New services and Mask Making 101

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New services andMask Making 101
Li/Bachman Foundry Team UC Irvine August 2005
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The Li/Bachman MEMS foundry
Existing standard services
New services to add

1. LithographyFor surface patterning, special
   processes
2. Silicon/metal etchUsually deep etch, primarily
   for PDMS molds
3. SU-8 processing Basic structures, for molds,
   small plastic parts, etc.
4. ElectroplatingFor metal parts.
5. CoatingsPECVD, metallization, for specialty
   applications

1. Mask-makingMasks for lithography
2. E-beam lithographyUltra-high resolution
   patterning
3. Steel cutting Meso-scale precision cutting in
   steel sheets
4. Precision injection moldingFor microfluidic
   devices
5. Electrodeposited polymerFor micro-adhesives,
   polymer MEMS

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The Li/Bachman MEMS foundry
Mask arrives from vendor
Foundry team performs fabrication process
Part is characterized and delivered to customer
Customer is billed for services
The Li/Bachman foundry service is the prototype
service for a larger MEMS foundry that will be
implemented in the INRF. The foundry is built
around several module processes that are
repeatable and reliable.
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Photomasks
A photomask is flat transparent surface with
opaque regions on it that define geometries to be
fabricated. Light is shone through the mask to
create lithographically defined regions on a
substrate.
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Mask fields
Masks can be printed on plastic or glass (or
quartz) materials. Generally speaking, we refer
to masks as dark field or bright (clear) field,
depending on whether the features are on an
opaque or transparent background.
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Film masks
Emulsion
Film
Film masks are made from transparent plastic
film polyester (Mylar) or acetate. The opaque
regions are made from a photosensitive silver
halide gel emulsion that is attached to one side.
This gel is hardened during development, but is
not scratch resistant.
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Glass masks
Glass masks are made from glass plates soda lime
(cheaper) or quartz (expensive). The opaque
regions are made from a metal layer that must be
etched away to reveal the mask pattern. Etching
requires the use of a photoresist layer. The
glass/metal is more resistant to scratches than
film.
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Glass transparency
Rayotekwww.rayotek.com
Transmission properties for glass are suitable
for UV exposure. For deep UV lithography (below
250 nm), quartz or fused silica masks must be
used.
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Film transparency
P. Crews. JAIC 1989, Volume 28, Number 2, Article
5 (pp. 117 to 125)
Transmission property of polyester is suitable
for long wavelength UV exposure only. Polyester
blocks some UV light, so is suitable for UV
lithography as long as the mask is thin (exposure
time may need to be increased).
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Mask writers
Laser imagesetter(does not require clean room)
E-beam writer(requires clean room)
Masks are made using laser imaging, photoimaging,
or e-beam imaging. In all cases, an electronic
drawing is created by moving a very thin beam of
light or electrons over the mask surface, then
developing the image. This is called rendering
the mask. The beam width is the aperture.
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Pellicles
Pellicle
Pattern
Glass
If the mask is to be used in a projection system,
then it can be covered with a transparent
pellicle. The projector has lenses that focus
the mask image on a far-removed substrate. The
pellicle keeps the mask clean of particles. Any
particles that land on the pellicle itself will
be out of focus.
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Contact printing
Glass
Pattern
Resist
Substrate
If the mask is to be used in a contact system,
then the emulsion or metal must be placed against
the photoresist layer during exposure. Any
particles on the pattern side will show up in the
photoresist pattern, so the mask should be
cleaned carefully before each use.
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Dies and streets
A mask may contain several copies of a single
pattern, arrayed in a grid, designed to be cut
out later. Each of these is called a die. Dice
are usually separated by streets which are
spaces for the cutting tool (diamond dicing
saw) to cut the dice.
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Step-and-repeat reticles
Source
Lens
Reticle
Lens
Diepattern
Substrate
A projection system can perform a step and
repeat operation to place many dice images on
the photoresist. In this case, the mask has a
special name reticle. Often a reticle is
designed at a larger size (such as 5X), the
reduced further in the step-and-repeat stage of
the exposure.
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Fiducials
Mask patterns may include additional features
that aid in positioning and alignment during
exposure. These are called fiducialsthey are
specific to the alignment system used by the
foundry company.
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Alignment marks
dark field
Alignment marks are used to help align a mask
with the previously defined layer. Alignment
marks must provide guides for x, y, and theta
alignment. For theta alignment, marks should be
well separated. Dark field masks should open
windows for alignment marks.
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Quality measures

