- Email: [email protected]
Machining of three-dimensional microstructures in silicon by electro-discharge machining
ELSEVIER
Sensors and Actuators
A 67 ( 1998)
159-165
Machining of three-dimensional microstructures in silicon by electrodischarge machining Dominick Reynaerts *, Wim Meeusen, Hendrik Van Brussel Katholieke
Universiteit
Lumen,
Division
of Productiorz
Engineering,
Machine
Design and Automation,
Celestijnenlaan
3OOB, B-3001
Heveriee,
Belgium
Abstract Currently, nearly all mlcrocomponents are fabricated by microelectronic production technologies like etching, deposition or other photolithographic techniques. In this way, the main emphasis has been put on surface micromrchanics. The major challenge for the future will be the development of real three-dimensional microstructures. Electra-discharge machining (EDM) is a so-called non-conventional machining technique, whereby material is removed through the erosive action of electrical discharges (sparks) provided by a generator. As shown in this paper, EDM proves to be a versatile technique that is very well suited for machining complex microstructures. Several examples, especially microparts with three-dimensional features, are given. It is demonstrated that the EDM process is independent on the silicon crystal orientation. This means that a wafer can be machined in any direction with respect to the wafer’s top plane. EDM, unlike most micromachining processes, is not a batch process. On the other hand, is it well situated to be used as a rapid prototyping technique or for machining complex parts. 0 1998 Elsevier Science S.A. All rights reserved. Keywords:
Silicon
micromachining;
Microelectromechanical
system (MEMS);
Elecuo-discharge
machining
ing-derived j techniques. First, it requires a low installation
1. Introduction
cost and small job overhead
In electro-dischargemachining (EDM), materialremoval startswhen the generatorappliesa voltage betweenthe workpiece (the silicon wafer) and a tool electrode (a tungsten wire j [ 1,2]. This voltage is high enough to produce a spark between the two electrodes.The sparkmelts a smallmaterial volume
on each of the electrodes.
The dielectric
fluid that
fills the gap betweenthe electrodesremovespart of this material. The remaining part solidifies again on the surfaceof the electrodes.Through appropriatesetting of the machiningparameters,the material removal on the tool electrode (the electrode wear) can be kept at leastanorder of magnitudesmaller than the material removal on the workpiece electrode. The net result of the spark is then a small crater on both workpiece and tool electrode. By applying a large number of sparks, large material volumes canbe removed. Since the sparkswill occur were the electrode and the workpiece are closest together, the final workpiece shapedependssolely on the shapeof the electrode and on the relative movement (if any) between the electrode and the workpiece. As already discussedin earlier papers[ 3-71, EDM hasseveral advantages for machining silicon comparedto the more traditional (etch* Corresponding author: Tel.: l 32- 16-322-640; E-mail: [email protected] 0924-3247/98/$19.00 PIISO924-4247(97)01724-X
0
1998 Elsevier
Fax:
+ 32- 16-322-987;
Science S.A. All rights reserved.
(such as designing
masks, etc.).
Secondly, EDM is very flexible, thus making it ideal for prototypes or small batchesof products with a high added value. Finally, EDM can easily machine complex threedimensional shapes.Shapesthat prove difficult for etching are relatively easy for EDM. In previous work by the present authors [ 81, theseproperties of EDM were already demoustratedby severalexamples.In this paper, this work is further continued towards real three-dimensionalstructures.First, it is demonstratedthat there is no influence of the crystallographic directions of the silicon on the EDM process.A secondinvestigation concernsthe machining of a planewith an arbitrary angle with respect to the top plane of a wafer. This technique is illustrated with several examples.Finally, a micropart machined on six sideswill be presentedas an example of real three-dimensional micromachining.
2. Influence of the crystallographic directions of the silicon on the EDM process To study the influence of the crystallographic directionsof the silicon on the EDM process, the following test was devised. In a polycrystalline wafer, a numberof identical test
D. Reyorrts
160
et al. /Sensors
and Actmor~:
A 67 (1998)
3. Mechining
159-165
planes at an arbitrary
angle
3.1. Introchlction
Fig. 1. Top view of a conical hole through a silicon wafer. The cone angle is 60”. The hole’s diameter is 521 pm (in the top plane) and 129 pm (in the bottom plane). Table 1 The influence of different crystallographic orientations on the results of the EDM process (the variation on the measurement of the electrode wear is approx. 5% of the average electrode wear) Summary Standard
deviation/average
Duration
Electrode
0.005
0.05
wear
h=R-- dm
structures were made using the same settings of the machine. Fig. 1 shows that this structure is a conical hole through a silicon wafer with a cone angle of 60”. Care was taken to ensure that these structures were made in different parts of the wafer, with a different crystallographic orientation. Each test structure consisted of five holes through the wafer. For each test the time required to drill the holes and the wear of the electrode were measured. No significant variations in these parameters were found. Table 1 summarizes these results. In total 24 test structures were made. For each structure the total time and electrode wear were calculated. Then the average and standard deviation were calculated. It appears that the influence of the crystallographic directions on these parameters is minimal.
