tools are categorized into 4 classifications of material and process compatibility:

Metallic Contamination
Outgassing Contamination
Elevated Temperature
Backside Cleanliness

In each category, higher numbers (e.g 4) are more stringent and controlled than lower numbers (e.g. 1).

Going into a tool with a higher number classification may require specific cleaning of a sample, or it may not be allowed at all. In general, it is good practice to start with a new sample on the most stringently rated tool, and work to less stringently rated tools and processes.

Matrix

Metallic Contamination

Level 4 - RCA Clean

Wafers must have been cleaned at the RCA Cleaning Hood.

Level 3 - Metal Ion Free / Low Diffusion Metal Films

Sample must be thoroughly cleaned in UPW to remove any metal ion residual. The only metallic films that may be present on the sample are those which have low diffusion probability into silicon and other common substrates.

Level 2 - Volatile Etch Products

Sample must not form any involatile etch products during the process which may be sputtered off and contaminate the chamber. For plasma etch systems, a very general primer on determining etch byproduct boiling points can be found here: Tutorial on Etching#General-Willitetch (reproduced below). Be sure to consult with BNC staff for any materials and etch products you have questions about, especially etch byproducts with a boiling point > 185 C. Note that "dec" stands for "decomposition". In general, this means the compound explodes into other compounds and that listed compound would not be removed from the chamber.

Tutorial on Etching

Materials can generally be etched in the RIEs as long as they form volatile byproducts, or products for which the vapor pressure (at the temperature of the etch) is higher than the pressure of the chamber.

Etching is very complicated and this will be massive oversimplification...but generally volatile byproducts can be determined from literature, or as a fallback, the CRC Handbook of Chemistry and Physics Online, (4) Properties of the Elements & Inorganics, Physical Constants of Inorganic Compounds: https://hbcp.chemnetbase.com/faces/documents/04_02/04_02_0001.xhtml. From there, click "Go to Interactive Table", and find products that may be formed (i.e. chloride, fluorides, oxides, depending on the gasses). A compound is deemed volatile if it has a boiling point at a reasonable temperature range for the temperature and pressure of the system. Note that at lower pressures, boiling points decrease, so these are just a good staring point reference.

As a VERY general rule of thumb, anything with a boiling point (t_bp) < 185 C will be volatile in the ICP RIEs.

As an example, aluminum chloride is volatile, and aluminum fluoride and aluminum are not.

Neither Copper chloride or copper fluoride is volatile, which is why it is not allowed in any chamber:

Byproducts of silicon are very volatile:

Many times different fluorides/chlorides of the same material will have drastically different boiling points. It's important to research which will be formed in the plasma. Titanium is a good example of this, with TiCl₂ and TiCl₃ being non-volatile, and TiCl4 being volatile:

Level 1 - Standard Semiconductor Concern

Sample should have been with semiconductor grade solvents or UPW prior to use in the tool.

Outgassing Contamination

Level 4 - High Vacuum (<10^-5 Torr)

Substrates:

Silicon
Glass (Quartz, Fused Silica, Pyrex)
GaAs
Sapphire

Deposited films:

Inorganic oxides/compounds
Metals, except:
- Arsenic
- Beryllium
- Cadmium
- Gallium
- Indium
- Magnesium
- Mercury
- Phosphorus
- Tin
- Gd

Photoresists:

Positive Photoresist (DNQ Based):

- AZ Series (AZ1518, AZ5214-E, AZ9260)
- S1800: S1805 g2, S1813, SC1827
- SPR: 220-3.0, 220-7.0

Negative Photoresist:

- LOR: 3A, 3B, 5A, 30B
- PGMI SF 8

EBL Resists:

Positive:

- PMMA 950 (A2 A4 A6 A8 A10), 495 (A2 A4)
- ZEP A

Negative:

- ma-N: 2401, 2403
- HSQ: HR-1541 2%, 4%, 6%, FOX-15, FOX-16

Level 3 - High Vacuum, Limited flex (<10^-5 Torr)

Everything permitted in Level 4, as well as:

Substrates:

Polycarbonate
PEEK
ABS plastic

Properly outgassed thin films of

PDMS:

Sylgard 182, 184 (not vacuum compatible, silicone based)

Polyimide

- Polyimide
- Kapton, no adhesive
- Kapton Tape (not approved for vacuum use in evap/sputs)

Epoxy

- Su-8
- OmniCoat

Parylene

Level 2 - Low Vacuum (<Atmosphere to 10^-3 Torr/1 mTorr/1 Pa)

At pressures down to 10^-3 Torr/1 mTorr/1 Pa and the substrate temperature during the process, substrate and any deposited materials must be stable solids, and should not pose a risk of outgassing contamination to the tool for the process you intend to run.

