76527, Introduction to Microsystem Technology

Microfabrication

(ADDITIONAL MATERIALS )

1.Introduction

Microfabrication technologies currently employed to micro electromechanical systems(MEMS) have originated from microelectronics technology. The microelectronics technologyis well developed and suitable for mass production. However, the technology has itslimitations. For example, the structure produced has low aspect ratio and often limited to theuse of silicon, and equipment required is generally expansive. In the mean while, it has becomeclear that other technologies from different fields could also contribute significantly to thedevelopment of MEMS system. Many technologies are capable of fabricating 3-D mechanismsrequired for applications such as microrobotics. These technologies include:

• LIGA (named from the German words “LIthographie, Galvanoformung, Abformung”) andother photolithography-related methods.

• Material removal methods such as excimer laser machining [1], electrical dischargemachining [2], ion beam milling [3] and ultrasonic machining [4].

• Material deposition techniques such as laser-assisted chemical vapour deposition (LCVD)[5], ultraviolet stereo lithography [6] and localized electrochemical deposition [7].

In the following text, we will address some of these material removal methods and materialdeposition techniques.

2.Material Removal Technologies

Material removal technologies include excimer laser machining, electrical dischargemachining, ion beam milling, ultrasonic machining, etc. In this section, excimer lasermachining and electrical discharge machining are briefly introduced.

2.1 Excimer Laser Micromachining

The excimer (excited dimer) laser was invented in 1975, in which a diatomic molecule such as or was used as the lasing material. Since then rare gas halide lasers have become more

common but are still referred to as excimer lasers. Excimer lasers produce relatively wideN2 H2

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beams of ultraviolet laser light. One interesting application of these lasers is their use inmicromachining organic materials (plastics, polymers, etc.). This is because the wave lengthsof excimer laser are more compatible with the chemical bond energies in organic compoundsand tend to produce less thermal damage, so the material adjacent to the area machined is notmelted or distorted by heating effects.

The organic materials on the work piece are removed at each pulse when the laser is pulsed onand off. The amount of material removed depends on the material properties, the pulse lengthand the intensity of the laser light. When the intensity of the laser is bellow a certain thresholdthat depends on the material, the laser light has no effect. By counting the number of pulses, itis possible to control the depth of the cut. Cuts of hundreds of microns can be made using theexcimer laser.

The shape of the structures produced is controlled by using a chrome on quartz mask, like themasks produced for photolithography. In the simplest system the mask is placed in contactwith the material being machined, and the laser light is shone through it. A more sophisticatedand versatile method involves projecting the image of the mask onto the material. Material isselectively removed where the laser light strikes it. Structures with vertical sides can becreated. By adjusting the optics it is possible to produce structures with tapered side walls, asshown in Fig. 1.

Excimer lasers have a number of applications beyond those mentioned here. One area ofapplication is in machining the cornea of the eye to change its optical properties; correcting forshort sight.

2.2 Micro Electro Discharge Machining (EDM)

Micro electro discharge machining (EDM) is a “non-conventional” machining techniquewhich make possible to achieve precision machining of micro holes, micro shafts and a varietyof complex shapes. EDM is a process whereby material is removed through the erosive actionby control the electrical discharges (sparks) between a tool electrode and the workpieceelectrode. These electrodes are separated by the dielectric fluid. Every discharges melts a smallamount of material on both electrodes. Some of the melted material is removed by thedielectric fluid while the remaining part resolidifies on the surface of the electrodes. By properchoosing the machining parameter, the material removal on the tool electrode can be kept atleast an order of magnitude smaller than the material removal on the workpiece [2]. One of thefirst micro EDM experiment reported was performed at the end of 60s [8]. However, thetechnique was not investigated further until later 80s.

FIGURE 1. Microstructure created by excimer laser micromachining [9].

