Repair Instructions for IRON OXIDE PHOTOPLATES
There are two basic situations for repair of photomasks. The first situation involves the initial production of photomasks. The second situation involves photomasks that need repair as a result of damage during use.
2. Repair During Initial Photomask Production
2.1 After all processing steps have been performed from pre-baking through resist removal, the photomasks are 100% visually inspected under Class 100 Clean Room conditions. (refer to Drawing C-100371 Processing Procedures, Ferroxoplates Sensitized with Shipley Photoresist, Positive Working Type S-1400-30 or S-1800).
2.2 Defect Causes
Defects which exist in the photo images are usually caused by the following:
(A) Defects In The Iron Oxide Coating Prior To Mask Fabrication
These are usually minimum in number since the Iron Oxide Plates are thoroughly inspected prior to mask fabrication.
(B) Defects In The Photoresist Coating
Defects in the photoresist coating are usually the result of resist impurities and/or contamination by the environment.
(C) Defects In The Master Plates
Defects in the master plates are usually the most prevalent and must be repaired prior to exposure.
2.3 Inspection And Repair
2.3.1 Inspection and repair are usually performed at the same time under Class 100 Clean Room Conditions.
2.3.2 Repair of Iron Oxide images such as broken or torn images, pin holes, or intrusions is performed by the use of an epoxy-based semi-paste ink. The ink is a single compound and h as excellent resistance to solvents, chemicals and abrasion when cured in the recommended procedure. The ink is available in a variety of colors and can be matched to the color of the Iron Oxide. Our experience, however, indicates that if the ink has more contrast, it will be easier to observe the repair operation.
2.3.3 Tools & Equipment Required For Repair Operations
(A) Light Table: A suitable light table is required having transmitted light.
(B) Stero Microscope: A stero microscope on a long boom having a variable magnification.
(C) Epoxy Ink: Manufactured by Markem Corporation, Kean NH. 7224 Ink Series in 4 ounce tubes. Orange color 754 Product No. 8055300. Brown color 330-B Product No. 8054941.
(D) Epoxy Thinner Type XN - Product No. 8000138: Manufactured by Markem Corporation, Kean, NH. Added as needed to prolong working time.
(E) Epoxy Cleaner Type 320: Recommended for removal of uncured ink from tools, etc.
(F) Glass Palette: A piece of glass approximately 10cm x 10cm x 6mm used to mix inks.
(G) Thickness Monitor Mask: In essence a piece of metal shim stock 25 microns thick with a 3mm x 6mm aperture used to create a controlled thickness ink pool.
(H) Squeegee, Doctor Blade or Spatula: Used to squeegee the ink in the thickness monitor mask.
(I) Probe Pens: Precision needle points ground to various tapers used to transfer ink from palette to repair area on photomask.
(J) Straight Edge Block: Used as a guide for trimming excess cured epoxy from repair areas.
2.3.4 Establishing The Controlled Ink Thickness Pool
Using the glass palette and the thickness monitor mask, apply ink into the aperture and squeegee the excess ink using a spatula. Carefully remove the mask. This should leave a raised deposit on the palette approximately 6mm x 3mm x 25 microns thick.
2.3.5 Transferring The Ink From The Palette To The Repair Area On The Photomask
Ink is transferred from the controlled thickness ink pool on the palette to the photomask by the use of a pen probe. It is important to dip the pen probe into the ink in a vertical position so that the ink on the top of the pen is only 25 microns in depth. This will limit the volume of ink transferred.
2.3.6 Applying The Ink To The Area To Be Repaired
The pen will transfer only sufficient ink to patch very small defects. Do not apply more than necessary to cover the defect. When patching an edge defect, be sure to allow sufficient over patch so that this can be trimmed off after curing.
2.4 Curing The Ink
After all inspection and repairs have been performed, the photomask is baked in a convection oven at a temperature of 180 C for a period of 45 minutes. This baking cycle has two functions, one to cure the epoxy ink and the other to enhance the Iron Oxide Film hardness and chemical resistance.
NOTE: Temperatures should be raised gradually and reduced gradually to eliminate thermal shock to the glass.
2.5 Trimming The Excess Cured Epoxy Ink From Edge Defects.
Using the straight edge block and a sharp pen probe, the excess epoxy can be trimmed very accurately to conform to the edge of the photo image.
2.6 Final Washing
Wash in photo flow, rinse in de-ionized water and alcohol vapor dry.
3. Repair Of Used Photomasks
In some cases it may be necessary to repair photomasks that have been damaged during use. The procedure for repair is similar to that described above, except that the final baking cycle is still 180 C for 45 minutes. Photomasks should be thoroughly cleaned, however, prior to the repair operation.