Fig. 2. Drawing
of the test structure
(left)
In general, structures can be made by EDM in two ways. Either an electrode is used that has the negative shape of the cavity that needs to be made, and is made to ‘sink’ into the unfinished workpiece. In this way, also referred to as diesinking EDM, nearly any shape of cavity can be made. The major problem of this technique is the manufacturing of the electrode, which is sometimes a rather complex part. Also, wear will be not equally distributed over the electrode. Another solution is to move an electrode with a simpler shape with respect to the workpiece. In this case the electrode can be a cylindrical tool or any other ruled surface. Due to the analogy with the milling process, this technique is also referred to as EDM-milling. As for traditional milling, the dimensional accuracy and the surface quality of the finished surface are determined by different factors: l the geometry of the milling tool; 0 the distance between two subsequent paths of the milling tool; 0 the number of times the same surface is machined. For a spherical milling tool, this can be expressed by the following formula:
for EDM-milling
with h the height of the scallop, i.e., the peak-to-peak distance of the features remaining on the surface, R the radius of the milling tool and L the distance between two subsequent milling paths. For a milling tool with a radius of 75 pm and a distance between two paths of 35 pm, this would result in a final surface roughness (peak-to-peak) of 2 km. However, the EDM--milling process is not completely comparable to traditional milling. Therefore, an experimental verification was preferred. To test the attainable surface quality using this method, a simple test structure was designed. Fig. 2 shows that the structure consists of two planes inclined under 45”, separated by a thin membrane. This structure could for example be used as a micromirror.
and photograph
of the finished
structure
(right).
D. Reymerts
+8.2m
er al. / Ser~~ors
.,
pm
-E.EBB Iin w--+-+a.847
-
Fig. 4. Roughness
.,
r-.+-2---,
A 67 (1998)
161
159-165
;
‘.:
_ -----y-
“’
.;.. ,.
Rbf---yx&--+e.zaE
ran
Fig. 3. Roughness
+a.647
and Actrtazors
plot of the polished
,... +9. EEb’MI
mm
plot of the plane under 45” (mirror
surface)
in the direction
Fig. 5. Top view of two spur gears (left) and view of two assembled gears (right), In the left picture, field of view of the picture, hence some teeth appear to be cut off. Individual gear diameter is 3 mm.
Fig. 6. Top view of one bevel gear (left) and side view of two assembled gears (right). the picture; hence some teeth appear to be cut off. Gear diameter is 4.3 mm.
The test structure was machined using several different machining strategies.A machining strategy is defined asthe description of the path the electrode is made to follow over the workpiece. The best strategy yielded a roughnessof the mirror surfacesof 0.014 p.m R, in the direction of the electrode movementand 0.127 km R, in a direction perpendicular
mm
top surface of the wafer.
of the electrode
movement.
the two gears taken together
In the left picture,
the gear is slightly
are slightly
larger than the
larger than the field of view of
to the electrodemovement. This last value is much lower than the surfaceroughnessoriginally calculatedby the above formula. Thesevalues shouldbe comparedto a measuredroughnessof 0.011 Frn R, for the polishedtop surfaceof the wafer. Fig. 3 showsthe roughnessplot of the polished top surface of the wafer.
162
D. Reymens
et al. /Sensors
(4
and Actuatorx
A 61 (I9981
159-165
To compare, Fig. 4 shows the roughness plot of the plane under 45” (mirror surface) in the direction of the electrode movement. These results show that, while the micro-EDM technique at first sight is based on a very rough removal process, a polished-like surface can be obtained. 3.2. 2Txamples: rnnchini~zg of bevel gears
(b)
(cl
(f)
Fig. 7. Machining
procedure
Fig. 8. Miniature
for a micro-die.