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There are two kinds of EDM: die-sinking EDM and wire EDM. The wire EDM uses a wire thatcuts through the workpiece. The wire is typically made of tungsten or copper of diameter 25-300 . The die-sinking EDM uses a tool electrode having the complementary form of thedesired workpiece, and “sinks” into the workpiece. The material of the electrode is normallycopper or graphite while the dielectric fluid is mostly oil. Comparing these two EDM methods,the wire EMD is cheaper and easier to renew the tool than the die-sinking EMD using acomplex electrodes. However, wire EDM can only make ruled surfaces and the wire may bendduring machining which results in a loss of accuracy. The machinable materials are usuallyelectrically conductive, but semiconductor materials, such as silicon, can also be machined bymicro EDM. Hard materials such as cemented carbide can also be micromachined withoutdifficulty.

In order to implement a practical micro EDM it is very important to minimize the singledischarge energy and to set out a very high discharge frequency. By reducing the dischargeenergy to about , the capacitance of the relaxation generator for discharging the pulses toabout , and limiting the stray capacitance using ceramic components for the EDMmechanical structure.

Micro EDM micromachining can drill precision micro holes (diameter of 15 to 300 , radialdeviation of 0.5 or less, surface roughness smaller than ), such as those required forgas and liquid orifices in aerospace and medical applications, pin holes for X-ray and nuclearfusion measurements, ink-jet printer nozzles and electron bean gun apertures. Holes canusually be machined to a depth of 3 to 10 times that of the hole diameter. The speed of EDMcan be quite high, e.g., a hole with a diameter of and deep can be eroded in 1to 2 in silicon material with to .

In order to drill microholes, microelectrodes must be fabricated first. This can be done by usinga wire EDM and reserving the polarity of the electro-discharge (known as Wire ElectrodeDischarge Grinding). The wire EDM method can also be used for fabrication of micronozzleswith irregular inner section, as shown in Fig. 3. Micro EDM is capable of machining shapes

FIGURE 2. Schematic representation of different type of EDM [13].

µm

10 7– J10 pF

µmµm 0.1µm

160µm 380µmmin 0.47 0.51µm Ra

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more complex that microholes and microshafts merely by implementing multi-axis motionmechanisms for the electrodes.

3.Material Deposition Techniques

Material deposition techniques includes laser-assisted chemical vapour deposition, ultravioletstereo lithography, localized electrochemical deposition, etc. In this section, laser-assistedchemical vapour deposition, ultraviolet stereo lithography and localized electrochemicaldeposition are briefly discussed.

3.1 Laser-assisted Chemical Vapour Deposition (LCVD)

Chemical vapour deposition (CVD) is a process where two gases enter a chamber and reactunder a suitable energy source, e.g., heat. If the reaction proceeds by a vapour-liquid-solid

FIGURE 3. Basic process from micronozzle fabrication using EDM [13].

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phase transformation, the gas is deposited as a liquid and solidifies. A solid film could beformed directly if the reaction proceed by a vapour-solid transformation.With focused energysource, e.g. laser, a film can be deposited locally. Thus, it is possible to control the depositionprocess.

The LCVD method is able to fabricate continuous thin rods and fibres by pulling the substrateaway from the stationary laser focus at the linear growth speed of the material while keepingthe laser focus on the rod tip, as shown in Fig. 4. LCVD was first demonstrated for carbon andsilicon rods. However, fibres were able to grown from hundreds of substrates including silicon,carbon, boron, oxides, nitrides, carbides, borides, and metals such as aluminium. The LCVDprocess can operate at low and high chamber pressures. The growth rate is normally less than

at low chamber pressure (<<1 bar). At high chamber pressure (>1 bar), highgrowth rate (>1.1 mm/s) has been achieved for small-diameter (< ) amorphous boronfibres.

The LCVD process can directly fabricate simple free-standing microstructure from the vapourphase. Fig. 5 shows microsprings made by LCVD process with a fibre diameter of < anda coil diameter of < . By using a two beam setup, it is possible to create three-dimensional fibre growth process that permits the direct one-step fabrication of complex freestanding microstructure, shown in Fig. 6.