Level 1 - Atmosphere

At atmospheric pressure and the substrate temperature during the process, substrate and any deposited materials must be stable solids, and should not pose a risk of outgassing contamination to the tool for the process you intend to run.

Elevated Temperature

Level 4 - > 450 C

At desired process temperatures above 450° C , substrate and any deposited materials must be stable solids, and should not pose a risk of outgassing contamination to the tool for the process you intend to run.

Level 3 - 120 to 450 C

At desired process temperatures between 120° C and 450° C , substrate and any deposited materials must be stable solids, and should not pose a risk of outgassing contamination to the tool for the process you intend to run.

Level 2 - <120 C

At 120° C, substrate and any deposited materials must be stable solids, and should not pose a risk of outgassing contamination to the tool for the process you intend to run.

Level 1 - Room Temperature

At room temperature, substrate and any deposited materials must be stable solids, and should not pose a risk of outgassing contamination to the tool for the process you intend to run.

Backside Cleanliness

Level 3 - Dry, with no metals/metal residue or polymer residue

Backside of sample must be totally dry, as dried with an N2 gun.
Backside must be free of any polymers or photoresist, including both uniform films and any non-uniform residue.
Backside must have been rinsed thoroughly with UPW to remove any metallic ion residue (which can be present after development in metal ion containing photoresist developers, or in an acid clean such as Piranha, Nanostrip, etc.). The backside must also be free of any deposited metals, or metal flakes as a result of liftoff.

Level 2 - Dry, with no polymer residue

Backside of sample must be totally dry, as dried with an N2 gun.
Backside must be free of any polymers or photoresist, including both uniform films and any non-uniform residue.

Level 1 - Dry

Backside of sample must be totally dry, as dried with an N2 gun.

Special

Some tools have specific concerns not otherwise covered under this scheme. For example, the STS ASE etcher is sensitive to negative photoresist (Particularly SU-8, due to molybdenum contamination), as well as polymers, polimide, and other resins.

MJB3, MJB4, MA6, and MLA150: Restrictions involving piece and substrate sizes? Contact Joon Park for more info.

SB6e bonder: No parylene at elevated temperatures. This may already be included in the classification scheme, and the special mark is only a holdover from a special concern on the tool page. Contact Joon Park for more info.

AOE: Special restrictions on wafer bonding and approved mask materials, see here for more info: /wiki/spaces/BNCWiki/pages/6227013

XeF2: Samples should be free of water residue, or an insoluble fluoropolymer (like teflon) will form. Either dehydration bake samples or rinse then in IPA before placing them in the chamber.

Fiji: Samples and processes need to be approved by Zhihong Chen.

Parylene CVD: Parylene is generally not allowed in most tools in the cleanroom where elevated temperatures and/or sub-atmospheric pressures will be used. If you would like to use parylene in a system at elevated temperature and/or sub-atmospheric pressures, this use must be approved on a tool-by-tool basis and sample-by-sample. Contact Joon Park for training on the bio vacuum oven for proper outgassing.

Undetermined: Committee must approve

Plastics

Polystyrene
Poly(vinyl alcohol)
Poly(ethyleneimine)
Spin on teflon
Polypropylene carbonate

PMMA Copolymer

MAMA(8.5)MAA EL 11

Elvacite acrylic resins

NIL Resists?

Nxr3032
Ormolayer stamp
Nxr 3020
Ormoprime 08

NOT ALLOWED SUBSTRATES:

Any Silicone Based Materials

PCB/FR4

Draft Table

Metallic Contamination	Level 1 - Standard Semiconductor Concern
Outgassing Contamination	Level 4 - High Vacuum (<10^-5 Torr)
Elevated Temperature	Level 2 - <120 C
Backside Cleanliness	Level 2 - Dry, with no polymer residue

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