Microsystem Tool for Microsystems Characterization Profile Measurement of High Aspect-ratio Microstructures Jean-Bernard POURCIEL*a, Eric LEBRASSEURa, Tarik BOUROUINAa, Takahisa MASUZAWA**b, Hiroyuki FUJITA**b a LIMMS /CNRS and bInstitute of Industrial Science, University of Tokyo. ABSTRACT A microsystem for the measurement of profiles of high aspect-ratio microstructures has been developed. This microsystem uses a silicon micro-probe with a sharp tip at its end and an integrated piezoresistive strain gauge force sensor. The probes are from 500 µm to 1 mm long with a cross-section of 20x20µm2; they were previously mainly designed for the characterization of narrow and deep micro-holes having a radius as small as 50µm. The profile measurement method has been extended to the characterization of other microstructures. In a first part of this paper, we explain the method based on an original algorithm to measure profiles with the greatest precision and reproducibility. In a second part we give some information about the capabilities for horizontal and vertical profiles measurement, concave and convex surfaces profiles plotting. We conclude with some experimental results for several types of profiles. Keywords: Stylus Profiler, profile measurement, microstructures, metrology for microsystems.
1. INTRODUCTION Microfabrication techniques such as Micro-Electro-Discharge-Machining1 or Micro-Ultrasonic Machining2 allow the micromachining of micro-holes, typically some tens of micrometers in diameter and about 1mm in depth. On the other hand, LIGA and deep RIE can also lead to very high aspect ratio-structures. Manufacturing of such high aspect structures has led to a new problem of characterization. Indeed, the existing characterization tools, including stylus surface profilers and optical profilers, are not suitable for the measurement of vertical profiles, especially inside narrow and deep structures. On the other hand, instruments based on SPM methods are dedicated to planar surfaces and are limited in terms of scanning area and/or scanning speed. One solution to this rather new characterization need consists of using long and thin probes, whose dimensions are typically 1 millimeter in length with a cross-section area of about 20x20µm2. An actuating element as well as a sensing element is associated to the probe so that it can be moved or vibrated, with the simultaneous detection of the contact with the sidewall. Starting from this, several possible implementations are possible, depending on the nature of the actuating and sensing elements and also on the measurement methodology. A first solution was successfully tested in a previous work. It is the so-called VibroScanning Method (VS-method)3,4,5. In this method, the probe is vibrated by means of a piezo-actuator near the surface to be measured. The contact is detected through the electrical contact between the probe and the surface. The VS-method has shown a measurement resolution of 500nm, the main limitation being due to the stability of the electrical contact. More recently, Yamamoto et al.6 have reported profile measurements of high aspect-ratio microstructures using a method, derived from the VSmethod. In this case, the probe is in tungsten carbide and it is coated with PZT thin films for vibration sensing. The method is based on the detection of a resonance frequency shift, which is induced by the strain caused by the mechanical contact between the probe and the side. For this reason it was called the Resonant mode VibroScaning method (RVS-method). In what follows, we present the method7, which uses both new measurement algorithm and new probes. This method is based on the use of force measurement for the detection of the mechanical contact. This is done using a micro-probe with integrated piezoresistive sensor. Integration of a sharp tip at the end of the probe is aimed to improve the lateral resolution. The measurement setup is completely automated for fast and safe measurements. Then we describe how the system has been used to plot profiles of various type of microstructures. *
[email protected]; phone +81 3 5452 6036; fax +81 3 5452 6088; http://www.fujita3.iis.u-tokyo.ac.jp/~limms/; Institute of Industrial Science, University of Tokyo, 4-6-1, Komaba, Meguro-ku, 153-8505 Tokyo Japan; ** Institute of Industrial Science, University of Tokyo, 4-6-1, Komaba, Meguro-ku, 153-8505 Tokyo Japan;
2. MATERIAL AND METHOD 2.1 Experimental setup A silicon micro-probe is inserted into the hole to be characterized. It is moved towards the sidewall using an external piezoactuator, while its deflection due to the mechanical contact is measured by an integrated force sensor (figure 1). Iterations for different heights lead to the vertical profile. In order to prevent the probe to be broken, the position of the hole is determined first, before the introduction of the probe. Indeed, a key issue in this method is the safe manipulation of the probe. If this is not achieved with enough precision, it may break. To alleviate this problem, the experimental setup was completely automated. A sequence of probe insertion in a hole and approach to a side is shown in figure 2.