1. ResolutionThe aperture size or pixel
   sizeminimum addressable position.
2. Critical dimension (CD)Smallest feature
   separation or line width that can be patterned.
3. Registration (layer-to-layer)Alignment tolerance
   when overlaying a mask on a patterned layer
4. Array alignmentPrecision of features from one
   end of the mask to another. Errors are caused by
   optics, plotting errors, rounding errors

Masks come in different qualities, depending on
the type of masks and the process used to make
them.
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Resolution vs. CD
Mask layout
Actual layout
Edges are rough
Critical dimension is a function of the aperture
size. The edges of a geometry are rough, and
several pixels are needed to form a circle.
Therefore, CD is usually roughly 6-10 times the
aperture, or resolution.
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Pattern defects
Pattern inspection (before and after lithography)
is critical for success. Dust and scratched can
contribute to poorly defines patterns.
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Mask specifications
Positive, Right Reading Emulsion UPPositive
(digitize dark), RREU
Positive, Right Reading Emulsion DOWNPositive
(digitize dark), RRED
Negative, Right Reading Emulsion UPNegative
(digitize clear), RREU
Negative, Right Reading Emulsion DOWNNegative
(digitize clear), RRED
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Rendering process
During rendering, the mask-writers computer
converts the graphic information into simple
shapes that it can raster or draw, usually
rectangles and lines. This is called
fracturing. Not all geometries are readily
fractured.
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Drawing formats

1. Postscript (EPS)A modern vector-based graphics
   format for the publishing industry. Almost any
   shape is possible.
2. Gerber 274X Legacy format for the printed
   circuit board industry. Simple shapes and lines
   only.
3. GDS-IIFormat for the integrated circuit (chip)
   industry. Simple shapes only.
4. DXFFormat for architecture and engineering.
   Simple outlines only. Not consistently
   interpreted among applications

Several common drawing formats. Each has
different structure and strategy for defining
geometric shapes. Conversion software is not
readily available.
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Available masks and processes
Name Aperture CD Cost File format Industry
Imagesetter 5080 dpi (film) 5 µm 30 µm 25/ 8x10 Postscript Publishing
Laser Photoplot (film) 0.625 µm 6 µm 70 / 8x10 Gerber 274-X PCB
Optical mask writer (glass) 0.4 µm 3 µm 800 / 5x5 GDS-II IC
E-beam mask writer (glass) 0.05 µm 0.7 µm 25k/5x5 GDS-II IC
Injection mold (Ni/poly) 0.4 µm 3 µm ??? DXF Specialty
Stencil sheet (steel) 5 µm 50 µm 200 Gerber 274-X PCB
Several industries use optical lithography, and
masks may be purchased from vendors in each
industry. Typical resolutions are shown here.
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Li/Bachman mask types

1. Lo-Res polymer filmPolyester film mask printed
   at 5080 dpi (5 µm aperture). Low cost, general
   purpose mask. Charge 25 per mask.
2. Hi-Res polymer film Polymer film mask printed at
   0.625 µm pixel size. Medium cost, general purpose
   mask. Charge 60 per mask.
3. Hi-Res glass maskGlass mask printed at 0.626 µm
   pixel size. Higher cost, reusable mask on chrome
   with antireflection coating. Charge 150 per
   mask (5 x 5 inch).
4. Stencil sheetStainless steel sheet with laser
   cut openings. Smallest opening allowed is 50 µm.
   Stencil thickness can range from 5 mil to 8 mil
   (125 - 200 µm). Charge 200 per stencil.
5. Hi-Res injection molded chipsCD injection
   molding process applied to microfluidic chips.
   Special service to be determined. Charge TBD.

