Abstract
We present the design, fabrication and characterization of a compact 2D stepping microdrive for pinhole array positioning. The miniaturized solution enables a highly integrated compact hyperspectral imaging system. Based on the geometry of the pinhole array, an inch-worm drive with electrostatic actuators was designed resulting in a compact (1 cm2) positioning system featuring a step size of about 15 µm in a 170 µm displacement range. The high payload (20 mg) as required for the pinhole array and the compact system design exceed the known electrostatic inch-worm-based microdrives.
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1. Introduction
Hyperspectral imaging (HSI) is the time-parallel measurement of numerous spectral signatures of object points [1]. It is an attractive method for quality assurance, e.g. analysis of blend homogeneity of pharmaceutical products [2] or remote sensing in agriculture [3]. A novel HSI system based on the confocal chromatic principle enables imaging with high spatial and high spectral resolution [4–7] at a compact size. By using hyperchromatic lenses in combination with a movable pinhole array which is a rectangular matrix of small (20–40 µm) circular apertures, complete images for different spectral bands can be recorded (see figure 1). A diffractive lens with large longitudinal chromatic aberration is used to focus the spectral components of the signal at different distances along the optical axis. At the focal plane of a specific wavelength each pinhole produces a circle of confusion with the focused wavelength in its center. On the used CCD detector only this central regions are evaluated. The measured intensity is mainly caused by the wavelength in focus, while the fraction of the defocused components is negligibly low. The current focused wavelength is selected by the tunable focal length of the optical system. By tuning the focal length of the optical system the wavelength in focus is selected, enabling a sequential analysis of the spectrum. By positioning the pinhole array in two directions with a step size smaller than the pinhole diameter the complete object plane can be measured (see figure 2 and supplementary video) (stacks.iop.org/JMM/25/074002/mmedia).
Figure 1. Optical functional principle of the HSI system.
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Figure 2. (a) Static pinhole array and (b) actuated pinhole array highlighted from behind and captured with 20 s exposure time. (c) Captured motion of the pinhole array with a high speed camera (AVI, 6054 KB).
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Standard image High-resolution imageThe focus of this paper is a micromechanical stepping drive for the stepwise positioning the pinhole array in two directions. By using a standard monochrome CCD Sensor (Sony ICX415AL, diagonal image size 8 mm) and a 1:1 imaging for minimizing optical aberrations, a glass-based pinhole array size of about 5.5 × 7 mm2 resulting in a weight of about 20 mg is used. The minimum pinhole diameter is chosen to be 20 µm, the single step size for the stepwise movement has to be at about 10–15 µm to achieve a homogenous and complete image of the full object plane. The largest distance between the single pinholes is 180 µm; therefore the displacement range of the micro-stepping drive has to be ±90 µm.
An ultra-compact system with a size comparable to the CCD sensor can be achieved allowing for further miniaturization.
Existing microsystem solutions do not meet this combination of requirements (see table 1). Many published 2D drives show a smaller displacement range [11–17]. In many cases the device lacks an optical aperture for light transmission [10, 13, 14, 16] or the positioned surface (stage) is too small [10, 12, 14, 17]. Moreover the relatively high payload addressed in this contribution was not a design aspect in the listed references. The stages with optical application goal have typically a circular aperture smaller than 1 mm [11, 15].
Table 1. Overview of published 2D microdrives and relevant characteristics.
| Static displacement range | Optical aperture | Surface of the stage | |
|---|---|---|---|
| Olfatnia et al [10] | 180 × 180 µm2 | Not transparent | 180 × 180 mm2 |
| Laszczyk et al [11] | (x) 56 µm; (y) 74 µm | ø800 µm | — |
| Beverly et al [12] | (x, y) 110 µm | — | ca. 30 µm2 |
| Gu et al [13] | (x, y) ±10 µm | Not transparent | 1600 × 1600 µm2 |
| Takahashi et al [14] | (x, y) 19 µm | Not transparent | 1 × 1 mm2 |
| Wu et al [15] | ±46.7 µm | ø200 µm | ca. ø200 µm |
| Kim et al [16] | 36 × 36 µm2 | Not transparent | 5 × 5 mm2 |
| Sun et al [17] | 3.7 µm | — | ca. 3 × 0.2 mm2 |
In the first step, a linear micromechanical stepping drive was implemented, where a special pinhole array design guarantees a complete imaging of the object plane [8, 9]. To further enhance the spectral separation with a higher pinhole distance, a 2D microdrive is needed.
2. Design of the 2D microdrive
A combination of horizontal and vertical (force directed normal to the substrate surface) electrostatic actuators are used to achieve an in-plane actuation and fixing of the pinhole array. The stepping mechanism is performed in six substeps as explained in figure 3. The colors black and red (hatched) represent different electrical potentials generating the electrostatic force for actuation.