Fig. 5 shows that micro-EDM of silicon easily allows spur gears to be machined. Two of these were placed on micromach.ined steel axes and were successfully made to rotate against one another. Several authors [ 9, lo] have reported on the fabrication of silicon microgears as possible components for a future generation of microrobots. Most of these silicon gears had straight teeth, thereby severely compromisingthere
silicon die (side surfaces are 350 pm
X
350 pm, hoies are 46 p,rn in diameter)
D. Reynaens
et al. /Sensors
and Actuators
lifetime and performance. Contrary to many other microgears, the flanges of these teeth are not straight lines, but rather have an involute [ 1I] shape to secure a constant rate of angular velocity between the driving and the driven axis and to ensure that there is no slip. The production of spur gears was the basis for machining bevel gears. These require a milled surface. Fig. 6 shows a finished bevel gear. Two of these gears were again assembled and were successfully driven by a small motor. As the starting material was a wafer only 350 km thick, the gears look rather slender. The size of both the spur and the bevel gears was chosen so that a standard electrode with a diameter of 150 km would just fit in the space between the teeth. By reducing the size of the electrode on the electro-discharge machine itself, smaller gears can be made. The smallest electrode the authors have been able to produce was 12 p,rn in diameter. 4. Machining
on six faces
4.1. Machining a micro-die All examples shown above still can be considered as 2.5 dimensional machined parts. The final aim of this research is to achieve a real three-dimensional micromachining technique. Therefore, a die was selected as a prototype piece to be machined on six faces. This is accomplished by first machining the front, left side and right side with respectively six, four, and three eyes (Fig. 7 (a) ) Next, the two eyes on the top face are machined (Fig. 7(b) ) . Then the wafer is reclamped to machine the five eyes on the (former) bottom side (Fig. 7(c) ) . The remaining single eye on the sixth face requires a special machining procedure. Fig. 7(d) shows that first a large hole is machined behind the die. This hole allows the entry of the electrode for machining the last eye. The finishing of the sixth face at the same time releases the part (Fig. 7(e), (0). Fig. 8 shows the six faces of the resulting micro-die. The side surfaces are 350 km X 350 km; the holes are 36 p,rn in diameter. This rather academic example shows that it is possible to machine six faces in any orientation. The holes in the top and
A 67 (1998)
159-165
163
bottom surfaces were made using a cylindrical electrode; the holes in the four side surfaces were made using a special electrode shown in Fig. 9. This electrode was shaped against a copper piece with the negative form of the final electrode. This type of machining is thus a special case of the die-sinking EDM mentioned above. As can be seen in Fig. 8, the alignment of the holes is not perfect. The normal alignment procedure for the electrodischarge machine used (touching the workpiece with the electrode at low voltage) is not applicable because of the low conductivity of the silicon. Also, the last face is rather rough because machining this face at the same time releases the final workpiece. The released die will also be flushed away by the dielectric fluid. An additional clamping device to clamp the die during this last machining step would give much better results. As alignment and clamping procedures do not exist for a micromachining environment, it is of major importance that these two topics will be addressed in the future. 4.2. Machining a resonant beam structure The aim of this example is to machine a resonant beam accelerometer structure as shown in Fig. 10. This accelerometer consists of a seismic mass (m) suspended on eight beams: four suspension beams (sb) and four resonant beams (rb) . The sensing principle is based on a shift in resonance frequency of the resonant beams induced by the acceleration forces. A major problem is the machining of a monolithic sensor structure in silicon.
Fig. 10. Resonant beam accelerometer design. The seismic mass (m) is suspended on four support beams (sb) and four resonating beams (rb).
164
D. Reynaerts
et al. /Sensors
and Actuator.;
A 67 (1998)
159-165
55.8 vrn
Fig. 11. Electrode
for machining
Fig. 12. Double-beam
To be able to manufacture this sensor by EDM, a test structure with only two beams was attempted. To machine this structure a special-shaped electrode shown in Fig. 11 was used. This shape is obtained by machining the electrode against a tungsten wire of 50 p,m diameter. Fig. 12 shows the resulting double-beam structure. The parallelism between top and bottom surfaces of the beam that can be obtained by this technique is about 1 to 300. The surface roughness of these surfaces is 0.94 km l?,. Another problem that is still to be solved is the dimensional accuracy of the machined beams.
5. Conclusions Microsystem technology is now focused on structures and systems that can be manufactured in one set-up using one processing technology. If the macro-world can be used as an example, eventually most microsystems will be assembled from parts made using the most appropriate manufacturing method. The authors feel that EDM can become the method of choice for complex three-dimensional workpieces. This paper shows the applicability of micro-EDM to machine three-dimensional microstructures in silicon. One of the major advantages of silicon micro-EDM is its power to machine structures with complex shapes that prove difficult
stzucture
a double-beam
machined
structure.
by EDM.
for other machining methods. Several examples, especially microparts with three-dimensional features, are given. It is demonstrated that the EDM process is independent of the silicon crystal orientation. This means that a wafer can be machined in any direction with respect to the wafer’s top plane, EDM should not be seen as a competitor to the other micromanufacturing techniques (etching, LIGA) , but rather as a complement to these techniques.