P iez o A c tua tor C on tro l V o ltage
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Figure 1 : Diagram for Computer control of the plant and view of the experimental setup
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Figure 2 : a) approach of the probe, b) insertion in a microhole, c) motion to the side, d) contact detection
This probe is attached to the YZ part of a stage while the sample to be measured is placed on the X stage. The XYZ stage is in closed loop with a minimum step value of 50 nm. This stage is connected to a PC by a serial communication port, allowing a real-time control. In order to move the probe towards the sample, the X stage is used to rough motion; precise and quick displacements are provided by means of a large piezo actuator attached to the probe. The latter is controlled via a piezo voltage amplifier, by an analog output coming from a digital to analog conversion board in the PC. The force signal is amplified and input to an analog to digital converter of the acquisition board. Real-time force data acquisition and control of the stage and the piezo actuator are allowed by the use of a home-made software. A graphic user interface has been developed to control the whole experiment and visualize the results (fig. 3).
Figure 3: Computer interface for automated contact detection and profile drawing
2.2.Algorithm for contact point detection and profile measurement The cantilever is approached by fixed steps towards the surface to be measured until touching this surface. Then, the movement is continued for some steps. The movement is produced by an external piezo actuator attached to the cantilever. The step value is 1 µm. The procedure is repeated for adjacent points of the line along which we want to draw the profile. In order to draw a profile, we have to do establish a reference line from which the profile depth will be measured. In our case, the reference is the zero point (starting point) of the piezo actuator movement, also corresponding to a zero applied voltage. 10
S ig n al V o lta g e (V )
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Figure 4: Illustration of the operation principle of the proposed method. Figure 4 illustrates the principle of the method. While the tip is moved without touching the surface, the output voltage remains constant (almost nil). When the surface is reached, the voltage increases. The constant values are on a horizontal straight line D1 (average of the measurements). The tip relative displacement during the contact phase is less than 10 µm; so the system response can be regarded as linear. The voltage values during the contact phase are on a second straight line D2 having a finite slope. By using a simple least squares method (numerically by computer software) it is possible to find the parameters of this line. After a complete scan we can calculate the coordinates of the crossing point CP between the two lines D1 and D2, which will give us a precise indication of the distance between the zero point and the surface. After a step down along the Z-axis, the procedure is repeated in order to determine the next contact point. Then, for each position in the Z-axis, the result for the contact point value is drawn on a curve. In this way, the complete profile is measured.
3. RESULTS AND DISCUSSION ABOUT THE TOOL 3.1. Fabrication of silicon micro-probes The core of the micro-probe is a thin and long cantilever beam manufactured by double-side dry etching of a SOI wafer. Typical dimensions are a cross-section area of 20x20µm2 and a length of 1mm. A first version of such a micro-probe
includes a piezoresistive force sensor (fig.5.b). It consists of a p-type Wheastone full bridge (typically four resistors with a value of around 1 N REWDLQHG E\ ERURQ LRQ LPSODQWDWLRQ RQ QW\SH VXEVWUDWH )LJXUH D VKRZV D SLFWXUH RI D ready-to-use micro-probe mounted on its package, which is also used as a holder. In order to ensure a good lateral measurement resolution, the probe also integrates a sharp out-of-plane tip (fig.5.c). It is fabricated by RIE etching in the beginning of the process prior to resistor fabrication.
3.3 Measurement characteristics 3.3.1 Reproducibility In order to obtain some results on the reproducibility of the method, we have made successive profiles for a same hole. As the EDM method causes a particulate lay inside the machined hole, it is difficult to retrieve exactly the same profile drawing. However, as shown on figure 6, one can see that we obtain a rather good reproducibility of the measurement.