Figure 3. Concept of the stepping mechanism. (a) Initial position pinhole array. (b) The actuator chip pulls itself to the pinhole array. (c) The pinhole array is released from the clamping chip and is fixed to the central frame of the actuator chip. (d) The actuator chip translates the pinhole array one step. (e) The pinhole array is pulled to the clamping chip. (f) The pinhole array is released from the actuator chip, while locked to the clamping chip. The moved frame of the actuator chip returns to its initial position.
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Standard image High-resolution imageA combination of silicon-on-insulator (SOI), Borofloat33® and silicon wafer is used. The functional layout is shown in figure 5. The system consists of three layers (see figure 5):
The actuator chip (1) (figures 5 and 6) is capable of a 2D in-plane motion. Symmetric comb drives (D1) pull the spring-suspended central frame in the desired direction. The comb drive structures have a symmetrical layout for the bidirectional operation in the x- and y-direction. The main geometrical properties of the comb drives are summarized in table 2 and defined in figure 4. The frame displacement is a single step (±10 µm) and is limited in each direction by mechanical stops. A numerical optimization was performed to maximize the out-of-plane (cz, z-direction) stiffness, while minimizing the in-plane stiffness (cx, cy, x- and y-direction) of the suspension springs. The calculated overall stiffness of the actuator frame suspension springs are listed in table 3. Isolated areas on the frame (A1 and C1) allow generating different electrical potentials across the frame. Hence, there are four mechanical contact areas between pinhole array and the frame. Only two of the four areas (C1) at a time are electrically contacting the pinhole array between substeps (b)–(e). The frame (A1) clamps the pinhole array in the vertical direction electrostatically whereat the corresponding mechanical contact areas on the pinhole array (C2) are separated electrically from the pinhole surface to avoid a short circuit.
Figure 4. Geometrical parameters of the electrostatic comb drive: (a) cross section of one comb electrode, (b) upper view of two comb electrode pairs.
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Figure 5. Schematic design of the micro-stepping drive; the functional parts of each element are highlighted.
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Figure 6. Focus stacked photograph of the fabricated actuator chip.
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Standard image High-resolution imageTable 2. Geometric parameters of the in-plane comb electrodes and out-of-plane vertical electrodes.
| Geometric property | Value |
|---|---|
| Comb electrode thickness (t) | 22 µm |
| Comb electrode height (h) | 100 µm |
| Comb electrode length (l) | 1020 µm |
| Comb electrode gap (g1) | 20 µm |
| Comb electrode gap (g2) | 60 µm |
| Comb electrodes per direction | 42 |
| Vertical electrode surface | 9.9 mm2 |
| Initial vertical gap | 10 µm |
Table 3. Main properties of the designed stepper drive.
| Displacement range | 200 × 200 µm2 |
| Step size | 10 µm |
| Driving voltage | <130 V |
| Vertical payload | 200 µN |
| Size of the system | 14 × 10.7 mm2 |
| Size of the pinhole array | 6 × 7.5 mm2 |
| Free optical aperture | 5 × 6.5 mm2 |
| Smallest structure gap | 10 µm |
| cx | cy | cz (N m−1) | 68.3 | 67.4 | 293.6 |
The pinhole array (2) has an electrical contact between the top and the bottom side; hence the top side and the bottom side are electrical short-circuited—except the two separated pads (C2, red) which have the electric potential of the center frame (A1, red) during substeps (b)–(e). The two green pads (A2) have an electrical contact to the pads C1 of the actuator chip during the substeps (b)–(e), thus they are responsible for a defined potential of the pinhole array throughout the whole stepping mechanism.
The clamping chip (3) fixes the pinhole array electrostatically with vertical clamping electrodes (B3, orange). The main properties of the vertical clamping electrodes are listed in table 2. The pinhole array is in mechanical and electrical contact to the elevated pad frame (A3, green) of the clamping chip during substeps (a), (b), (e) and (f), thus leaving an air gap between the vertical clamping electrodes of the clamping chip and the pinhole array.
The spacers (4–7) are cut from the same substrate as the pinhole array to reduce thickness and wafer bow differences. Tight thickness tolerances are crucial to keep the actuation voltages low and guarantee a reliable clamping mechanism. They also limit the displacement area of the pinhole array; therefore an accurate positioning is needed in the mounting process. The two longer spacers are featuring an additional 5 µm thin layer to increase the distance between the clamping chip and the actuator chip. Therefore, the simultaneous mechanical contact of the pinhole array with both the actuator chip and the clamping chip is avoided and it can be moved frictionless in substep (d) by the actuator. The main properties of the 2D stepping microdrive are summarized in table 3. The 3D-design and mounting steps are illustrated in figure 7.
Figure 7. Assembly of the micro-stepping drive (3D CAD models).
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Standard image High-resolution image3. Technology and processing
The fabrication process is shown in figure 8. The actuator chip itself is based on an SOI-substrate featuring a 100 µm device layer, 2 µm buried oxide layer (BOX) and 400 µm handle layer thickness (a). The pinhole array and the spacers are fabricated using 200 µm Borofloat33® wafers (b). For the clamping chip, a standard silicon wafer with 300 µm thickness is used (c).
