Photoresist

Positive Photoresist

A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The unexposed portion of the photoresist remains insoluble to the photoresist developer.

Negative Photoresist

A negative photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.

Differences between Positive and Negative Resist

Note: This table is based on generalizations which are generally accepted in the Microelectrionmechanical systems(MEMS) Fabrication industry.

Types of Photoresist

Based on the chemical structure of photoresists, they can be classified into three types:

Photopolymeric, photodecomposing, photocrosslinking photoresist.

Photopolymeric Photoresist

Photopolymeric photoresist is a type of photoresist, usually allyl monomer, which could generate free radical when exposed to light, then initiates the photopolymerization of monomer to produce a polymer. Photopolymeric photoresists are usually used for negative photoresist, eg. methyl methacrylate.

Photodecomposable Photoresist

Photodecomposing photoresist is a type of photoresist that generates hydrophilic productunder light. Photodecomposing photoresists are usually used for positive photoresist. A typical example is azide quinone, eg. diazonaphthaquinone (DQ).

Photocrosslinking Photoresist

Photocrosslinking photoresist is a type of photoresist, which could crosslink chain by chain when exposed to light, to generate an insoluble network. Photocrosslinking photoresist are usually used for negative photoresist.

(1) SU-8

(2) Off-Stoichiometry Thiol-Enes (OSTE) polymers

Self-assembled Monolayer (SAM) Photoresist

For SAM photoresist, first an SAM is formed on the substrate by self-assembly. Then, this surface covered by SAM is irradiated through a mask, similar to other photoresist, which

generates a photo-patterned sample in the irradiated areas. And finally developer is used to remove the designed part (could be used as both positive or negative photoresist).

Resolution

Resolution is defined as the ability to differ the neighboring features on the substrate. Critical dimension (CD) is a main measure of resolution.

The smaller the critical dimension is, the higher resolution would be.

Contrast

Contrast is defined as the gradience from exposure portion to unexposure portion. The Higher contrast is, the more obvious difference between exposure portion and unexposure portion would be.

Sensitivity

Sensitivity is defined as the minimum energy that is required to generate a well-defined feature in the photoresist on the substrate. Unit: mJ/cm².

The sensitivity of a photoresist is important when using deep ultraviolet (DUV) or extra-deep ultraviolet (EUV).

Viscosity

Viscosity is defined as a quantity of the photoresist, expressing the magnitude of internal friction. Specific gravity (SG) is a measure of viscosity of a photoresist. Unit: poise or cps, 1 cps = 0.01 poise.

When it is needed to produce a thicker layer, the photoresist with higher viscosity will be preferred.

Adherence

Adherence is defined as the adhesive strength between photoresist and substrate. The inefficient adherence of photoresist to substrate may lead to the deformation of the generated features.

Anti-etching

Anti-etching is defined as the ability of a photoresist to resist the high temperature, different pH environment or the ion bombardment in the possess of post-modification.

Surface Tension

Surface tension is defined as the tension that induced by a liquid tended to minimize its surface area, which is caused by the attraction of the particles in the surface layer.

In order to better cover the surface of substrate, photoresists are required to possess relatively low surface tension.

DNQ-Novolac photoresist

One very common positive photoresist used with the I, G and H-lines from a mercury-vapor lamp is based on a mixture of diazonaphthoquinone (DNQ) and novolac resin (a phenol formaldehyde resin). DNQ inhibits the dissolution of the novolac resin, but upon exposure to light, the dissolution rate increases even beyond that of pure novolac. The mechanism by which unexposed DNQ inhibits novolac dissolution is not well understood, but is believed to be related to hydrogen bonding (or more exactly diazocoupling in the unexposed region). DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26N tetramethylammonium hydroxide (TMAH) in water).

Chemical structure and mechanism of DNQ-Novolac photoresist are shown.

Epoxy-based polymer

One very common negative photoresist is based on epoxy-based polymer. The common product name is SU-8 photoresist, and it was originally invented by IBM, but is now sold by Microchem and Gersteltec. One unique property of SU-8 is that it is very difficult to strip. As such, it is often used in applications where a permanent resist pattern (one that is not strippable, and can even be used in harsh temperature and pressure environments) is needed for a device. Mechanism of epoxy-based polymer is shown in 1.2.3 SU-8.

Off-Stoichiometry Thiol-Enes(OSTE) polymer

In 2016, OSTE Polymers were shown to possess a unique photolitography mechanism, based on diffusion-induced monomer depletion, which enables high photostructuring accuracy. The OSTE polymer material was originally invented at the KTH Royal Institute of Technology, but is now sold by Mercene Labs. Whereas the material has properties similar to those of SU8, OSTE has the specific advantage that it contains reactive surface molecules, which make this material attractive for microfluidic or biomedical applications.

Chemical structure and mechanism of OSTE polymer as negative photoresist are shown.

Microcontact Printing

Microcontact printing was described by Whitesides Group in 1993. Generally, in this techniques, an elastomeric stamp is used to generate two-dimensional patterns, through printing the “ink” molecules onto the surface of a solid substrate.

Step 1 for microcontact printing. A scheme for the creation of a polydimethylsiloxane(PDMS) master stamp. Step 2 for microcontact printing A scheme of the inking and contact process of microprinting lithography.

Fabrication of printed circuit boards (PCB)

Printed circuit boards is one of the most important applications of photoresist. The general is that by applying photoresist, exposing image to ultraviolet rays, and then etching using iron chloride, cupric chloride or an alkaline ammonia etching solution to remove the copper-clad substrate.

Well known photoresist materials used in the PCB industry:

FR-4 (woven glass and epoxy) - most common material today

FR-5 (woven glass and epoxy)

FR-6 (cotton paper and epoxy)

G-10 (woven glass and epoxy)

CEM-1 (cotton paper and epoxy)

CEM-5 (woven glass and polyester)

A PCB sample

Patterning and etching of substrates

This includes specialty photonics materials, Micro-Electro-Mechanical Systems(MEMS), glass printed circuit boards, and other micropatterning tasks. Photoresist tends not to be etched by solutions with a pH greater than 3.

MEMS microcantilever resonating in an SEM

Microelectronics

This application, mainly applied to silicon wafers/silicon integrated circuits is the most developed of the technologies and the most specialized in the field.

A 12-inch silicon wafer