Acknlowledgements This research is sponsored by the Fund for Scientific Research Flanders (Belgium) (F.W.O.), project G029&95N, and by the Inter University Attraction Pole IUAP3/24. D. Reynaerts is a postdoctoral fellow of the Fund for Scientific Research - Flanders (Belgium) (F.W.O.). The authors wish to thank A. Verbruggen, P.-H. ‘s Heeren, and fi4EC for their contribution to this research.
References [I] [2]
C. van Osenbruggen, Micro spark erosion (in Dutch), Philips Technisch Tijdschrift 20 (1969) 200-213. R. Snoeys, F. Staelens, W. Dekeyser, Current trends in non-conventi’snal material removal processes, Annals CIRP 35 ( 1980) 467480.
D. Rrynaerts
et al. /Sensors
and Actuators
of sil[31 D. Reynaerts, P.-I-i. Heeren, II. Van Brussel, Microstructuring icon by electro-discharge machining (EDM) -partI: theory, Sensors and Actuators A 60 (1997) 212-218. M. Fujino, Self-aligned machining and [41 H.H. Langen, T. Masuzawa, assembly of high aspect ratio microparts into silicon, Proc. MEMS ‘95, Amsterdam, The Netherlands, Jan. 1995, pp. 187-191. The present and future developments of EDM and [51 K. Kobayashi, ECM, Proc. 1 lth Int. Symp. ElectroMachining, Lausanne, Switzerland, 17-21 April, 1995, pp. 2947. 161 T. Higuchi, K. Furutani, Y. Yamagata, K. Takeda, Development of a pocket-size electro-discharge machine, Annals CIRP30 ( 1991) 203205. M. Fujino, Modular method for micro[71 H.H. Langen, T. Masuzawa, parts machining and assembly with self-alignment, Annals CIRP 44 (1995) 173-176. of 181 D. Reynaerts, P.-H. ‘s Heeren, H. Van Brussel, Microstructuring silicon by electro-discharge machining (EDM) - part II: applications, Sensors and Actuators A 61 ( 1997) 379-386. mechanisms and 191 M. Mehregany, Y.C. Tai, Surface micromachined micromotors, J. Micromech Microeng. 1 (1991) 73-85. micromotors, [ 101 K. Suzuki, H. Tanigawa, Single crystal siliconrotational Proc. IEEE Micro Electra Mechanical Systems Workshop, Nara, Japan, 1991, pp. 15-20. ill1 R.C. Junivall, KM. Marshek, Fundamentals of Machine Component Design, John Wiley, New York, 1991.
Biographies Dominick Reynaerts graduated as a mechanical engineer (K.U. Leuven, 1986). He started his activities as a research assistant at the Division of Production Engineering, Machine Design, and Automation (PMA) of K.U. Leuven in 1986. Within the framework of the Erasmus student-exchange program, he stayed as a Ph.D. student at the Scuola Superiore S. Anna in Pisa in 1990. In 1993 he became a research manager of the PMA Division. He received a Ph.D. degree in mechanical engineering in 1995 from KU. Leuven. Currently, he is a postdoctoral fellow of the Fund for Scientific Research Flanders (F.W.O.) and assistant professor at KU. Leuven.
A 67 (1998)
159-165
165
His current research interests include the design and control of multi-fingered robot grippers, shape memory alloy actuators, precision mechanics and micromechanical systems. He is a member of IEEE. Wim Meem-en graduated as a mechanical engineer (K.U. Leuven, 1997) and, as a research assistant at the PMA Division of K.U. Leuven, he is currently working towards a Ph.D. degree in mechanical engineering. His research interests include manufacturing and assembly methods for microstructures, applications of micromechanical systems and metrology. Hendrik Van Brussel is a mechanical engineer (HTI-Oostende, Belgium, 1965) and an electronics engineer (K.U. Leuven, Belgium, 1968). He received a Ph.D. degree in mechanical engineering in 1971 from K.U. Leuven. From 1971 until 1973 he was ABOS-expert at the Metal Industries Development Centre in Bandung, Indonesia, where he set up an engineering research centre, and associate professor at ITB, Bandung. In 1973 he became lecturer at K.U. Leuven and is now full professor in automation and head of the Department of Mechanical Engineering. He was a pioneer in robotics research in Europe and an active promoter of the mechatronics idea as a new paradigm for concurrent machine design. He has published more than 200 papers on different aspects of robotics, mechatronics and flexible automation. His present research interests are shifting towards holonic manufacturing systems and precision engineering, including microrobotics. He is a fellow of SME and IEEE and in 1994 he received an honorary doctorate from the ‘Politehnica’ University in Bucarest, Romania and from RWTH, Aachen, Germany. He is also a corresponding member of the Royal Academy of Sciences, Literature and Fine Arts of Belgium and an active member of CIRP (International Institution for Production Engineering Research).
