Latest Micromachining
Technology — Part 4
Reproduction of Small Shapes — Molding
Technologies
Professor Kazuo Sato
Department of Micro System Engineering, Nagoya University
In this last part of this series of lectures, I would like todeal with technologies to transfer microscopic shapes, which
are fabricated by the methods discussed previously, in othermaterials. These technologies feature two outstanding effects
as follows: (1) The mass production of microscopic parts is possible. (2) Micromachining using various types of materials including silicon becomes possible. These technologies will be thebasis for a number of applied micromachine technologies to beintroduced into industries of different genres.
1. Transcription of etched shapes
By etching a pattern on a silicon single crystal substrate
and molding the shape on the surface, a 3-D structure of adifferent material from the substrate can be formed. For example, nonplanner structures can be created with films suchas silicon dioxide or silicon nitride. Fig. 1 shows an AFM-probe
chip made of a silicon dioxide film with a thickness of 1.2 µm.

Fig. 1 AFM-probe chip made of silicon dioxide film 1.2 micron thick having a bend tip.
To produce a structure with a bend tip, steps equivalent to bendcross-sections are formed on the surface of the silicon substrate.
Next, the entire surface is oxidized. Then the surrounding oxide film and silicon substrate are selectively etched away leaving the profile of the probe.
It is also possible to transcribe the shape on a silicon single
crystal substrate, where deep recesses are machined by an etching method, in polycrystalline silicon film. In the second partof this lecture series, an example of a film structure with a Tshaped cross-section in connection with the technology to produce a high-rigidity structure with a film was introduced. In22Fig. 2 Cross-sectional view of a fabrication process of a honeycomb structure made of polycrystalline silicon film.
addition to these, a honeycomb type film structure is also available. Fig. 2 shows the cross-sectional view of the fabrication
process of a honeycomb type film structure. A film structure isseparated after processing a silicon single crystal substrate witha large depth-to-aperture ratio (aspect ratio) by reactive ionetching (RIE: Refer to the first part of this lecture series), laminating a sacrificial layer such as a silicon dioxide film and structure material such as polycrystalline silicon film in order overthe substrate surface entirely, including inner surfaces of recesses, and finally etching away the sacrificial layer selectively.
Through this procedure, a honeycomb structure with a thickness of 100 µm is produced with a film with a thickness of 1 to2 µm.
In the field of biochemical technology that is expected to be
one of the micromachine applications, disposable micro fluid
devices are currently in demand. To meet such application purposes, the shape of a micro fluid channel created on a silicon
substrate by an etching method is transcribed in styrol resinetc. for mass-replication. The technology to transcribe microscopic shapes, or replication technology, itself is not new. For
example, in the field of spectroscopic devices, the technology oftranscribing a surface profile of diffraction grating which ismechanically engraved on an aluminum film with a pitch of
0.1 µm in resin material has already been established.
2. Reproduction of shapes developed in photoresist
Fine patterns are developed in a thick photoresist layer by
exposure to light to create micro cavities. Metal is depositedinside the cavities by electroplating method to produce micro
metal parts.
2.1 LIGA process
The LIGA process was developed by Karlsruhe ResearchCenter, Germany, in 1986. LIGA stands for the three processes
mentioned below, using initials of German words.
(1) Cavity formation by lithography (Lithografie in German)(2) Molding of cavity by metal electroplating (Galvanoformungin German)(3) Reproduction of metal mold shape in resin by injection molding (Abformung in German)
Fig. 3 illustrates the cross-sectional view of steps (1) and(2) in the LIGA process above. This process features the use ofFig. 5 Example of a molded plastic part made by using theLIGA process (Courtesy of Brother Industries, Ltd.).

Synchrotron radiation
Mask substrate
Mask pattern
PMMA photoresist
Metal substrate

PMMA (after developing)Electroplated metal(Ni)Finished productFig. 3 Cross-sectional view of the LIGA process.
synchrotron radiation (SOR) in place of ordinary UV light. With
SOR used, exposure of the 2-D mask pattern is deeply and accurately made in PMMA-system photoresist with a thicknessof several millimeters. By developing this pattern, a cavity withan aspect ratio in excess of 100 is formed. A metal structure asshown in Fig. 4 can be processed by electroplating the inside of
the cavity and reproducing the shape. The structure in thisphase can be directly put into practical use. However, by performing injection molding of plastics (Fig. 5) with the structure of a die, and reproducing the shape with electroplatingfurther applied, or using the structure for ceramic molding (solgel method), the structure can be applied to various types ofmaterials.
The high-precision processing accuracy of the transcription technology as represented by the aspect ratio is used insuch applications as optical communication devices, for example
3-D grating (diffraction grating) or optical waveguide, andchemical analysis devices with fine fluid channels.
The challenge in using the LIGA process is the limitationthat it cannot produce anything without SOR equipment. It istrue that the original die must be prepared by the use of SORequipment. However, once the master mold is prepared, thefollowing processes can be set for mass production. In Japan, aresearch team at Himeji Institute of Technology following theteam at Ritsumeikan University has successfully started theLIGA process using radiation equipment.
2.2 High aspect ratio UV photoresist
Now available is a new type of photoresist that allows forhigh aspect ratio exposure and development by ordinary UV
exposure techniques, though the processing accuracy is lower
than that of the LIGA process. For example, SU-8 photoresist
developed by IBM can be exposed to a layer depth of several
hundred microns by the UV light method. The transcription
process using plating technique of the shape of the cavity after
exposure and development process is identical to that used in
the LIGA process.
Though even oxygen plasma cannot easily decompose thephotoresist resin for removal, the fact that UV exposure can beused is an advantage. Besides, the transparency of the resin isvery high allowing the product to be used directly as an opticalcomponent material regardless of the transcription technique.
Summary
With transcription technologies of microscopic shapes, 3-Dstructures can be made of films. In addition, microscopic shapes
can now be created from such materials as metal, resin, andceramic. Increased material diversity is expected to expand theapplication range of micromachine technology.

Fig. 4 Micromotor structure made of Ni by the LIGA process(Courtesy of Prof. S. Sugiyama of Ritsumeikan Univ.).

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