3.2 Micro Stereo Lithography

It is very difficult to fabricate high aspect ratio 3-D complex microstructure with curve surface.LIGA process, which can create micro structure with excellent aspect ratio, cannot fabricategeneral 3-D structures having curved surfaces. The micro EDM process, which principally canbe implemented similar to a multi-axis numerical controlled micromachining tool, cannotmachine some materials (e.g., dielectric materials).

FIGURE 4. Linear and potentially continuous growth of an advanced inorganic fibre by high-pressure LCVD with a single laser [10].

100µm s⁄20µm

10µm350µm

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The micro stereo lithography (IH process [12]) introduced in 1992, based on the principle ofmacro stereo lithography, is an approach to solve those problems. The basic principle of the IHstereo lithography process is generating a 3-D structure by constructing 2-D sliced thin planeshardened from ultraviolet (UV) radiation curing polymer liquid by UV radiation. Either a He-Cd laser or a Xenon lamp can be used as light source, whereas the light beam is controlled by aXYZ stage in order to draw a 2-D shape directly from a CAD system, as shown in Fig. 7. Theresolution of the IH process is . 3D structures, such as a bending pipe( ) and a micro coil spring (diameter of ) has been made by thistechnology. Moreover, Mass-IH process has been developed. The above mentioned processesare very quick and flexible 3D micro fabrication proxies compared with silicon process,because no mask process required. However, the yield is not sufficiently high.

Recently, the super IH process [6] was introduced. The super IH process can solidify UVpolymer from a pinpoint in 3D space by focusing a laser beam inside the polymer. The method

FIGURE 5. LCVD springs: (a) A 3D system and a rotation goniometer; (b) Photograph of boron microsprings made with goniometer [11].

FIGURE 6. (a) Three-dimensional direct-write principle; (b) Three-dimensional free-standing alumina microstructure fabricated directly from the vapour phase by LCVD. The fibre segments, each having a

diameter of 20 , were continuously formed and joined in this two-laser LCVD process[10].

(a) (b)

(a) (b)

µm

5µm100 100 1000µm×× 50µm

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breaks the limitation of layer by layer process of conventional stereo lithography and is able tomake freely moveable mechanism without any support parts nor sacrificial layers, e.g., freelymoveable wheel and gear. The resolution of the method has attained to less than by usingthe simple desktop apparatus. In the mean while, the yield rate is much higher thanconventional stereo lithography because of the 3D fabrication inside the UV polymer. Theproduction speed could be several minutes to tens of minutes depends on the structurecomplexity.

FIGURE 7. Schematic diagram of (a) IH process and (b) super IH process [6].

FIGURE 8. Comparison of solidification process (a) conventional micro stereo lithography needs base layer; (b) super IH process needs no base [6].

FIGURE 9. Process to make moveable gear and shaft (a) conventional micro stereo lithography needs support structure (b) super IH process can make gear without support [6].

(a) (b)

1µm

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3.3 Localized Electrochemical Deposition

The localized electrochemical deposition uses a tip of sharp pointed electrode placed in aplating solution and brought near the surface where deposition is to occur. Potential is appliedbetween the tip and the substrate. The electric filed are confined to the region beneath the tipand thus the deposition is localized. The tip can move on the forming surface to create thedesired structure. If the substrate is electrically continuous, it is principally able to form anystructure geometry, with the resolution determined by the size of the electrode. In the meanwhile, the electrochemical nature can also be used to perform etching and polishing tasks.

The electrode tip has disk or cone like shape, the diameter of which determine the resolution ofthe microstructure to be built as well as the deposition rate. The deposition rate is affected by

FIGURE 10. Micro quoits made by super IH process: (a) SEM image of a micro quoits made of solidified polymer; (b) fabrication process of the quoits [6].

FIGURE 11. (a) The concept of localized electrochemical deposition; (b) The apparatus including the current feedback loop used for position control [7].

(a) (b)

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various phenomena such as mass transport, electron transfer, electrical potential, chemicalpotential and crystal growth. Microstructure of diameter of has been made. Thedeposition rate can be several micrometers per seconds achieved using polycrystalline nickel.