Sensors and Actuators
A 67 ( 1998)
159-165
Machining of three-dimensional microstructures in silicon by electrodischarge machining Dominick Reynaerts *, Wim Meeusen, Hendrik Van Brussel Katholieke
Universiteit
Lumen,
Division
of Productiorz
Engineering,
Machine
Design and Automation,
Celestijnenlaan
3OOB, B-3001
Heveriee,
Belgium
Abstract Currently, nearly all mlcrocomponents are fabricated by microelectronic production technologies like etching, deposition or other photolithographic techniques. In this way, the main emphasis has been put on surface micromrchanics. The major challenge for the future will be the development of real three-dimensional microstructures. Electra-discharge machining (EDM) is a so-called non-conventional machining technique, whereby material is removed through the erosive action of electrical discharges (sparks) provided by a generator. As shown in this paper, EDM proves to be a versatile technique that is very well suited for machining complex microstructures. Several examples, especially microparts with three-dimensional features, are given. It is demonstrated that the EDM process is independent on the silicon crystal orientation. This means that a wafer can be machined in any direction with respect to the wafer’s top plane. EDM, unlike most micromachining processes, is not a batch process. On the other hand, is it well situated to be used as a rapid prototyping technique or for machining complex parts. 0 1998 Elsevier Science S.A. All rights reserved. Keywords:
Silicon
micromachining;
Microelectromechanical
system (MEMS);
Elecuo-discharge
machining
ing-derived j techniques. First, it requires a low installation
1. Introduction
cost and small job overhead
In electro-dischargemachining (EDM), materialremoval startswhen the generatorappliesa voltage betweenthe workpiece (the silicon wafer) and a tool electrode (a tungsten wire j [ 1,2]. This voltage is high enough to produce a spark between the two electrodes.The sparkmelts a smallmaterial volume
on each of the electrodes.
The dielectric
fluid that
fills the gap betweenthe electrodesremovespart of this material. The remaining part solidifies again on the surfaceof the electrodes.Through appropriatesetting of the machiningparameters,the material removal on the tool electrode (the electrode wear) can be kept at leastanorder of magnitudesmaller than the material removal on the workpiece electrode. The net result of the spark is then a small crater on both workpiece and tool electrode. By applying a large number of sparks, large material volumes canbe removed. Since the sparkswill occur were the electrode and the workpiece are closest together, the final workpiece shapedependssolely on the shapeof the electrode and on the relative movement (if any) between the electrode and the workpiece. As already discussedin earlier papers[ 3-71, EDM hasseveral advantages for machining silicon comparedto the more traditional (etch* Corresponding author: Tel.: l 32- 16-322-640; E-mail: [email protected] 0924-3247/98/$19.00 PIISO924-4247(97)01724-X
0
1998 Elsevier
Fax:
+ 32- 16-322-987;
Science S.A. All rights reserved.
(such as designing
masks, etc.).
Secondly, EDM is very flexible, thus making it ideal for prototypes or small batchesof products with a high added value. Finally, EDM can easily machine complex threedimensional shapes.Shapesthat prove difficult for etching are relatively easy for EDM. In previous work by the present authors [ 81, theseproperties of EDM were already demoustratedby severalexamples.In this paper, this work is further continued towards real three-dimensionalstructures.First, it is demonstratedthat there is no influence of the crystallographic directions of the silicon on the EDM process.A secondinvestigation concernsthe machining of a planewith an arbitrary angle with respect to the top plane of a wafer. This technique is illustrated with several examples.Finally, a micropart machined on six sideswill be presentedas an example of real three-dimensional micromachining.
2. Influence of the crystallographic directions of the silicon on the EDM process To study the influence of the crystallographic directionsof the silicon on the EDM process, the following test was devised. In a polycrystalline wafer, a numberof identical test
D. Reyorrts
160
et al. /Sensors
and Actmor~:
A 67 (1998)
3. Mechining
159-165
planes at an arbitrary
angle
3.1. Introchlction
Fig. 1. Top view of a conical hole through a silicon wafer. The cone angle is 60”. The hole’s diameter is 521 pm (in the top plane) and 129 pm (in the bottom plane). Table 1 The influence of different crystallographic orientations on the results of the EDM process (the variation on the measurement of the electrode wear is approx. 5% of the average electrode wear) Summary Standard
deviation/average
Duration
Electrode
0.005
0.05
wear
h=R-- dm
structures were made using the same settings of the machine. Fig. 1 shows that this structure is a conical hole through a silicon wafer with a cone angle of 60”. Care was taken to ensure that these structures were made in different parts of the wafer, with a different crystallographic orientation. Each test structure consisted of five holes through the wafer. For each test the time required to drill the holes and the wear of the electrode were measured. No significant variations in these parameters were found. Table 1 summarizes these results. In total 24 test structures were made. For each structure the total time and electrode wear were calculated. Then the average and standard deviation were calculated. It appears that the influence of the crystallographic directions on these parameters is minimal.