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Li/Bachman mask policy

1. Users send encapsulated postscript (EPS)
   fileUsers draw their masks using high quality,
   low cost commercial software and our templates,
   then export to EPS at highest allowed resolution.
   User submits job to Li/Bachman foundry by web and
   specifies mask type.
2. Layout and inspection Foundry worker places mask
   drawings on layup editor and performs inspection.
   Up to four masks may be placed in a single layup
   document.
3. Export to EPS formatIf masks have errors, they
   are returned to user for changes. If not, mask
   layout is exported as EPS file.
4. Format conversionUsing commercial conversion
   software, masks are converted to appropriate
   format (EPS, Gerber 274X, DXF, GDS-II).
5. Final inspectionLook at converted file using
   viewer for last minute inspection.
6. Mask printedMask is sent to vendor by email or
   web submission. User is billed for mask process,
   depending on type of mask ordered.

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Mask inspections Lines
User should convert lines to closed polylines
with zero-width (or near zero-width) lines for
best results. Make sure geometries that are
supposed to touch, really do touch.
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Mask inspections Multi-polygons with overlap
Complex shapes can be drawn by overlapping
simpler closed polygons. Absolutely NO white
square on black square constructions. Non
Postscript languages dont know the difference
between white and black.
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Maskmaking software

1. Recommended drawing software (export to
   EPS)Macromedia FreehandAdobe IllustratorDeneba
   Canvas
2. Recommended layup softwareDeneba Canvas
   (preferred)Adobe Illustrator (OK, but has
   limited zoom)
3. Recommended conversion softwareBay Technology
   LinkCAD
4. Recommended viewing softwareEPS Adobe
   Illustrator or GSView (www.cs.wisc.edu/ghost/gsvi
   ew/)Gerber PentaLogix ViewMate
   (www.pentalogix.com/) DXF Brava DWG viewer
   (www.infograph.com/products/dwgviewer/)

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Preferred mask vendors

1. Page-One (www.pageonedigital.com)Desktop
   publishing, pre-press for printing
   industry.Accepts EPS format, email.Must specify
   5080 DPI (max resolution) for 5 µm pixels.Use
   for low res polymer film masks.
2. International Phototool Company
   (www.photoplotstore.com)PCB industry, laser
   photoplots.Accepts Gerber 274X format, web
   upload.Must specify 1/40.64th mil, 0.625 um
   grid.Use for hi res polymer film masks.Use
   for hi res soda-lime glass masks.

We currently use two mask vendors for our
photomasks. We have accounts already set of for
both of these vendors. Identify the account name
as UC-Irvine / Mark Bachman when ordering.
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Preferred specialty vendors

1. Stencils Unlimited (www.stencilsunlimited.com)Las
   er cutting service for PCB industry.Accepts
   Gerber 274X format, web upload.Prints up to 20
   x 20Use for stencil sheets.
2. Ritek, Ltd. (www.ritek.com)Largest CD/DVD
   manufacturer in Taiwan.Special arrangement with
   UCI/INRF.Accepts DXF format, email
   upload.Details still in developmentUse for hi
   res injection molded chips.

We will begin to use new vendors for specialty
processes. This is still in progress.
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Coming attractions

1. Mark will prepare softwareInstall design, layup,
   conversion, and inspection softwareMake
   templatesBuild web siteDevelop tracking system
2. Richard will begin accepting mask filesPerform
   inspectionPerform layup, concersion and
   inspectionSupervise foundry work
3. Foundry team will begin job trackingMonitor job
   statusMonitor work performedAccurate billing
   system

We will begin the new foundry service by the Fall
quarter 2005. That is less than one month away!