4.Comparison of Micromachining Technologies

The are many microfabrication methods available. Each of them has its strengths and specialfeature. Among them, LIGA process is one of the most important techniques in spite of itsexpensive device and manufacturing cost. An incomplete comparison of differentmicrofabrication and micromachining technologies is shown bellow (based on [14]).

FIGURE 12. Microstructure made by Local electrochemical deposition: (a) Nickel column ( diameter); (b) Nickel spring [7].

TABLE 1. Micromachining technologies comparison

TechnologyMinimal dimension

AccuracyAspect ratio

Geometrical freedom

Roughness Feasible materials

LIGA ++ ++ ++ +- + metals, polymers, ceramic materials

Etching +- + - - + metals, semiconductors

Excimer-laser - - +- + - metals, polymers, ceramic materials

Micro-milling - + +- + + metals, polymers

Diamond cutting + + + -- ++ non-ferro metals, polymers

Micro stereo lithography

-(+) -(+-) ++ ++ - polymers

Micro EDM + + ++ ++ + metals, semiconductors, ceramics

LCVD + +- ++ + - metals, semiconductos

10µm

(a) (b)

10µm

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5.References:

1. Van Dyke, L.S., Brumlik, C.J. and Martin, C.R., UV laser ablation of electronicallyconductive polymers, Synthetic Metals, 52 (1992), pp. 299-204.

2. Reynaerts, D., Heeren, P-H. and Brussel, H.V., Microstructuring of silicon by electro-discharge machining (EDM) - part I: theory, Sensors and Actuators, A60 (1997), pp. 212-218.

3. Dizon, R., Han, H., Russell, A.G. and Reed, M.L., An ion milling pattern transfer techniquefor fabrication of three-dimensional micromechanical structures, Journal ofMicroelectromechnical Systems, VOL. 2, NO. 4, December 1993, pp. 151-158.

4. Sun, X.-Q., Masuzawa, T. and Fujino, M., Micro ultrasonic machining and its applicationsin MEMS, Sensors and Actuators A 57 (1996), pp. 159-164.

5. Wallenberger, F.T., Nordine, P.C. and Boman, M., Inorganic fibers and microstructuresdirectly from the vapor phase, Composites Science and Technology 51 (1994), pp. 193-212.

6. Ikuta, K., Maruo, S. and Kojima, S., New micro stereo lithography for freely movable 3Dmicro structure, Proc. of IEEE International Workshop on Micro Electro MechanicalSystem (MEMS’98), 1998, pp. 290-295.

7. Madden, J.D. and Hunter I.W., Three-dimensional microfabrication by localizedelectrochemical deposition, Journal of Microelectromechanical Systems, VOL. 5, NO. 1,March 1996, pp. 24-32.

8. van Osenbruggen, C., Micro spark erosion, Philips Technisch Tijdschrift, 20 (1969), pp.200-214.

9. Bank, D., Introduction to microengineering, http://www.ee.surrey.ac.uk/Personal/D.Banks/umintro.html.

10.Wallenberger, F.T., Rapid prototyping directly from the vapor phase, Science, VOL. 267,March 1995, pp. 1274-1275.

11.Westberg, H., Thermal laser-assisted chemical vapor deposition, Summary of Dissertations375, Faculty of Science, University of Uppsala, Sweden, 1992.

12.Ikuta, K. and Hirowatari, K., Real three dimensional micro fabrication using stereolithography and metal molding, Proc. of Micro Electro Mechanical Systems, Ft.Lauderdale, FL, 1993, pp. 42-47.

13.Masuzawa, T., Kuo C.-L. and Fujino, M., A combined electrical machining process formicronozzle fabrication, Ann. CIRP, 43, 1994, pp. 189-192.

14.Heeren, P.-H., Reynaerts, D., Brussel, H.V., Beuret, C., Larsson, O. and Bertholds, A.,Microstructuring of silicon by electro-discharge machining (EDM) - part II: application,Sensors and Actuators, A 61 (1997), pp. 379-386.