The spin speed between and rpm allows the formation of a uniform film. At these speeds, the centrifugal force causes the liquid to flow to the edges, where it builds up until expelled when the surface tension is exceeded.
The resulting polymer thick- ness, T, is a function of spin speed, solution concentration, and molecular weight measured by intrinsic viscosity. The spin curves for various photore- sists can be obtained from the manufacturer. The spinning process is of pri- mary importance to the effectiveness of pattern transfer.
The quality of the resist coating determines the density of defects transferred to the device under construction. Introduction to Microfabrication Techniques 9 Fig. Resulting patterns after exposure and development of a positive- and nega- tive-tone photoresist.
The opaque image on the mask is transferred as is onto the posi- tive photoresist. The image is reversed in the case of a negative photoresist. Exposure and Postexposure Treatment Pattern transfer onto a photoresist is done by shining light through the mask see Fig.
One typically uses the g-line nm or i-line nm of a mercury lamp. In general, the smallest feature that can be printed using projec- tion lithography is roughly equal to the wavelength of the exposure source.
The action of light on a photoresist either increases or decreases the resist solubility depending on whether it is a positive or negative photoresist, respec- tively. Thus, for a positive-tone photoresist, the opaque pattern on the mask will determine the features remaining in the resist layer after development see Fig. Conversely, after development of a negative photoresist, the clear pat- tern of the mask determines the remaining photoresist features Fig.
The profile of the photoresist side walls see Fig. Figure 4 illustrates the use of a lift-off profile in the lift-off process. The resist wall profile can be controlled by adjusting resist tone, exposure dose, developer strength, and development time, as well as by other means.
The three different photoresist profiles. The overcut profile is the profile that is normally obtained from a positive-tone photore- sist. The vertical profile achieves the best pattern fidelity, but is relatively difficult to obtain. Post-exposure treatment is often desired because the reactions initiated dur- ing exposure might not have run to completion.
To halt the reactions or to induce new ones, several post-exposure treatments can be used: postexposure baking, flood exposure with other types of radiation, treatment with reactive gas, and vacuum treatment. Development, Descumming, and Postbaking During the development process, selective dissolving of resist takes place see Fig.
Development can be done using a liquid wet development , a gas, or plasma dry development. Positive resists are typically developed in aqueous alkaline solutions e. Unwanted residual photoresist sometimes remains after development. Descumming is a procedure for removing this unwanted photoresist with a mild plasma treatment.
In this process, highly energetic oxygen ions react and essentially burn away the unwanted photoresist. Hard baking also improves the hardness of the film. Improved hardness increases the resistance of the resist to subsequent etching and deposition steps. Introduction to Microfabrication Techniques 11 Fig. Example of lift-off sequence using negative resist as sacrificial layer. This method is used in cases in which the metal is difficult to etch e.
Pattern Transfer In cases in which the photoresist is a permanent part of the final device e. In most other cases, the sacrificial photoresist pattern is used as a mask for etching subtractive or deposition additive on the underlying substrate a subtractive process; see Fig. In a subtractive process, the resist acts as a protective barrier to the etching agent, which can be a liquid solution, a gas, or plasma.
After pattern transfer, the resist can be removed for further process steps. Similarly, pattern transfer can involve a deposition technique: chemical vapor deposition or e-beam evaporation. Soft Lithography Soft lithography 2—4 techniques incorporate an imprint step, in which the topography of a template defines patterns created on a substrate. In soft lithog- raphy, a patterned elastomer is used as a stamp, mold, or mask to generate micropatterns and microstructures instead of using a rigid photomask.
These methods include replica molding, micro-contact printing, micromolding in cap- illaries, and micro-transfer molding 5. The method is low in cost and, unlike photo- lithography, soft lithography is applicable to almost all polymers and, thus, many materials that can be prepared from polymeric precursors.
Because soft materials are used, deformation of the stamp or mold, low reproducibility owing to distortion , and defects yield are problems that prevent this technology from being a viable manufacturing technique, but it is widely used in research set- tings 2. An example of soft lithography frequently used in creating microfluidics is polydimethylsiloxane PDMS molding.
SU-8 is a chemically amplified negative photoresist with high transparency. The high transparency allows light to penetrate through thick layers of photoresist, thus creating near vertical sidewall profiles. As a form of soft lithography, SU-8 structures have been used as molds for microfluidic applications. The PDMS is cured, removed, and then pressed or bonded onto a flat substrate to create microchannels.
The materials and fabrication methods used in MEMS are much more varied than those used for IC fabrica- tion in which one deals principally with Si, oxides, and metals patterned using photolithography. In contrast to the IC industry, in which the devices are care- fully packaged and protected from the environment, MEMS devices, such as pressure or glucose sensors, often must have surfaces that are directly exposed to the environment in which they are sensing.
Because of the irreversible chemical reactions involved and contamination considerations, BioMEMS devices tend to be dis- posable. Nontraditional Materials Different materials that are being used or investigated for use in MEMS processes include polymers, ceramics, nitinol shape memory alloys , biomaterials, and carbon.
Patterned SU-8 photoresist is converted into car- bon electrodes by subjecting the photoresist to high temperatures in an inert environment in a process called pyrolysis. C-MEMS electrodes can be easily patterned into complex three-dimensional geometries that were previously dif- ficult or expensive to fabricate using conventional carbon electrode fabrication methods.
In addition, the electrochemical properties of carbon make it an excel- lent electrode material 6. High-aspect-ratio carbon structures see Fig. SEM micrograph of a high-aspect-ratio SU-8 negative photoresist structure taken before structure was pyrolyzed into carbon. The resulting electrodes and inter- connects were used for battery research. Introduction to Microfabrication Techniques 15 ratio carbon structures are being investigated for their potential use in batter- ies, and chemical and biosensing applications.
Conclusion In this chapter, we introduced lithography as an important tool for microfabrication. The application domain of lithography has greatly expanded beyond the IC world to encompass fabrication of mechanical and biological devices. The fact that some DNA arrays are nowadays made through lithogra- phy illustrates this point.
With the advent of MEMS, different materials and high-aspect-ratio fabrication techniques have also been introduced. The field of microfabrication is so vast that it would be impossible to give a comprehen- sive review in the context of one short chapter. The reader is therefore referred to textbooks 7,8 and recent reviews for a more thorough investigation. References 1. Xia, Y. The country you have selected will result in the following: All instructor resources are now available on our Instructor Hub.
Other editions — View all Fundamentals of Microfabrication: Madou is very thorough in the coverage of material in the fabrication of MEMs. What are VitalSource eBooks? Add to Wish List. Ten microfabricatuon discuss in detail topics such as lithography, pattern transfer, wet and dry bulk micromachining, surface micromachining, and LIGA.
From inside the book. This book brings an understanding of manufacturing options, material choices, and size Fundamentals of Nanoelectronics George W. With this movement have come new types of applications and rapid advances in the technologies and techniques needed to fabricate the increasingly miniature devices that are literally changing our world. This website uses cookies to improve your experience while you navigate through the website.
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