Tetramethylammonium hydroxide (TMAH) is a quaternary amonium salt mostly used in the production of integrated circuits (IC), printed circuit boards (PCB), and flat panel displays (LCD), and photolithography is the most common process used to manufacture these devices.
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In this process, a photoresist developer is used to transfer a pattern on a substrate. The chemicals used in the semiconductor industry must be exceptionally pure because even traces of contaminants have a negative effect on electrical properties.
The development stage is a critical step in photolithography, and in order to be successful, this process must be optimized in order to increase production efficiency. This Process Application Note presents a method to monitor the TMAH concentration in the developer solution via online process titration. This is a multiparameter analytical technique that can accurately monitor TMAH using a combined pH electrode.
For the production of semiconductors it is of the utmost importance to use extremely high purity chemicals. The presence of impurities (even trace concentrations) can significantly affect the material’s electrical properties. The same applies for the concentrations of the chemicals used during the production process. In back end of line (BEOL) processing, the photolithography process uses light to print thin film patterns from a photomask (an opaque plate with openings for light) on a micrometer (or smaller) scale with a light-sensitive photoresist chemical applied thinly on the silicon wafer.
After a certain exposure time, the printed circuit is developed and the photoresist can be stripped away in preparation for the next steps (Figure 1). Tetramethylammonium hydroxide (TMAH, N(CH3)4OH) is an alkaline ingredient in photoresist developer kept at a concentration between 2.38– 2.62% in many applications (Figure 2). TMAH is highly effective in stripping off the acidic photoresist as it becomes soluble in the developer. TMAH-based photoresist developers have replaced many traditional developers (such as KOH and NaOH) since these processes increasingly need to be metal-ion-free.
A concentrated solution of TMAH (25%) is diluted in the Chemical Central Supply System (CCSS) and the appropriate percentage is then added to the production line. Used TMAH developer containing the photoresist residue is returned and more TMAH is added to adjust the concentration. Once the amount of residue has reached a certain level, the waste is removed. A purification unit can be used to minimize TMAH in the waste stream.
From: On Behalf Of Matthieu Nannini, Dr.
Sent: May-19-11 8:29 AM
To:
Subject: Re: [labnetwork] TMAH
Here's an article relating the tragic event.
http://joh.med.uoeh-u.ac.jp/pdf/E50/E50_2_01.pdf
From: On Behalf Of Robert M. Hamilton
Sent: May-18-11 4:43 PM
To: Matthieu Nannini, Dr.
Cc:
Subject: Re: [labnetwork] TMAH vs KOH
Matthieu Nannini,
At the Berkeley NanoLab we support both KOH and TMAH use at a dedicated work process station. We have not had issues with these processes. Having said this, we have added the following caution statement about TMAH exposure to our lab and safety manuals after our Office of Environmental Health & Safety shared data about fatal TMAH exposures in Taiwan:
"TMAH (Tetramethylammonium hydroxide) is widely used as a photoresist developer (2-3%) and for the anisotropic etching of silicon (10-15%). TMAH is a strong base and hazardous by ingestion, inhalation, skin (dermal) exposure and eye contact. In addition to alkalinity-related chemical burn, dermal exposure to TMAH may also result in respiratory failure and/or cardiac arrest. A study of case reports of Taiwan semiconductor factory injuries linked exposure of 25% TMAH to three cases of heart failure. It is important to treat TMAH skin exposure by flooding the effected area with water for at least 15 minutes and to report all exposures."
More generally, caustic burns (KOH or TMAH) can be worse than acid burns. It is difficult to cleanse the orbit of an eye when exposure occurs and caustic quickly penetrate tissue. While 15 minutes may seem an inordinate amount of time for a topical exposure to a chemical, trauma specialists who we've consulted tell us such long rinses have definite value.
Sincerely,
Bob Hamilton
Robert M. Hamilton
Marvel NanoLab
University of CA at Berkeley
Rm 520 Sutardja Dai Hall
Berkeley, CA -
Hello Mattieu,
IMHO, from a health perspective, TMAH appears far more dangerous than KOH. We recently prepared a lab-wide notification concerning TMAH and its known but often overlooked dangers based on skin adsorption as opposed to ingestion as an identified route of exposure.
For more TMAH Developerinformation, please contact us. We will provide professional answers.
"Materials Safety Bulletin
September 24,
TMAH: New Hazard Awareness Concerning an Old Chemical Tetramethylammonium hydroxide (TMAH) is widely used in micro- or nanofabrication as an etchant and developer. In MSD, TMAH is typically one of several ingredients in commercial etching/stripping mixtures, although it may also be used as a pure chemical.
While it has long been known to be very toxic if ingested, recent industrial experience indicates that skin exposure may result in serious injury/illness or even death. Since , there have been 3 recorded fatalities from skin exposure to TMAH solutions as dilute as 25%. Two of the recorded fatalities occurred due to heart attack despite immediate decontamination and prompt medical care. Thus skin, exposure to >1% TMAH over a few percent of the body must be treated as a life-threatening event. MSDSs may be out of date and not properly describe this high dermal toxicity.
When handling this material, the minimum set of personal protective equipment includes:
safety goggles (not glasses), disposable nitrile gloves, a buttoned lab coat, leg covering and closed-toe shoes. The disposable gloves may only provide brief protection and must be replaced if they become wetted. More protective gloves are the Stansolv or Tri-Ionic glove models sold by MAPA.
In the event of a splash, contaminated clothing must be removed and the wetted area thoroughly washed with soap and water, using the emergency shower if necessary.
Call 911 and summon emergency medical help.
You should not handle TMAH containing materials when working alone.
Tri-Ionic clean room gloves provide excellent protection from TMAH exposure."
David A. Bunzow
User Facilities Program Manager
The Molecular Foundry
Materials Science Division
Lawrence Berkeley National Laboratory
1 Cyclotron Road MS 67-
Berkeley, CA
On 5/18/ 9:20 AM, Matthieu Nannini, Dr. wrote:
Dear all,
Having only one base dedicated bench running TMAH @ 85C I have now to make a choice between TMAH and KOH since new researcher are asking for KOH. Maintaining the 2 chemicals requires resources that I don't have.
Process wise, TMAH is better with oxide masks but KOH is better with SiN masks. KOH also etches faster and nicer.
My questions are in terms of safety: could you share your experiences with KOH vs TMAH in terms of ease of use, safety, etc...
Thanks
-----------------------------------
Matthieu Nannini
McGill Nanotools Microfab
Manager
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