Fig. 2. Drawing
of the test structure
(left)
In general, structures can be made by EDM in two ways. Either an electrode is used that has the negative shape of the cavity that needs to be made, and is made to ‘sink’ into the unfinished workpiece. In this way, also referred to as diesinking EDM, nearly any shape of cavity can be made. The major problem of this technique is the manufacturing of the electrode, which is sometimes a rather complex part. Also, wear will be not equally distributed over the electrode. Another solution is to move an electrode with a simpler shape with respect to the workpiece. In this case the electrode can be a cylindrical tool or any other ruled surface. Due to the analogy with the milling process, this technique is also referred to as EDM-milling. As for traditional milling, the dimensional accuracy and the surface quality of the finished surface are determined by different factors: l the geometry of the milling tool; 0 the distance between two subsequent paths of the milling tool; 0 the number of times the same surface is machined. For a spherical milling tool, this can be expressed by the following formula:
for EDM-milling
with h the height of the scallop, i.e., the peak-to-peak distance of the features remaining on the surface, R the radius of the milling tool and L the distance between two subsequent milling paths. For a milling tool with a radius of 75 pm and a distance between two paths of 35 pm, this would result in a final surface roughness (peak-to-peak) of 2 km. However, the EDM--milling process is not completely comparable to traditional milling. Therefore, an experimental verification was preferred. To test the attainable surface quality using this method, a simple test structure was designed. Fig. 2 shows that the structure consists of two planes inclined under 45”, separated by a thin membrane. This structure could for example be used as a micromirror.
and photograph
of the finished
structure
(right).
D. Reymerts
+8.2m
er al. / Ser~~ors
.,
pm
-E.EBB Iin w--+-+a.847
-
Fig. 4. Roughness
.,
r-.+-2---,
A 67 (1998)
161
159-165
;
‘.:
_ -----y-
“’
.;.. ,.
Rbf---yx&--+e.zaE
ran
Fig. 3. Roughness
+a.647
and Actrtazors
plot of the polished
,... +9. EEb’MI
mm
plot of the plane under 45” (mirror
surface)
in the direction
Fig. 5. Top view of two spur gears (left) and view of two assembled gears (right), In the left picture, field of view of the picture, hence some teeth appear to be cut off. Individual gear diameter is 3 mm.
Fig. 6. Top view of one bevel gear (left) and side view of two assembled gears (right). the picture; hence some teeth appear to be cut off. Gear diameter is 4.3 mm.
The test structure was machined using several different machining strategies.A machining strategy is defined asthe description of the path the electrode is made to follow over the workpiece. The best strategy yielded a roughnessof the mirror surfacesof 0.014 p.m R, in the direction of the electrode movementand 0.127 km R, in a direction perpendicular
mm
top surface of the wafer.
of the electrode
movement.
the two gears taken together
In the left picture,
the gear is slightly
are slightly
larger than the
larger than the field of view of
to the electrodemovement. This last value is much lower than the surfaceroughnessoriginally calculatedby the above formula. Thesevalues shouldbe comparedto a measuredroughnessof 0.011 Frn R, for the polishedtop surfaceof the wafer. Fig. 3 showsthe roughnessplot of the polished top surface of the wafer.
162
D. Reymens
et al. /Sensors
(4
and Actuatorx
A 61 (I9981
159-165
To compare, Fig. 4 shows the roughness plot of the plane under 45” (mirror surface) in the direction of the electrode movement. These results show that, while the micro-EDM technique at first sight is based on a very rough removal process, a polished-like surface can be obtained. 3.2. 2Txamples: rnnchini~zg of bevel gears
(b)
(cl
(f)
Fig. 7. Machining
procedure
Fig. 8. Miniature
for a micro-die.
