Photomask Fabrication

Unit 2 Photomask Fabrication

By Dr. Ghanshyam Singh

Sharda University

Photomask FabricationA mask, or photomask, is a glass or quartz plate coated on one side with chrome.

What is Conventional Mask??

Conventional Mask pattern generator uses step-and -repeat and contact printing photolithography method to transfer the circuit layout designs from CAD into the photomask for circuit requiring minimum line width of < 1m.

What is Modern mask??Modern mask pattern generation systems use precision lasers or electron beams to image the design of one layer of an integrated circuit (IC), or chip, onto the mask especially for circuit requiring minimum line width of <1m. After the design has been exposed on the mask, the pattern is etched into the chrome, and the mask is inspected.

Mask-making process Flow CIRCUIT Layout

Digitizing

Flow A Pattern Generation (reticle)

CIRCUIT Layout

Flow B Reticle Flow C Master plate

Master Plate processing

Contact print substrate

Working plate processing

Wafer exposure (Contact, proximity, projection)

Reticle Processing

Wafer exposure (stepper)

Mask Processing

Wafer exposure (proximity, Projection)

Conventional Modern

Laser/ E-beam direct write

Conventional Photomask Fabrication

1. Circuit Layout 2. Data Digitising 3. Photomask Coat Materials and Sizes 4. Pattern Generation5. Step & repeat reduction into master

copy mask plate6. Contact print working mask plate

Circuit Layout

In the circuit design process, patterns which represent the circuits are created by the chip designer. These patterns are then sent via magnetic media or electronically to the mask shop where the pattern data is prepared for mask manufacturing.

Data Digitising

Early photomasks were cut by hand in a material called rubylith, a sandwich of a clear backing layer and a thin red layer of Mylar. The red layer was cut with a stylus and peeled off, leaving the desired pattern in red. The original rubylith copy of the mask was 100 to 1000 times larger than the final integrated circuit and was photographically reduced to form a reticle for use in a step-and-repeat camera. Today, computer graphics systems and optical pattern have largely supplanted the use of rubylith. An image of the desired mask is created on a computer graphics system.

Photomask Coat Materials and Sizes

The primary material used to make a mask is a quartz substrate that has a layer of chrome on one side. The chrome layer is covered with an anti-reflective coating and a photosensitive resist. Mask sizes range from three to nine inches square, but most masks produced

today are five or six inches square.

Pattern Generation

Once the image is complete, files containing the commands needed to drive a photolithography pattern generator are created on magnetic tape or disks. A pattern generator consists of a light source and a series of motor-driven shutters. The pattern generator uses 4 flash lamps to expose the series of rectangles composing the mask image directly onto a blank photographic plate called the reticle. The chrome-covered mask or reticle coated with a layer of photoresist is moved under the light source as the shutters are moved and opened to allow precisely shaped patterns of light to shine onto the resist creating the desired pattern.

Pattern Generation (Continued)

The reticle or mask is processed through the development, develop inspect, chrome etch, resist stripping, and final inspection steps that transfer the pattern permanently into the chrome layer as follow:(a) Etch and resist stripping (b) Defect Inspection (c) Repair (d) Metrology Inspection (e) Cleaning (f) Final Inspection

Step & Repeat reduction

After the creation of the reticle of the circuit image,the reticle pattern is transferred to a new resist-coated mask blank by a step-&-repeat camera in order to reduce the reticle images into the final size on a master copy mask plate. After each of the exposure steps, the reticle or mask is processed through the development, develop inspect, chrome etch, resist striping, &final inspection that transfer the pattern permanently into the chrome layer of the master copy mask plate. This process gives a master copy of the actual circuit features.

Contact print working mask plate

The master copy mask plate is then used to create multiple working mask plates in a contact printer using photolithography. This tool brings the master copy mask into contact with a resist-coated mask blank and has a UV light source for transferring the image from the master copy plate into the working copy plate. After the exposure steps the reticle or mask is processed through the development, develop inspect, chrome etch, resist stripping, & final inspection that transfer the pattern permanently into the chrome layer of the working mask plate. Inspections are again very critical since any undetected mistake or defect has the potential of creating thousands of scrap wafers.

Advanced Photomask Fabrication

For ULSI fabrication, the minimum features or line widths of the devices in the circuits are reduced from 1m to approaching 0.1 m or lower. This requires modern mask pattern generation systems that use precision lasers or electron beams or writers to image the design from CAD of an integrated circuit (IC), or chip, onto the mask. This method is fast, direct and requires fewer processing steps, but the image writer systems used are much more expensive.

Advanced Photomask Fabrication

X-ray mask fabrication

Target parameter: <0.15m minimum feature size; then the width of the absorber pattern should be controlled within less than 10nmConventional method: (a) A silicon wafer is usually used as a starting material,(b) The silicon wafer is coated with layer of the membrane material such as silicon nitride or silicon carbide.(c) The back of the silicon wafer is patterned lithographically to protect the outer ring. The membrane material at the backside of the silicon wafer is then etched away,(d) The silicon wafer is etched through backside in a long silicon etch that remove most of the silicon wafer with the remaining wafer material forming a ring beneath the membrane.(e) The ring is then bonded onto a glass ring for additional strength and mechanical stability.

X-ray Mask Fabrication(Continued)

(f) The next process is to deposit the absorber layer on the membrane. A thin gold or tantalum is deposited by electroplating on the completed mask blank, followed by a thick stencil resist layer which is baked. Thin layers of chromium and imaging resist are deposited subsequently on the stencil resist.

(g) The top imaging resist layer is exposed, developed and used to pattern the chromium in chromium etch solution.

(h) The patterned chromium acts as hard etch mask during the RIE etch of the thick stencil resist.

(i) The chromium is then stripped and a layer of gold is then deposited on the resist trenches by electroplating.

(j) Finally the resist is stripped and the gold/tantalum base is removed leaving the patterned gold absorber layer on the membrane.

Problems in X-ray mask fabrication

Distortion of mask (membrane and absorber) patterns

a) placement errors associated with e-beam writing on the resist patterning that is transferred to the mask membrane and absorber patterns.

b) non-uniform clamping of the maskc) different thermal expansion of the different

layers that may change the stress on the mask materials during processing and thus different degree of relaxation.