SU-8 BAKING

EQUIPMENT 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.

Need an advice about SU-8 baking?

Feel free to contact us at:

contact@elveflow.com

Ask me your question

(We will answer within 24 hours)





Email (Mandatory)

Your needs

SU-8 baking Process Chart

 

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: ovens or hot plates?

Ovens

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.

SU8 baking  Oven non homogenous temperature

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

Hot plates

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 for a hot plate and an oven [3]. Furthermore, baking times can be significantly shorter with a hot plate (see Figure 3).

SU8 baking Baking time vs thickness

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

 

Selection criteria

Ovens 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

The two options are viable, and the selection may be simply dictated by the availability of equipment in the clean room. Hot plates, however, usually permit to achieve an accurate control of the heating process more easily. Thereby, they appear more suitable for persons with no prior (or limited) experience in order to obtain good and repeatable results. Meanwhile, the risk of trials and errors can be minimized. Moreover, manufacturers of SU-8 photoresists often provide bake recommendations for hot plates. Therefore, we will considerer the use of hot plates in the next steps of this tutorial.

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. Indeed 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:

Soft baking temperature ramp profileFigure 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).

SU8 backing time depending of thicknessTable 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)

SU-8 post exposure baking time depending of thicknessTable 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.

soft-lithography-SU-8-PDMS-microfluidic-chips

Begin with Soft-lithography ?

Do your own  SU8 Mold and PDMS chips

> We install everything & train your team in 1 week

> Plug and play soft-lithography protocol

> Get the highest resolution without clean room 

 

Tutorials : Introduction about soft lithography

Definition Microfluidic

Soft-lithography definitions

When you talk about microfluidic some words can be new, we gather here some common and relevant definitions ...
Read More
Microfluidic-fabrication-technics-short

Introduction about soft-lithography for microfluidics

Unlike photolithography, soft lithography can process a wide range of elastomeric materials, i.e. mechanically soft materials ...
Read More
PDMS-membrane-short

PDMS membrane: thickness of a spin coated PDMS layer

The final PDMS layer thickness mainly depends of spin-coating speed and duration ...
Read More
Photolithography-mask-short

Introduction about photomask in microfluidics

A photolithography mask is an opaque plate or film with transparent areas that allow light to shine through a defined pattern ...
Read More
Microfluidic Wafer SU8 Mold

SU-8 mold lithography

Here you can find a complete over view of a SU-8 mold fabrication process ...
Read More
PDMS-chip-short

PDMS softlithography

Here you can find a complete over view of a PDMS chip replication ...
Read More

Tutorials : How to choose your soft lithography instrument

Microfluidic 3D Printer

Microfluidic 3D printer

Every following technology are based on the same system of additive process, every object is build layer by layer after being sliced by an informatics systems ...
Read More
How-to-choose-plasma-pdms

PDMS Soft lithography : Plasma cleaner

A plasma cleaner to bond your PDMS chip, you will find here the relevant points to think about ...
Read More
UVLamp-short-HowToChoose

SU-8 photolithography : UV sources

An UV Lamp to exposed your SU-8 photoresist, you will find here the relevant points to think about ...
Read More
Spin-coater-short-HowToChoose

SU-8 photolithography : Spin coater

A spin coater to create thin layer of photoresist or PDMS, you will find here the relevant points to think about ...
Read More
Hot-plate-short-HowToChoose

SU-8 photolithography : Hot plates

A hot plate to bake your SU-8 photoresist, you will find here the relevant points to think about ...
Read More
Photolithography-mask-HowToChoose

SU-8 photolithography: photomask

Basically you have the choice between glass or plastic photolithography mask, but how to do choice? you will find here some information to help you in the decision ...
Read More

Tutorials : How to get the best soft lithography  process

PDMS microfluidic light

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 ...
Read More
Soft Lithography SU-8 baking

Soft Lithography: SU-8 baking

Replicating PDMS-based structures first requires the fabrication of a SU-8 master mold that will serve as a patterned template for PDMS casting ...
Read More
Spin-coater-short-HowToChoose

SU-8 photolithography: Spin-coating

How do you do to have a successful spin coating? Here you will find the tips and tricks to do it ...
Read More
Hot-plate-short-HowToChoose

SU-8 photolithography: Baking

How do you do to have a successful photoresist baking? Here you will find the tips and tricks to do it ...
Read More
UVLamp-short-HowToChoose

SU-8 photolithography: UV exposure

How do you do to have a successful SU-8 exposure? Here you will find the tips and tricks to do it ...
Read More
How-to-choose-plasma-pdms

Soft lithography: Glass/PDMS bonding

How do you do to have a successful PDMS bonding with a plasma cleaner? Here you will find the tips and tricks to do it ...
Read More

Tutorials : Microfluidic device fabrication

Photolithography-mask-short

Fabrication of glass and film photomasks

The photolithography mask is an important tool in soft photolithography processes, we explain here how they are made ...
Read More

Influence of your microfluidic laboratory environmental parameters on your photomask

PDMS chips, soft lithography ... Do it yourself ! More information about the SoftLithoBox®   Environmental and mechanical conditions can affect the ...
Read More
Microfluidic-foundries-short

Microfluidic Foundries

If you don’t fabricate your microfluidic device by yourself, it is important to choose the right manufacturer to fabricate your microfluidic chips. Here is a list of microfluidic foundries ...
Read More
Microfluidic-fabrication-technics-short

Microfluidic fabrication technics

It exist different technics to fabricate microfluidic devices but the main can be resume by Etching, Thermoforming, Polymer ablation and Polymer casting ...
Read More

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.

References:

[1] http://www.scme-nm.org/index.php

[2] http://www.brewerscience.com/processing-theories/bake-plate/

[3] http://www.microchem.com/Prod-SU8_KMPR.htm

[4] R. Yang and W. Wang, UV Lithography of ultrathick SU-8 for microfabrication of high aspect ratio microstructures and applications in microfluidics and optical components, Bio-MEMS: Technologies and applications, Chapter 2, CRC Press.

[5] http://www.gersteltec.ch/su-8-Photoresists/

[6] B. H. Ong et al., Photothermally enabled lithography for refractive index modulation in SU-8 photoresist, Opt. Lett., 31, 1367-9.

[7] J. M Sidorova et al, Microfludic assisted analysis of replicating DNA molecules, Nat. Protoc., vol 4, nb 6, pp 849-61.