Material restrictions

This wiki page covers the LNF policies for materials restrictions or LNF segregation policies. The LNF users are expected to be familiar with these protocols to minimize the risks associated with them.

Introduction

Working with chemicals, different materials, and processing systems involves different risks that need to be considered. The LNF, same as all the well-established cleanrooms, follows a series of guidelines to minimize the risks related to micro and nanofabrication.

Our first concern is safety, and all our protocols prioritize it. Our priority list goes as:

1. Safety of users and staff

2. Safety of equipment

3. Avoid contaminating samples

Our main priority at LNF is the safety of all users and staff. And the integrity of our equipment cannot be compromised over the samples. If there is a need to run a non-approved or new material (e.g., a new chemical), there needs to be explicit approval. To learn more about requesting approval for materials, please keep reading. Also, keep in mind the list of approved materials is specific for each tool. When in doubt, always create a helpdesk ticket to verify with the tool engineer.

Materials classification

In general, the materials used in the cleanroom can be classified as CMOS (complementary metal-oxide-semiconductor) compatible or non-compatible.

What is a CMOS-compatible material? CMOS refers to the standard fabrication process for semiconductor devices (i.e., metal oxide field-effect transistors, a.k.a MOSFET). In other words, it refers to the process used in the semiconductor industry to fabricate integrated circuits. CMOS devices’ performance can be strongly deteriorated due to contaminants that affect the terminals’ doping or the electric gates’ performance. Therefore, a CMOS-compatible material does not affect the performance of CMOS devices.

Some examples for CMOS-compatible materials are Silicon, amorphous Silicon, Silicon oxide, Silicon nitride, Aluminum, Some photoresists such as SPR-220, etc.

Some examples of non-CMOS-compatible materials are Gold, Nickel, Zinc, photoresist developers with potassium ions such as AZ-400K, etc.

A non-CMOS-compatible material should never be run in a system that is exclusive to CMOS materials. This presents the high risk of passing contaminants to other samples and changing the performance of the tool. Refer to the wiki page to check the approved materials for the different systems at the LNF, these lists are specific for each system. When in doubt, make a tool ticket to ask directly to the tool engineer.

LNF systems (equipment) classification

The LNF uses a wide variety of processes and materials, some of which can cause contamination to others' processes. To reduce the possibility of contamination, the lab places material restrictions on each tool.

There are four primary levels of material restrictions at the LNF described in detail below. They are CMOS-Clean, Semi-Clean, Metals, and General. The materials listed in each section are examples of what may be allowed in a tool with that level. A complete list may be found on individual tool pages.

You can find the material classification of a tool on the wiki. For example, the STS Pegasus 4 is a Semi-Clean tool.

CMOS-Clean

CMOS-Clean tools are considered the lowest level of possible contamination and thus have the most significant level of restriction. For a sample to run in a CMOS-Clean tool, it must have only CMOS-compatible materials and only ever be processed in CMOS-Clean tools. Some of the CMOS-Clean tools also require an additional clean to the samples before the process, such as a Pre Furnace Clean. Please see individual tool pages for details.

Here is an example list of materials allowed in CMOS Clean tools. Silicon wafers used in CMOS Clean tools must be Prime Grade. - Silicon (prime grade wafers)

- Amorphous silicon

- Silicon oxide

- Silicon nitride

Semi-clean

Semi-clean tools present the next level of material restrictions. This level includes all materials in the CMOS-Clean group and some low-mobility metals and films made in Semi-Clean tools. CMOS-Clean and Semi-Clean wafers may be processed in these tools. To go back to a CMOS Clean tool after process in a Semi-Clean tool, additional cleans may be necessary. For more information, see individual tool pages.

Here is an example list of materials potentially, but not necessarily, allowed in Semi-Clean tools.

- Aluminum oxide

- Chromium

- Titanium

- Photoresist SPR-220/955

- Photoresist KMPR

Please note that even though a semi-clean tool allows semi-clean materials, there can be further restrictions. For example, the pegasus tools allow aluminum to be present in the samples; however, it cannot be exposed to the plasma. When in doubt, refer to the specific system wiki page for more information or create a helpdesk ticket.

Metals

Metals classification is the lowest level of material segregation in the cleanroom. Many materials are allowed in these tools, including the CMOS-compatible and semi-clean materials. However, any sample processed in a Metals tool is not allowed back into a Semi-Clean or CMOS-Clean tool without express permission from LNF staff by creating a helpdesk ticket. Additional cleans will be required.

Here is an example list of materials potentially, but not necessarily, allowed in Metals-classified tools.

- Pyrex glass

- Gold

- Silver

- Nickel

- Copper

- Zinc

- Photoresist AZ-10XT

General

General class tools allow most Metals class materials; however, these systems have a low potential for cross-contamination. Therefore, the segregation class of a Semi-Clean or CMOS clean wafer is not altered by it. Most metrology and lithography equipment falls in this category.

Types of sample cleaning

Different contaminants can be present in a sample: particles, organic residues, remaining photoresist, ions, etc. Depending on the type of contaminant and the system that will process the sample, the samples will require a cleaning process.

For more details about sample cleaning, refer to our LNF wiki page for cleaning.

You can also find helpful information in our LNF tech talk.

Things to consider when planning a new process

When designing a new process flow that involves the LNF, it is crucial to keep in mind the materials’ restrictions for each one of the systems. In general, samples processed in a “dirty” tool should not be processed in a “clean” tool.

Some of the things that you should be aware of:

- Does the process present a health risk to users or staff?

- Does the process flow consider going from “dirty” systems to “clean” systems?

- Does the process have a risk of contaminating the tool and affecting others’ processes?

- In what other systems have the samples been?

- Do the samples need to be clean before a run?

- Is there a risk for the sample to get contaminated from a “dirty” tool?

- Is the sample compatible with a vacuum chamber? (does it degas?)

- What is the thermal budget of the samples?

- Is there more than one option (different systems) to run the process?

Examples of poorly designed processes

Example 1

A sample has been coated with Pt and patterned using the ion mill. There is a need to do a SiO2 etching in the sample and the Glass Etcher tool is considered as an option.

Problem: the ion mill tool is classified as metal tools, and the glass etcher is classified as a semi-clean tool. This would be a violation of the materials segregation protocols.

Notes: to etch the oxide, the plasmatherm could be considered or the ion mill.

Example 2

A new exotic material is planned to be etched, and the LAM tool is considered as an option.

Problem: If the material is not listed as an approved material for the system, it is not allowed to process it unless there is explicit permission.

Note: A request to process the new material needs to be made via the LNF ticketing system. The LNF staff will evaluate the safety of the proposed process.

Example 3

A sample with GaN has been processed using AZ400K developer. The sample has been cleaned with acetone and IPA and is ready to be dry etched in the LAM

Problem: The AZ400K developer is not a CMOS-compatible material. Now the sample is contaminated with ions that can affect other people samples.

Noteː Before going into the LAM an RCA cleaning (which includes an ionic cleaning) needs to be done on the sample.

How to request working with non-approved materials

If you need to process samples with a non-approved material, you can request approval following the next steps. Also, consider that new materials’ authorization is conditioned to reviewing its risks to other users and the tool’s performance.

- Gather information about the risks involved in running your sample

- What is the toxicity of your samples? Is there a health risk?

- There will be a generation of any waste? How does the waste need to be disposed of?

- What other tools have processed your sample? Am I always going from clean to less-clean tools?

- Fill up the following form: Request Form.

- Make a help desk ticket to the tool you want to use