GENERAL STEPS INVOLVED IN MAKING A SET OF MASKS

• Device Data Preparation
• Processing and Mask Layer Design
• Mask Image Transfer Exposure to the Mask Medium
• Development for Mask Pattern
• Exposed Metal Etching
• Stripping/Cleaning/Drying
• Inspection and Measurement under microscope

The photo masks are essentially optical fabrication tools for the device. First the design drawing package for the device is prepared. This is available in the form of a 3D solid model or a conventional Plan, Front View and Side View drawings in true scale. Based upon prior wafer processing experience, a sequence of masking steps are designed so that one or more raw wafer substrates can be micromachined to yield the designed device. This iterative process reduces the masking steps into a number of overlapping masking layers, each modifying the substrate incrementally in desired locations. Often the wafers are processed from both sides, and hence, a front-to-back pattern alignment has to be done. This alignment has to be nearly perfect (within 1 micron near the outer edges of the wafer). When multiple substrates are bonded together, the facing wafer-to-wafer alignment is also very critical. A device can have as many as ten masking layers for single sided processing, or twice as many for double sided processing. For example, the ANDAS integrated sensor design shown in the figure above required 12 masking layers.

Once the key parameters of the masking layer are determined, the mask artwork data file is generated. Each mask has a unique ID, which may include a device name, mask layer number, production date, or some other helpful information for the operator (e.g. RIGHT READING FACE DOWN) printed outside the pattern area along the edge of the mask. Other data required for the mask includes:

• Step and repeat device patterns within the wafer size;
• Mask Layout preparation;
• Blocking of device patterns landing on vacuum holes of the Aligner chuck;
• Insertion of Alignment Marks and Structures for the double sided alignments, if any
• Insertion of Alignment Marks for the successive alignments on the same side;
• Insertion of critical etch depth and dimension bar patterns;
• In-process wafer measurements and testing sites and structures;
• Mask Processing (exposure and development) Rules Verification (separation of various types structures in close proximity e.g. rules for a series of alternating opaque and clear strips);



Next, a Layout drawing for the mask is prepared

If the mask art work is constructed in true scale dimensions, any dimensional error made in the artwork gets transferred to the mask in the same scale, i.e. 1 micron error in the artwork shows up as 1 micron error. To make the masks more accurate, the artwork is generally prepared in 10X or 100X scale; and is optically reduced to 1/10 or 1/100 scale on the mask. This reduces any error made in the artwork by the same factor on the mask, i.e. 1 micron error in the artwork reduces to 0.1 or 0.01 micron on the mask. Thus, the mask layer artwork can take the form of :

• 10X or 100X Black and clear Transparency plots made by large format HP printer;
  
• If the artwork is hand drafted, we can still use Rubylith or Amberlith to make the 10X or 100X film for the mask;
• Gerber Autoprep 2000 is employed for digitizing and laser preparation of large format mask film made from hybrid composites (plotted drawings, Rubylith, laser printed transparencies)

Exposure for Mask Image Transfer to the Mask Medium

Next, the artwork as prepared above is back illuminated with proper wavelength light:

• A 48"X48" light board (with high UV content fluorescent lights mounted in a reflective chamber
  with a diffuser in front) is used to backlit the transparency. The transparency is aligned with
  marks on the light board so as the center of the layout approximately coincides with the center of
  the mask blank;
• A large format camera is used to capture the transparency image directly on to a photomask blank. Depending on the photoresisit coated on the blank, the image on the mask is either inverted negative or positive of the original print.
• The picture on the left shows a 4”X5” film holder converted to adapt a 4”X4” mask blank.
  
• The picture on the right shows a large format camera being loaded with a mask blank.

The set-up shown has been designed to produce a 1/10 inverted image of the transparency layout. A special lens is used that provides uniform reduction of the illuminated object devices from the edges and corners to the center of the layout. The large format film cassettes have been modified to accept mask blanks with photoresist side facing the lens. For best reproduction, the focus, exposure time, aperture and f-stops are pre-determined using trial 4”X5” films. These camera settings are also influenced by the type of photoresist (and its softbake time) applied on the mask blank.

The process of loading the mask blank into the film holder, its insertion behind the camera bellows, and the exposure are all performed in safelite conditions.

Development for Mask Pattern

Depending upon the photoresist coated on the mask plate blank, a suitable developer is used to develop the mask. If a positive photoresist is involved; wherever it is exposed to UV light it gets dissolved by the developer. Only the photoresist in unexposed regions is left revealing the metal coated on the mask blank. This remaining photoresist serves as a protective cover for the metal underneath it. The mask is cleaned and dried. For protective handling, the photoresist is hardbaked.

Metal Etching

In this step, the mask with hardbaked photoresist pattern (with the exposed through metal) is placed in a chemical bath appropriate for etching the metal. Careful processing controls are exercised to make sure that the metal is neither under etched (making the clear glass pattern translucent or making the clear openings narrower than the design) nor over etched (undercutting the photoresist boundaries or making the clear region dimensions wider than the design). After the metal is etched, the clear glass pattern of the mask appears which can be seen by shining light through.

Stripping/Cleaning/Drying

Up to this point, the mask still has the protective photoresist saving the opaque regions of the mask. The mask is placed in a bath of chemical that completely dissolves this protective photoresist. Slight agitation and bubbling air separates the photoresist from the mask plate. Next, the mask is thoroughly cleaned and dried with nitrogen.

Inspection and Measurement under microscope

Finally, the mask is visually inspected against the shining light for obvious major defects and flaws. For submicron level anomalies, a high quality microscope with x-y stage micrometers is employed to inspect the mask. The etching pattern is inspected for under or over etching, smoothness of the pattern edges. The actual dimensions of the mask patterns are measured and verified.

Some Important Mask Making Caveats

• Careful considerations can reduce the errors introduced during successive image transfers to less than 0.01 micron.
• All masks in a set for a device should be made at the same time under the same conditions (temperature, strengths of chemicals used).
• Artwork for all mask layers should be made at the same time to avoid errors introduced by the printer or artwork making equipment.
• All exposure and mask development and metal etchings should be done under the same conditions.
• In a set of masks, generally a defect or error in a mask layout shows up in all succeeding masks. In most cases, all succeeding layers have to be also corrected.




For a brief slide show, click on Photomask Production.