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Optical Photolithography Overview

Part I:  Description of Process Steps: 

Photomasking or photolithography forms the core of device fabrication.  The equipment here are grouped in order of process function: Resist Coat; Exposure; Develop; and Oven Bake & Cure.  Broken down, the basic process steps of photomasking (and the SNF equipment used) are described below.

 

For a description of basic lithography problems, see the Process document found at Process > Process Modules > Photolithography > Common Photolithography Problems at SNF. The direct link is:

https://snf.stanford.edu/SNF/processes/process-modules/photolithography/common-photolithography-problems-at-snf

 

For more detailed descriptions about these process steps and their influence on pattern quality, see the Microchemicals website, located at:  http://www.photoresists.eu

 

·Singe and Prime: 

Singe bake at 150 C drives off moisture on the surface of the substrate which will interfere with the Prime step.  "Prime" is the vapor deposition coating of the substrate surface by hexamethyldisilazane (HMDS).  Surfaces that are normally hydrophillic (such as silicon with native oxide, silicon oxides and nitrides) become hydrophobic when coated with HMDS, which allows for better resist coverage and adhesion.

 

·Spin coat: 

Resist, in viscous liquid form, is dispensed onto a spinning substrate.  By controlling the dispense and spin speeds, resist spreads across the wafer in a uniform coat of desired thickness.  Additionally, solvent may dispensed on the wafer backside, in order to ensure the wafer backside is clear of resist, and on the frontside wafer edge, to remove resist on the edge (“Edge Bead Removal” or “EBR”).  Residual resist on the wafer backside and edges can result in processing problems later on, particularly during expose.  EBR is especially helpful in eliminating resist particulates (by removing “threads” of resist which form by spinning off at the  wafer edge) and prevents sticky resist problems in equipment with automatic wafer handling.  There are many formulations of resist, depending on the application and patterning need.   Check the process pages for more information about specific resist characteristics.

 

·Pre-bake: 

This drives off excess solvent and turns the resist from a liquid into a film.  Before pre-bake, wafers are considered to contain hazardous chemicals and may be handled only under exhausted areas of the lab.  Once wafers undergo pre-bake, they may be handled in the open.

 

·Expose/Align: 

Optical resists are patterned by exposure to UV illumination.  Exposure is performed through a "mask",  which is a transparent plate custom-patterned with UV-opaque features. This process is also called "alignment" since patterning of resist on a substrate already containing another pattern requires alignment of the mask to the existing pattern.  UV exposure tools generally fall into two classes:  "contact aligners" in which the mask material is brought into direct physical contact with the resist-coated substrate; and "reducing steppers" in which the mask image is optically reduced and projected onto the resist-coated substrate.

 

·Post-expose bake: 

This is used to “set” chemically-amplified resists, such as the 3612 resist, so that finer features and smoother vertical-wall profiles can be achieved.  This is optional – whether you need to do this depends on your process requirement – but is recommended for desired resolutions of less than 2 microns.

 

·Develop:  

Removes resist in UV-exposed areas (positive resists) or in UV-protected areas (negative resists).  The developer chemical required will depend on the specific resist used.  Generally less critical to litho quality than other steps.

 

·Post-develop bake:

To drive off moisture, harden the resist or reflow sharp resist profile corners.  This is built into tsome process runsheets (such as the EE410 process flow) but considered optional, depending on your process needs. 

 

Part II:  Equipment

Table I:  Litho Tool Process  Sequence

Here is a summary of the sequence of tools used for standard photo processing.  The shaded areas indicate tools with integrated process modules. 

 

Singe

Prime

Coat

Prebake

Expose

PE Bake

Develop

PD Bake

ovensinge

svgcoat2

svgcoat/2

svgcoat/2

evalign/2

hot plate

svgdev/2

svgdev/2

yes

yes

headway2

oven90c

karlsuss/2

svgdev

wbmiscres

hot plate

 

 

laurell-R/3

hot plate

asml

fusion

 

oven90c

 

 

 

 

ev-imprint

oven110c

 

oven110c

 

Similar processing and equipment sequence generally applies to Electron Beam patterning of substrates, except that the resist are dispensed manually (headway2 or laurell-r/3)– the developers are usually solvents (as opposed to aqueous alkaline solutions used in photopatterning) so Develop processing is done at wbebeam.

 

Table II:  Limits and Capabilities of Litho Tools

The specific equipment choice would depend on your substrate size/shape, the resist or other chemical used, and any other special process requirements.  Table II provides a general guide; for more specific detail, particularly with regard to acceptable chemistry, see the equipment and process documentation for that tool.

 

Coral Name

Pieces

4” or
100mm

6”or
150mm

Chemistry*

ovensinge

OK

OK

OK

No chemicals, no resist on substrates

yes

OK

OK

OK

HMDS only. No Chemicals or resist on substrates

oven90

OK

OK

OK

Resist coated substrates

oven110

OK

OK

OK

Resist coated substrates

hot plates

OK

OK

OK

Flexible

svgcoat/2

 

OK

 

Std resist (limited manual dispense chemistry)

headway2

OK

OK

OK

Flexible (SU8, polyimide OK)

laurell-R/3

OK

OK

OK

Flexible (no SU8,LOL2000 and  polyimide)

evalign/2

OK

OK

Ok

Flexible

karlsuss/2

OK

OK

Ok

Flexible

ev-imprint

OK

OK

 

Flexible

asml

 

OK

OK

Resist

svgdev/2

 

OK

 

Std Developer

wbmiscres

OK

OK

OK

Flexible

fusion

 

OK

OK

Resist

 

Many of these tools can also be used for processing substrates in a variety of treatments unrelated to photo- or ebeam- patterning.  Make sure to observe the equipment rules and consult with Staff members before trying anything that is not already documented at each of these stations.

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