Soft lithography: SU-8 baking

Equipement and protocols for heating SU-8 molds

Replicating PDMS-based structures first requires the fabrication of a SU-8 master mold that will serve as a patterned template for PDMS casting. During the fabrication of the SU-8 master, which is usually based on standard photolithography, the film of SU-8 photoresist must be baked several times, as seen in Figure 1.

Figure 1: photolithography standard protocol used to create SU-8 master molds: the SU-8 film used must be baked at least two times (schemes in the chart come from [1])

As a matter of fact, heating properly the SU-8 film is essential to ensure good surface properties of the mold. This short tutorial aims at providing some insights on the equipment and the protocols that must be followed in order to successfully bake SU-8 molds.

SU-8 Baking: oven or hot plate?

Oven

A convection oven constitutes a first option to heat a wafer coated with SU-8. It may be especially useful when one needs to deal with many SU-8 masters simultaneously. Despite this appealing feature, convection ovens also have noticeable drawbacks. In particular, it is difficult to guarantee a uniform temperature distribution inside the large volume of an oven (see Figure 2). As a consequence, if several wafers of SU-8 masters are placed in an oven, the baking time may actually vary according to the wafer position.

Figure 2: Illustration of the non homogeneous temperature distribution inside a conventional laboratory oven. Consequently, SU-8 layers placed inside an oven may need different baking times (image from [2]).

Hot plate

As an alternative, hot plates represent an ideal option for rapid prototyping. Although hot plates are intended to heat one wafer at a time, they ensure a much more uniform temperature across the whole wafer. Indeed, the heat transfer condition and ventilation are different between a hot plate and an oven [3]. Furthermore, baking times can be significantly shorter with a hot plate (see Figure 3).

Figure 3: Variations of (soft) baking time as a function of the SU-8 layer thickness [4].

Selection criteria

Oven and hot plates have their own advantages and limitations for baking SU-8 layers. A non exhaustive list of pros and cons are summarized on the chart [1]:

Table 1: Comparison of some criteria for baking SU-8 molds with an oven and with a hot plate

SU-8 baking: how to proceed?

SU-8 soft baking:

The SU-8 layer must be heated a first time after it has been applied to the surface of the wafer. This first heating step corresponds to the SU-8 soft baking process (see Figure 1). The SU-8 may be soft baked in two steps. Drastic temperature changes must be avoided for better photolithography results. Ideally, the wafer should be gradually heated in accordance with the temperature profile of Figure 4:

Figure 4: Typical temperature profile to be followed for SU-8 soft baking [5]

In a first step, the temperature of the SU-8 layer can be progressively increased from room temperature to 65°C at a rate of 2°C/min. The slope of the temperature ramp, however, may slightly differ depending on the SU-8 formulation used. The SU-8 film is heated at 65°C from 5 to 10 minutes depending on the photoresist thickness. The temperature is then elevated again at 2°C/min and then dwelled at 95°C. Higher temperatures should be avoided since they may activate a thermal cross-linking of the SU-8 even if the exposure process has not taken place [6]. The SU-8 is maintained at 95°C as long as necessary (see examples of baking times in Table 2).

Table 2: Soft baking times for various SU-8 types and thicknesses [3] (data valid for hot plates)

Afterwards, the wafer must be allowed to slowly cool to room temperature. Ideally, the temperature can be ramped down in a progressive manner similarly to the profile used for heating the SU-8. Alternatively, the power of the hot plate can be simply switched off.

For an accurate control of the temperature throughout the process, a programmable hot plate may be used to finely tune the temperature ramps. For thin layers of SU-8 (thickness < 100 µm), another option is to set two hot plates at 65°C and 95°C, respectively. The wafer can be baked first at 65°C, and then transferred to the 95°C hot plate [7].

SU-8 post exposure baking (PEB):

The SU-8 layer must be heated again after the exposure phase. This second heating step corresponds to the SU-8 post exposure baking process (see Figure 1). It is intended to accelerate the SU-8 polymerization. The protocol to be followed for SU-8 PEB is actually almost identical to the soft baking step one. The temperatures are the same; only the baking times are different (see Table 3).

Table 3: Post exposure baking times for various SU-8 types and thicknesses [3] (data valid for hot plates)

It can be noticed that the cross linking process might engender significant residual stress in the SU-8 layer. Because this residual stress is a major source of potential cracks, it is preferable to avoid a rapid cooling of the SU-8 after the PEB at 95°C (a cooling source should not be used).

SU-8 hard baking:

After development, the SU-8 master may be heated a third time (see Figure 1) to further cross link the SU-8 mold and make sure the SU-8 will not be damaged during the soft lithographic steps with PDMS. The temperatures involved during the SU-8 hard baking process are usually higher than for the soft baking and PEB processes. Typically, the SU-8 master is heated at a temperature ranging from 140°C to 200°C during 20-30 minutes. Longer baking times, however, may be required depending on the thickness of the SU-8 layer. Thanks to the good mechanical properties of SU-8, this step remains optional and is normally not necessary.

For more tutorial 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 precised. Article written by Guilhem Velvé Casquillas and Timothée Houssin.

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