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 this 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.

Level 1 - Standard Semiconductor Concern

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