Fig. 5 shows that micro-EDM of silicon easily allows spur gears to be machined. Two of these were placed on micromach.ined steel axes and were successfully made to rotate against one another. Several authors [ 9, lo] have reported on the fabrication of silicon microgears as possible components for a future generation of microrobots. Most of these silicon gears had straight teeth, thereby severely compromisingthere
silicon die (side surfaces are 350 pm
X
350 pm, hoies are 46 p,rn in diameter)
D. Reynaens
et al. /Sensors
and Actuators
lifetime and performance. Contrary to many other microgears, the flanges of these teeth are not straight lines, but rather have an involute [ 1I] shape to secure a constant rate of angular velocity between the driving and the driven axis and to ensure that there is no slip. The production of spur gears was the basis for machining bevel gears. These require a milled surface. Fig. 6 shows a finished bevel gear. Two of these gears were again assembled and were successfully driven by a small motor. As the starting material was a wafer only 350 km thick, the gears look rather slender. The size of both the spur and the bevel gears was chosen so that a standard electrode with a diameter of 150 km would just fit in the space between the teeth. By reducing the size of the electrode on the electro-discharge machine itself, smaller gears can be made. The smallest electrode the authors have been able to produce was 12 p,rn in diameter. 4. Machining
on six faces
4.1. Machining a micro-die All examples shown above still can be considered as 2.5 dimensional machined parts. The final aim of this research is to achieve a real three-dimensional micromachining technique. Therefore, a die was selected as a prototype piece to be machined on six faces. This is accomplished by first machining the front, left side and right side with respectively six, four, and three eyes (Fig. 7 (a) ) Next, the two eyes on the top face are machined (Fig. 7(b) ) . Then the wafer is reclamped to machine the five eyes on the (former) bottom side (Fig. 7(c) ) . The remaining single eye on the sixth face requires a special machining procedure. Fig. 7(d) shows that first a large hole is machined behind the die. This hole allows the entry of the electrode for machining the last eye. The finishing of the sixth face at the same time releases the part (Fig. 7(e), (0). Fig. 8 shows the six faces of the resulting micro-die. The side surfaces are 350 km X 350 km; the holes are 36 p,rn in diameter. This rather academic example shows that it is possible to machine six faces in any orientation. The holes in the top and
A 67 (1998)
159-165
163
bottom surfaces were made using a cylindrical electrode; the holes in the four side surfaces were made using a special electrode shown in Fig. 9. This electrode was shaped against a copper piece with the negative form of the final electrode. This type of machining is thus a special case of the die-sinking EDM mentioned above. As can be seen in Fig. 8, the alignment of the holes is not perfect. The normal alignment procedure for the electrodischarge machine used (touching the workpiece with the electrode at low voltage) is not applicable because of the low conductivity of the silicon. Also, the last face is rather rough because machining this face at the same time releases the final workpiece. The released die will also be flushed away by the dielectric fluid. An additional clamping device to clamp the die during this last machining step would give much better results. As alignment and clamping procedures do not exist for a micromachining environment, it is of major importance that these two topics will be addressed in the future. 4.2. Machining a resonant beam structure The aim of this example is to machine a resonant beam accelerometer structure as shown in Fig. 10. This accelerometer consists of a seismic mass (m) suspended on eight beams: four suspension beams (sb) and four resonant beams (rb) . The sensing principle is based on a shift in resonance frequency of the resonant beams induced by the acceleration forces. A major problem is the machining of a monolithic sensor structure in silicon.
Fig. 10. Resonant beam accelerometer design. The seismic mass (m) is suspended on four support beams (sb) and four resonating beams (rb).
164
D. Reynaerts
et al. /Sensors
and Actuator.;
A 67 (1998)
159-165
55.8 vrn
Fig. 11. Electrode
for machining
Fig. 12. Double-beam
To be able to manufacture this sensor by EDM, a test structure with only two beams was attempted. To machine this structure a special-shaped electrode shown in Fig. 11 was used. This shape is obtained by machining the electrode against a tungsten wire of 50 p,m diameter. Fig. 12 shows the resulting double-beam structure. The parallelism between top and bottom surfaces of the beam that can be obtained by this technique is about 1 to 300. The surface roughness of these surfaces is 0.94 km l?,. Another problem that is still to be solved is the dimensional accuracy of the machined beams.
5. Conclusions Microsystem technology is now focused on structures and systems that can be manufactured in one set-up using one processing technology. If the macro-world can be used as an example, eventually most microsystems will be assembled from parts made using the most appropriate manufacturing method. The authors feel that EDM can become the method of choice for complex three-dimensional workpieces. This paper shows the applicability of micro-EDM to machine three-dimensional microstructures in silicon. One of the major advantages of silicon micro-EDM is its power to machine structures with complex shapes that prove difficult
stzucture
a double-beam
machined
structure.
by EDM.
for other machining methods. Several examples, especially microparts with three-dimensional features, are given. It is demonstrated that the EDM process is independent of the silicon crystal orientation. This means that a wafer can be machined in any direction with respect to the wafer’s top plane, EDM should not be seen as a competitor to the other micromanufacturing techniques (etching, LIGA) , but rather as a complement to these techniques.