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Figure 5: Silicon probe: a) Overall view b) Magnified view of the integrated piezoresistive type force sensor c) Magnified view of the out-of-plane tip.
3.3.2 Accuracy The easiest way to do a rough estimation of accuracy is to perform measurements without any vertical motion of the probe that is several measurements of the same point. During our test, we have performed 200 measurements while the tip was always touching the same point on the surface. By that way, we can determine a bandwidth in which all measurement data are located. This bandwidth gives a good indication on the accuracy of the measurement. As shown on the figure 7, as results of this experiment, the accuracy bandwidth is around 70 nm. The contact point position is known at ±35 nm. This accuracy results from our first experiments using the proposed method. We intend to improve it by means of modifications on the electronic circuit and the software for signal processing.
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Depth (µm) 14 27 40 53 66 79 92 105 118 131 144 157 170 183 196
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Figure 6: Three successive profiles of the same micro-hole surimposed.
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Figure 7: Estimation of accuracy by measuring 200 times the same contact position
3.3.3 Self-compensation If we consider the event of a drift in the measurement system, then each curve that lead to the determination of the contact points (such as the one in figure 4) will be shifted along the vertical (force) axis with different magnitudes, which are of parasitic nature. But this vertical shift does not affect the determination of the contact point since the latter results from the crossing between the horizontal line and the sloped line. Of course, we suppose that there is no considerable drift during the measurement of a single curve, that is, during much less than one second. As a consequence, the method has self-compensation capability as regard drift.
4. USE OF THE SYSTEM FOR VARIOUS PROFILING ANGLES 4.1 Capability for tilting the measurement head In a previous work7 we have presented this new method originally designed to plot profiles inside deep microholes. As a continuation, we wished to demonstrate that we can use such a system to measure profiles when a conventional equipment is not appropriate. 4.1.1 Rotation for horizontal and vertical profile measurement As shown on the figure 8 and figure 9 the measurement system has rotation capabilities. A manual rotating device is installed near the Y axis of the stage. So, it is possible to adjust for an horizontal or a vertical position of the silicon probe above the sample to be measured. In the case of an horizontal profile measurement it could be interesting to have a weak angle between the cantilever and the surface of the sample to avoid contacts between the surface and the resistive bridge wire-bonding. This adjustment can be operate by means of this rotating device.
Tilt adjust.
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Figure 8 : Angle tilt and motion control
Figure 9 : Rotation for horizontal and vertical profilometry
1. 4.1.2 Adjustment for complex profiles measurement Sometimes it could be useful to draw profiles of vertical surfaces with a negative angle. In such a case, if the cantilever remains exactly vertical, the tip is not long enough to touch the surface after a vertical moving towards the lower part of the sample. It is allowed to tilt the cantilever with the purpose to place it in a parallel position with regard to the surface. It is then necessary to fix the amplitude of the piezo-actuator motion to be sure to touch the surface in the maximum depth position of the tip (figure 10).
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Figure 10 : Tilting the probe by a negative angle
4.1.3 Auxiliary adjustment An other angle adjustment system has been placed near the fixed part of the piezo-actuator. This system allows the orientation of the tip to be perpendicular to the surface. This manual rotation device is planned for the use of multicantilever probes in further developments 4.2 Concave and convex surface profile measurement 4.2.1 Concave surface profile measurement If the probe is placed exactly vertically in a hole having a concave surface (figure 11), due to the limited length of the silicon tip(around 7 µm), we can have no chance for a contact on the surface in the larger middle part of the surface. In such a case it is possible to use the same method than the one used for complex profiles measurement. We have to tilt the cantilever towards the surface and adjust the motion of the tip (piezo-actuator total amplitude) in order to have both a part of movement of the tip without any contact and a part with contact during the exploration of the whole depth of the profile (Figure 12). 4.2.2 Convex surface profile measurement In order to measure profiles of a convex surface, it needs only an adjustment of the piezo-actuator motion amplitude to make the method fit the maximum roughness of the profile(Figure 13).
