Cleaning Methods & Cleanroom Procedures By Christiaan Bruinink and Meint de Boer

Contents Importance of cleaning Types of contaminants Cleaning methods Considerations for selecting a cleaning procedure Cleaning procedures NanoLab policy Persons to contact Why should you care? FAQ Take home message

Importance of cleaning The NL-cleanroom is a multi-application cleanroom for research & pilot-production. Device performance: The NL-cleanroom offers processes for optical, mechanical, chemical, electrical and 3D-nanoshaping applications. Process reliability: The NL-cleanroom persues to provide reproducible processes and/or properties of materials for scientific research.

In the NL-cleanroom four types of contaminants can have a negative effect on the device performance and process reliability: Metal contaminants Organic contaminants Ionic contaminants Particle contaminants Types of contaminants

Sources: stripping of metals, plasma processing, physical contact with metal surfaces (substrate holders, tweezers, masks). Examples: gold, silver, copper, iron, chromium, cobalt, etc. Negative effects: Shortage in semiconductor devices Scattering and absorption of light in optical devices Cross-contamination of equipment Metal Contaminants

Organic Contaminants Sources: stripping in organic solvents, airborne molecules, dirty waferboxes, hotplates, substrate holders, tweezers, masks. Examples: residues of resists and organic solvents, dust, skin cells, etc. Negative effects: Bad adhesion of films Carbonization of organic traces at high temperatures Cross-contamination of equipment

Sources: etchants, glass substrates, Quick Dump Rinser (QDR), dirty beakers and wafer carriers. Examples: wet-chemical etching of silicon in KOH, stripping of metals, insufficient rinsing of substrates in QDRs, wet transport in dirty beaker. Negative effects: Shortage in semiconductor devices Scattering of light in optical devices Cross-contamination of equipment Ionic Contaminants

Sources: mechanical processing. Examples: silicon particles after dicing, encoding or cleaving of substrates, SiO 2 particles after chemical mechanical polishing (CMP). Negative effects: Defects in resist coatings Bad wafer bonding (microvoids) Scattering in optical devices Cross-contamination of equipment Particle Contaminants

Cleaning Purpose of cleaning: to remove of particulate and chemical impurities from the surface without damaging the surface of the substrate. Cleaning methods have been developed for cleaning of silicon substrates. Cleaning methods are wet-chemical processes.

Chemical agentContaminantsComments Ozone Steam (O 3 /H 2 O) Organics Metals Simultaneous removal of organic and metal traces HNO 3 Organics Metals 99% HNO 3 69% HNO 3 at 95 o C RCA-1 (NH 4 OH/H 2 O 2 /H 2 O) ParticlesPost CMP (SiO 2 particles) RCA-2 (HCl/H 2 O 2 /H 2 O) Ionics Metals Post KOH (alkali metal ions) Post metal stripping Cleaning Agents

UCL clean in Ozone Steam: compatible with front-end electronic processing, e.g. gate oxidation, B/P diffusion and ALD (equipment with UCL label). Pre-furnace clean in HNO 3 : compatible with high-temperature processing, e.g. LPCVD, annealing, dry and wet oxidation. In-line clean in HNO 3 : compatible with lithography, sputtering, etching, dicing, etc. Post clean in RCA solutions: obligatory after CMP (RCA-1) and KOH etching/metal stripping (RCA-2). Cleaning Methods

RCA-1 removes particles from the surface by dissolving the thin native SiO 2 layer and forming a new SiO 2 by oxidation of the silicon surface (self-cleaning effect). CAUTION: RCA-1 is active for about 15 mins and etches silicon at higher concentrations of NH 4 OH, therefore the exact composition of the solution is critical! RCA-1: NH 4 OH:H 2 O 2 :H 2 O = 1:1:5 (v/v %) at o C for 10 mins. RCA-1 (post-CMP)

RCA-2 removes metal and alkali-metal ions from the surface by oxidation and complexation. CAUTION: RCA-2 is active for about 15 mins. RCA-2: HCl:H 2 O 2 :H 2 O = 1:1:5 (v/v %) at o C for 10 mins. RCA-2 (post KOH)

Consult the Equipment database: Type of substrate Type of coating(s) on the substrate Process history of the substrate (equipment) Type of (possible) contaminants on the surface* * Excl. indiffusion of metals into a substrate or coating due to temperature processing. Considerations for Selecting a Cleaning Method

