Specific Process Knowledge/Lithography/Resist/UVresist

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UV resist comparison table

Comparison of specifications and feature space of UV photoresists.

Exposure dose

Spectral sensitivity of AZ resists represented as optical absorption.

During exposure of the resist, the photoinitiator, or photo-active component, reacts with the exposure light, and starts the reaction that makes the resist develop in the developer.

In a positive resist, it makes the resist become soluble in the developer. In a negative resist, usually assisted by thermal energy in the post-exposure bake, it makes the resist insoluble in the developer. The amount of light required to fully develop the resist in the development process, is the exposure dose.

The optimal exposure dose is a function of many parameters, including the type of resist, the resist thickness, and the sensitivity of the resist.

Resist sensitivity
The resist sensitivity is a measure of how efficiently it reacts to the exposure light. Spectral sensitivity is the sensitivity of the resist as a function of wavelength. It is usually given simply as the range from the wavelength below which absorption in the resist material makes lithography impractical to the wavelength at which the photoinitiator is no longer efficiently activated.

Within the sensitivity range, the optical absorption is commonly used as a measure of sensitivity. A high absorption coefficient signifies a high sensitivity, as the light is absorbed by the photoinitiator. Because of spectral sensitivity, the optimal dose of a given resist type and thickness is also a function of the spectral distribution of the exposure light, i.e. the equipment used for the exposure. Using a combination of experience, calculation and assumptions, it may be possible to estimate the dose for an exposure equipment, if the exposure dose is already known on another equipment.

Due to reflection and refraction at the interface between the resist and the substrate, the optimal dose may also be a function of the type of substrate used. Unless otherwise stated, the exposure doses given below are for standard silicon wafers.

Apart from the already mentioned factors, the optimal dose also depends on the developer chemistry and the parameters used in the development process. Finally, the requirements to the lithographic process in terms of resolution, bias (line broadening), etch selectivity, side wall angle, etc. may narrow down, or widen, the process window. The exposure doses given in the sections below should be used as a starting point for individual fabrication process development.

Due to the process of bleaching, where the absorption of the resist changes during exposure, the exposure dose is unfortunately not always constant at different intensities of the exposure light. The exposure time is thus not always a linear function of the exposure intensity.

Calculate exposure time

In the maskless aligners, the dose is set directly as a process parameter in the job. In mask aligners, on the other hand, the parameter that is set is the exposure time, i.e. how long the shutter is open during the exposure.

The exposure dose, D [J/m²], is given by:

D = I x t ,

where I [W/m²] is the intensity of the exposure light, and t [s] is the exposure time. As the intensity is specific to the spectral sensitivity of the sensor used to measure the exposure light, and the exposure time is specific to the spectral distribution of the exposure light (cf. spectral sensitivity), this dose is specific to the combination of exposure source and optical sensor.

Given an exposure dose, the exposure time, t, is calculated as:

t = D / I

It is important to keep in mind that this exposure time is valid only for a specific combination of exposure source and optical sensor, as well as for a specific development process.

Exposure dose for mask aligners

KS Aligner

The KS Aligner has an i-line notch filter installed. This results in an exposure light peak around 365 nm with a FWHM of 7 nm. Unless otherwise stated, the exposure doses given here are for standard silicon wafers.

	Date	Thickness	Dose	Development	Comments
AZ 5214E Old German version	Long ago	1.5 µm	72 mJ/cm2	Single puddle, 60 s	Positive process
	Long ago	2.2 µm	80 mJ/cm2
	Long ago	4.2 µm	160 mJ/cm2
AZ 4562 Old German version	Long ago	10 µm	510 mJ/cm2	Multiple puddle, 4 x 60 s	Multiple exposure with 10-15 s pauses is recommended.
AZ MiR 701 Old PFOA containing version	Long ago	1 µm	180 mJ/cm2	Single puddle, 60 s	PEB: 60 s at 110°C Preliminary results
	Long ago	2 µm	200 mJ/cm2	Single puddle, 60 s
	Long ago	4 µm	400 mJ/cm2	Single puddle, 60 s	PEB: 90 s at 110°C Preliminary results
AZ nLOF 2020	Long ago	2 µm	110 mJ/cm2	Single puddle, 60 s	PEB: 60 s at 110°C Side wall angle ~15° For smaller angle (~5°), develop 30 seconds instead

Aligner: MA6 - 2

The Aligner: MA6-2 has an i-line notch filter installed. This results in an exposure light peak around 365 nm with a FWHM of 7 nm. Unless otherwise stated, the exposure doses given here are for standard silicon wafers.

	Date	Thickness	Dose	Development	Comments
AZ 5214E Old German version	Long ago	1.5 µm	72 mJ/cm2	Single puddle, 60 s	Positive process
	Long ago	2.2 µm	90 mJ/cm2	Single puddle, 60 s
AZ 5214E Image Reversal Old German version	Long ago	1.5 µm	22 mJ/cm2	Single puddle, 60 s	Image reversal process. Reversal bake: 120s at 110°C. Flood exposure: 200 mJ/cm2
	Long ago	2.2 µm	25 mJ/cm2	Single puddle, 60 s
AZ MiR 701 Old PFOA containing version	Long ago	1.5 µm	169 mJ/cm2	Single puddle, 60 s	PEB: 60 s at 110°C
	Long ago	2 µm	~200 mJ/cm2	Single puddle, 60 s
	Long ago	4 µm	~280 mJ/cm2	Single puddle, 60 s	PEB: 60 s at 110°C Process adopted from process logs
AZ nLOF 2020	Long ago	1.5 µm	104 mJ/cm2	Single puddle, 30 s	PEB: 60 s at 110°C Use 60 s development for lift-off
AZ 5214E New Japanese version	2023-01-11 jehem	1.5 µm	70 mJ/cm2	Single puddle, 60 s
AZ 5214E Image Reversal New Japanese version	2023-01-11 jehem	2.2 µm	22 mJ/cm2	Single puddle, 60 s	Image reversal process. Reversal bake: 60s at 110°C. Flood exposure: 500 mJ/cm2
AZ 4562 New Japanese version	2021-12-08 jehem	10 µm	550 mJ/cm2	Multiple puddles, 5 x 60 s	Priming: HMDS Rehydration after SB: 1 hour (may not be necessary) Exposure: Multiple exposures with pauses, 5 x (10 s exposure + 10 s pause) Degassing after exposure: 1 hour (may not be necessary)
AZ MiR 701 New PFOA free version	2021-06-23 elkh	1.5 µm	~150 mJ/cm2	Single puddle, 60 s	PEB: 60 s at 110°C