Specific Process Knowledge/Lithography/Resist/UVresist

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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.

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. The spectral sensitivity is listed in the Resist Overview. Within this 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, see tables below. Using a combination of experience, calculation, and assumption, one may be able to estimate the dose for one equipment from the known dose 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 here 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.