Acknlowledgements This research is sponsored by the Fund for Scientific Research Flanders (Belgium) (F.W.O.), project G029&95N, and by the Inter University Attraction Pole IUAP3/24. D. Reynaerts is a postdoctoral fellow of the Fund for Scientific Research - Flanders (Belgium) (F.W.O.). The authors wish to thank A. Verbruggen, P.-H. ‘s Heeren, and fi4EC for their contribution to this research.
References [I] [2]
C. van Osenbruggen, Micro spark erosion (in Dutch), Philips Technisch Tijdschrift 20 (1969) 200-213. R. Snoeys, F. Staelens, W. Dekeyser, Current trends in non-conventi’snal material removal processes, Annals CIRP 35 ( 1980) 467480.
D. Rrynaerts
et al. /Sensors
and Actuators
of sil[31 D. Reynaerts, P.-I-i. Heeren, II. Van Brussel, Microstructuring icon by electro-discharge machining (EDM) -partI: theory, Sensors and Actuators A 60 (1997) 212-218. M. Fujino, Self-aligned machining and [41 H.H. Langen, T. Masuzawa, assembly of high aspect ratio microparts into silicon, Proc. MEMS ‘95, Amsterdam, The Netherlands, Jan. 1995, pp. 187-191. The present and future developments of EDM and [51 K. Kobayashi, ECM, Proc. 1 lth Int. Symp. ElectroMachining, Lausanne, Switzerland, 17-21 April, 1995, pp. 2947. 161 T. Higuchi, K. Furutani, Y. Yamagata, K. Takeda, Development of a pocket-size electro-discharge machine, Annals CIRP30 ( 1991) 203205. M. Fujino, Modular method for micro[71 H.H. Langen, T. Masuzawa, parts machining and assembly with self-alignment, Annals CIRP 44 (1995) 173-176. of 181 D. Reynaerts, P.-H. ‘s Heeren, H. Van Brussel, Microstructuring silicon by electro-discharge machining (EDM) - part II: applications, Sensors and Actuators A 61 ( 1997) 379-386. mechanisms and 191 M. Mehregany, Y.C. Tai, Surface micromachined micromotors, J. Micromech Microeng. 1 (1991) 73-85. micromotors, [ 101 K. Suzuki, H. Tanigawa, Single crystal siliconrotational Proc. IEEE Micro Electra Mechanical Systems Workshop, Nara, Japan, 1991, pp. 15-20. ill1 R.C. Junivall, KM. Marshek, Fundamentals of Machine Component Design, John Wiley, New York, 1991.
Biographies Dominick Reynaerts graduated as a mechanical engineer (K.U. Leuven, 1986). He started his activities as a research assistant at the Division of Production Engineering, Machine Design, and Automation (PMA) of K.U. Leuven in 1986. Within the framework of the Erasmus student-exchange program, he stayed as a Ph.D. student at the Scuola Superiore S. Anna in Pisa in 1990. In 1993 he became a research manager of the PMA Division. He received a Ph.D. degree in mechanical engineering in 1995 from KU. Leuven. Currently, he is a postdoctoral fellow of the Fund for Scientific Research Flanders (F.W.O.) and assistant professor at KU. Leuven.
A 67 (1998)
159-165
165
His current research interests include the design and control of multi-fingered robot grippers, shape memory alloy actuators, precision mechanics and micromechanical systems. He is a member of IEEE. Wim Meem-en graduated as a mechanical engineer (K.U. Leuven, 1997) and, as a research assistant at the PMA Division of K.U. Leuven, he is currently working towards a Ph.D. degree in mechanical engineering. His research interests include manufacturing and assembly methods for microstructures, applications of micromechanical systems and metrology. Hendrik Van Brussel is a mechanical engineer (HTI-Oostende, Belgium, 1965) and an electronics engineer (K.U. Leuven, Belgium, 1968). He received a Ph.D. degree in mechanical engineering in 1971 from K.U. Leuven. From 1971 until 1973 he was ABOS-expert at the Metal Industries Development Centre in Bandung, Indonesia, where he set up an engineering research centre, and associate professor at ITB, Bandung. In 1973 he became lecturer at K.U. Leuven and is now full professor in automation and head of the Department of Mechanical Engineering. He was a pioneer in robotics research in Europe and an active promoter of the mechatronics idea as a new paradigm for concurrent machine design. He has published more than 200 papers on different aspects of robotics, mechatronics and flexible automation. His present research interests are shifting towards holonic manufacturing systems and precision engineering, including microrobotics. He is a fellow of SME and IEEE and in 1994 he received an honorary doctorate from the ‘Politehnica’ University in Bucarest, Romania and from RWTH, Aachen, Germany. He is also a corresponding member of the Royal Academy of Sciences, Literature and Fine Arts of Belgium and an active member of CIRP (International Institution for Production Engineering Research).