Pre-furnace Clean

Cleaning sequence 1.Clean: removal of contaminant(s) from the surface 2.Rinsing: removal of chemical agent(s) from the surface - quick-dump rinsing - demi-water - resistivity > 10 M  cm - do not store the substrates in a beaker with demi-water 3.Drying: removal of surface water - semitool or single-wafer spinner - critical: avoid drying of substrate in air (traces or drying spots)

Lab policy Cleaning must be done < 4 hours before loading your substrates into any equipment. You are responsible to store the substrates in a clean wafer box. Conditions for transfer substrates between equipment without cleaning: - time interval is < 4 hours - transport in a clean wafer box

Lab policy All new materials and chemicals must undergo review prior to use in the cleanroom. This is to ensure that potential issues of cross-contamination and safety are carefully taken into consideration before approving new processing. ‘New’ includes not only materials or chemicals that have not been used in the cleanroom before, but also: - standard materials or chemicals from unknown source. - use of standard materials or chemicals in a different application lines (e.g. H2 LPCVD Si 3 N 4 in UCL).

Persons to Contact New materials Meint de Boer (process advice), Christiaan Bruinink (quality control), Peter Linders (safety) and the administrator of the equipment. New chemicals Peter Linders (safety), Marion Nijhuis and Samantha Ooijman (wet benches). Samples of unknown source Meint de Boer (process advice) and Christiaan Bruinink (quality control).

Why should you care? YOUR actions affect OUR process reliability. Any contamination you introduce in our cleaning lines due to YOUR actions can ruin not only YOUR device performance but also the results of OTHER users! Take responsibility of your actions: NEVER deviate from the cleaning procedures. DIRECTLY report a misstep to the staff. ALWAYS alert another user if he/she makes a misstep. ONLY work with approved process flows. DISCUSS any modification in the process flow with the staff. DISCUSS any rework in your processing with the staff.

FAQ How to remove traces of metals after wet-chemical stripping of metals?  RCA-2 (WB09) How to remove traces of alkali metal ions after etching silicon in KOH?  RCA-2 (WB09) How to remove particles after chemical mechanical polishing?  RCA-1 (WB09)

FAQ What is the procedure to prepare the RCA-1 or RCA-2 solution?  Contact the administrator for a training.  ALWAYS prepare a new cleaning solution. What is the right moment to prepare the RCA-1/RCA-2 solution?  Wafers are ready for cleaning. What is the right moment to clean in the RCA-1/RCA-2 solution?  Submerge your samples directly after adding the H 2 O 2 to the solution at a temperature of o C (bubbles).

FAQ How to remove traces of organics?  HNO 3 (WB14)  Ozone Steam (WB12) Where can you find the cleaning procedures?  Go to the Process flow database on the MIS website. Technology | Process flow database What to do if you cannot find a cleaning procedure for your substrate?  Contact Meint de Boer (process advice), Christiaan Bruinink (quality control) and the administrator of the equipment.

FAQ Why would you clean in private-use HNO 3 ?  HNO 3 is for free, so why bother?  I always use private-use HNO 3 for cleaning my wafers.  I do not trust the HNO 3 in the standard beakers (history).  I do not trust anyone in the cleanroom.  I do not trust my wafers.  I can not find a cleaning procedure in the database.  Contact Meint de Boer (process advice), Christiaan Bruinink (quality control), Marion Nijhuis and Samantha Ooijman (wet benches)

Take home message Don’t allow bad habits to propagate. Be conscientious about your own habits and don’t be shy about correcting other users. Don’t shortcut policy to get your own work done. We ALL make mistakes. Be professional – take responsibility. Inform the staff immediately so we can minimize the effect on other user’s processing. Ask questions to the staff if you have any doubts. Remember: This is a multi-application facility. Everything you do affects not only your work, but that of your fellow lab members.

Literature/Links Kern, W. (1994). Handbook of Semiconductor Wafer Cleaning Technology. Park Ridge, New Jersey: Noyes publications. Mitall, K.L. ( , volume 1-6). Developments in surface contamination and cleaning. Van Rossum, F. Ozon stoom cleaner (internal report). The Process Flow database and the Equipment database are found on the MIS homepage.