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Figure 11: Concave profile limitation
Figure 12 : Tilting the probe
Figure 13 : Convex profile
4.3 Experiental results for different applications 4.3.1 Profile inside of a EDM microhole By using the vertical profiler, we have measured the profile inside narrow and deep micro-holes. The micro-holes were 80 µm in diameter. The used cantilever had a cross-section of 20x20µm2 and was 1 mm long. The length of the tip was 7.2 µm. The device under test is a 5x5 array of through holes, micromachined by EDM in a 200 µm-thick stainless steel plate. A typical profile is given in figure 14. Contact position (µm) 0
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Figure 14 : Profile of the inner surface of a EDM microhole
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Profile drawing on silicon lenses
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Figure 15 : Probe operating above the lens and resulting profiles
A new development for silicon lenses is in progress in our lab. We have done some horizontal profile plots with the surface of silicon lenses at several stages of the production process. We give on the figure 15 a view of the probe position for an horizontal detection, the plotting of the resulting profile for a radius of the lens, and a measured detail of the surface obtained by reducing the horizontal scanning step and increasing the number of scans in order to have a better accuracy for the profile line. This profile is obtained before the polishing stage of the lens.
4.3.2 Profiles of complex silicon shapes
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Figure 16 : Use of the experimental set for plotting profiles of complex shapes
The profiler has been used to plot profiles in the case of complex silicon microsystem devices. Figure 16 shows the probe operating to detect the profile of a silicon pole and, on the same device, the profile of a hole. For the measurement of the pole profile, we had to adjust the tilt of the probe in order to make the cantilever to remain approximately parallel to the vertical surface of the pole.
ACKNOWLEDGEMENTS This work was performed in the framework of LIMMS, Laboratory for Integrated Micro Mechatronic Systems. It is a joint laboratory between CNRS, French “Centre National de la Recherche Scientifique” and the Institute of Industrial Science of the University of Tokyo. It is supported by CNRS, the Monbusho (Japanese Ministry of Education) and the JSPS (Japanese Society for the Promotion of Science).
REFERENCES 1. T.Masuzawa, C.I.Kuo, M.Fujino, ‘Drilling of Deep Microholes by EDM Using Additional Capacity’ Jpn. Soc. Prec. Eng. 24: (4) (1990) 275-276. 2. X.Q.Sun, T.Masuzawa, M.Fujino, ‘Micro ultrasonic machining and its applications in MEMS’ Sensors and Actuators A, vol. 57 (2) (1996) 159-164. 3. T. Masuzawa, Y. Hamasaki, T. Fujino ‘Vibroscanning method for nondestructive measurement of small holes’. Annals of CIRP 42 (1993) 589-592. 4. T. Masuzawa, B.J. Kim, C. Bergaud, M. Fujino,‘ Twin-probe vibroscanning method for dimensional measurement of micro holes’ 47th CIRP Gen. Assembly, Tianjin, 24-30 August (1997). Annals of the CIRP, Vol. 46, N°1, pp 437-440 5. B.J. Kim, T. Masuzawa, T. Bourouina, ‘Vibroscanning method for the measurement of microhole profile’. Measurement Science and Technology, Vol. 10, n° 8 (1999) 697 – 705. 6. M.Yamamoto, I.Kanno, S. Aoki, ‘Profile measurement of high aspect-ratio microstructures using a tungsten carbide micro-cantilever coated with PZT thin films’. Proceed. of IEEE MEMS2000, Myazaki, Japan Jan. 23-27, (2000) 217-22 7. E.Lebrasseur, J-B Pourciel, T.Bourouina, M. Ozaki, T. Masuzawa, and H. Fujita, Microsystem for Vertical Profile Measurement of High Aspect-ratio Microstructures, Transducers ‘01, Munich, June 2001,pp.1058-1061.
