Soft lithography: SU-8 coating

In soft lithography, the fabrication of a mold, often made in SU-8, is required for replicating PDMS microfluidic structures. For the fabrication of this SU-8 master, a standard photolithography process is commonly used. Therefore, a first key concern is to apply a coating of SU-8 photoresist layer on a silicon wafer, as shown in Figure 1.

Photolithography standard protocol used to create SU-8 master molds: a thin SU-8 layer must first be deposited on a silicon wafer (drawings in the chart are adapted from [1])

SU-8 coating is a critical step that can strongly influence the subsequent steps of photolithographic process. Indeed, any alteration in the uniformity and smoothness of the SU-8 film (e.g., presence of air bubbles) will lead to non-homogenous heating during baking and/or uneven exposure to UV light. Consequently, areas of the SU-8 film can polymerize at different rates. This short tutorial aims to provide some insights on the equipment and protocols that must be followed in order to coat SU-8 uniformly throughout the wafer surface.

Soft lithography SU-8 coating: spin or spray coating?

Photoresists can be applied on wafers via different techniques (e.g., photoresist electrodeposition). This tutorial, however, will only consider the two most predominant methods for coating wafers with SU-8 during the fabrication of microfluidic chips.

Photo of a spin coater.

Illustration of the spin coating process (images adapted from [1]).

Spray coating: basics

An example of spray coating unit is shown in Figure 3a. The spray system typically includes a spray nozzle that generates a distribution of micrometer-sized droplets. Similarly to the spin coater, the wafer to be coated is maintained on a chuck via a vacuum system. During the spray coating process, the wafer is rotated at low angular velocity (30-60 rpm) while the swivel arm of the spray coating unit is moved across the wafer (see Figure 3b).

Although the process can be automated with a unit such as the one shown in Figure 3a, one can note that spray coating can also be manually executed in a cost effective manner with portable, self contained aerosol spray cans.

Example of aerosol spray can for manual spray coating [4].

Spin coating versus spray coating: a comparison

Spin coating has many advantages. First, it is a very mature and robust technique. Thereby, excellent results can be achieved. SU-8 photoresists with various degrees of viscosity can be directly used with the spin coating process. A large range of thickness films (from ultrathin layers to layers with a thickness of hundreds of microns) can be obtained with a high degree of uniformity. Moreover, spin coating offers a good repeatability. Nevertheless, spin coating also has some limitations. In particular, spin coating only provides good results on flat wafers. In addition, spin coating generates a large amount of waste material since most of the photoresist (>95%) is thrown off the substrate during the spinning process.

For spray coating, since the photoresist droplets are supposed to stay where they are deposited, the amount of SU-8 wasted during spray coating can be significantly less than with spin coating. Furthermore, an interesting advantage of spray coating is that it can be used on non-flat or textured wafers (e.g., wafers with holes, striations, etc.). Nonetheless, to get a proper size distribution of the photoresist, low viscosity solutions are usually necessary [5]. This means than SU-8 photoresist must be diluted with solvents in order to obtain good coverage properties. Additional restrictions of spray coating are related to its inability to control the thickness of the deposited film as precisely as spin coating. Likewise, spray coating usually leads to a more irregular coating uniformity of the layer deposited (see Figure 5). Finally, it may be delicate to deposit SU-8 layers thicker than 20 µm with spray coating [6].

Example of coating uniformity obtained with an aerosol spray can for a 9µm SU-8 film [4]

The fabrication of microfluidic chips based on soft lithography techniques usually involves wafers with flat surfaces. Thereby, spin coating is certainly the most widespread method and is unanimously selected for microfluidics related works. This is why the remainder of this tutorial will focus on spin coating.

SU-8 spin coating: how to apply uniform SU-8 films for microfluidic molds?

Despite its apparent simplicity, the physics underlying spin coating process may be considered quite complex. Indeed, the radial flow of photoresist induced by the angular velocity of the wafer is actually combined to solvent evaporation and drying effects. Moreover, ambient conditions surrounding the spinning wafer (temperature, air flows from hoods, humidity rate, etc.) may affect the process.

Nevertheless, during the fabrication of microfluidic molds, the commercial spin coaters used are closed bowls that minimize susceptibility to unwanted air flows, humidity variations and other ambient conditions. It is also admitted today that the amount of SU-8 initially deposited on the wafer, the rate at which it is deposited, the history of rotational acceleration prior to the final acceleration and the total spin time have limited or no effects [7]. As a consequence, only two parameters will significantly influence the final result of the spin coating process: the spinning speed and the spinning acceleration.

A commercial spin coater allows to accurately set these two parameters. A basic program for a spin coater is shown in Figure 6.

Example of basic velocity profile for programming a spin coater [8]

In Figure 6, the first acceleration ramp towards the main spin speed must be controlled accurately. Indeed, the photoresist begins to dry from the first part of the process and up to 50% of the solvents can be lost the first few seconds. Because this acceleration provides a twisting force, it will help in the quick and correct dispersal of the SU-8 over the wafer. Typically, acceleration ramps are set from 100 rpm/s to 300 rpm/s.

The spin speed then affects the amount of centrifugal force applied to the photoresist. This high speed generally defines the final film thickness. The latter approximately decreases with the square root of the spin speed (in rpm), as illustrated in Figure 7.

Film thickness as a function of the spin speed for different SU-8 formulations

Such curves are provided by manufacturers for each type of photoresist. They must be used to select the appropriate spinning speed according to the thickness of SU-8 film required. The high spinning speed is typically maintained from 30s to 60s (hold time, see Figure 6) depending on the photoresist type. One can note that as the photoresist continues to dry, the viscosity of the photoresist increases until the centrifugal force of the spin process can no longer appreciably move it over the wafer surface. At this point, longer hold time will not significantly decrease the layer thickness.

As a general guideline, Table 1 provides orders of magnitudes to set acceleration ramps as well as spin speed in order to obtain different layers thickness with various SU-8 types.

Examples of acceleration and spin speed settings for various films thicknesses and SU-8 formulations

For more tutorials about microfluidics, please visit our other tutorials here: «Microfluidics tutorials». The photos in this article come from the Elveflow® data bank, Wikipedia or elsewhere if specified. Article written by D. Desmaële. and T. Houssin.

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