Figure 8. Fabrication process for the micro-stepping drive.
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Standard image High-resolution image3.1. Fabrication of the actuator chip
On both sides of the SOI substrate (a1) thin aluminum (Al) layers are deposited and patterned by wet chemical etching (a2). Afterwards the device layer (100 µm) is etched utilizing a deep reactive ion etching process (DRIE) with a photoresist mask (a3). During this step the comb actuator, the springs and frame geometry are defined at a time. The optical aperture and additional openings under the springs and the electrostatic actuators are generated with a second DRIE process of the backside (handle layer) (a4). During this process the backside Al layer is used as mask and the chips are separated by 'plasma dicing'. To protect the sensitive device layer structures in the plasma process during the backside opening, a thin (1 µm) silicon dioxide (SiO2) film is deposited by inductively coupled plasma chemical vapor deposition (ICP CVD) (a3).
The device layer is released with hydrofluoric acid vapor etching in which the protective oxide layer is gently removed, too (a4). After the BOX etching, needle-like silicon particles were observed on the backside of the device layer. They are formed during the first DRIE process due to notching. In the last step, a light isotropic silicon etch process was carried out in an SF6 plasma to avoid any risk of a short circuit due to the needles.
3.2. Fabrication of the pinhole array and the spacers
On both sides of the Borofloat33® substrate 200 nm Gold (Au) is deposited and patterned in a wet chemical etch process (b2). During this step the pinhole array characteristics (pinhole size and distance) are defined. Afterwards a nickel layer with 10 µm thickness is deposited on both sides with electroplating processes generating the mechanical and electrical contact areas between the single chips (b3). In the last step the substrate is diced to separate the pinhole arrays and the spacers for later assembling (b4). The total thickness variation of the used glass wafers is below 10 µm to enable a reliable vertical gap between the clamping and the actuator chip.
3.3. Fabrication of the clamping chip
The fabrication of the clamping chip starts with thermal oxidation and an additional ICP CVD deposition of a thick (5 µm) SiO2 layer on the foreside (c2). An Al layer is deposited (c2) on top of the thick SiO2 layer and both layers are patterned with the same photoresist mask (c3). For the Al layer a wet chemical etch process is used whereas the SiO2 layer is etched utilizing reactive ion etching (RIE). A second Al layer is deposited and patterned by wet chemical etching (c4). Hence two separately patterned Al layers are generated that are positioned in two different heights on the substrate which is essential for the later electrostatic clamping function of the pinhole array. The elevated Al layer is in mechanical contact with the pinhole array and provides a defined electrical potential on the pinhole array surface. The lower Al layer forms together with the pinhole array the vertical acting electrode for clamping the array. In the last fabrication step the central optical aperture is generated by a DRIE process from the backside using a photoresist mask (c4).
3.4. Assembly
The chips are mounted using a manual precision mounting device featuring a vacuum placer and a dispenser unit. Several precision stages are used to align the chips respective to each other. The vacuum placer unit is mounted on two linear stages (x, z), and on a two-axis tilt stage. The ground plate is mounted on a linear stage (y) and a rotation stage. The dispensing unit is mounted on a three-axis stage (x–z). All chips are positioned considering alignment marks on the clamping chip with the help of two microscope cameras. The required accuracy for assembling is <25 µm to avoid a short circuit between the contact pads of the pinhole array and the actuator chip. The spacers are connected with the clamping chip utilizing ultraviolet curing electrically insulating adhesive Dymax® 429 (figure 7(b)). A precisely controlled amount of adhesive gel is dispensed in designated cavities to avoid an additional vertical gap otherwise arising from the thickness of the adhesive film. The pinhole array is placed on the clamping chip between the spacers (figure 7(c)) and the actuator chip is finally glued to the spacers (figures 7(d) and figure 9). The chip stack is connected to a printed circuit board (PCB) with thermal curing electrically conductive adhesive (iKTZ 2AlF). The electrical contacts to the PCB are created using bonded gold wires. The assembled stepping microdrive is presented in figure 10.
Figure 9. Placing the actuator chip with a vacuum pipette on the spacers.
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Figure 10. Assembled stepping drive mounted on a custom PCB.
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Standard image High-resolution image4. Device control
The device is controlled with a series of predefined time-stamped voltages for all electrodes to move the pinhole array.
The necessary voltage courses of a single-step cycle are shown in figure 11. The vertical pinhole electrode (green signal) has a constant potential throughout the stepping cycle and was chosen as reference (0 V). At the beginning, the pinhole array is locked to the clamping chip at a potential of −60 V (see figure 3(a)). Afterwards, the potentials of the actuator chip (blue and red) are raised to +60 V, which pulls the centre frame vertically to the pinhole array (b). Then, as the potential difference between the pinhole and the vertical clamping electrode (yellow) equals zero (c), the pinhole is released from the clamping chip. Afterwards, the potential of a pair of comb drives is reduced to −60 V, exerting a force on the central frame with the gripped pinhole array in the desired direction (d). After the step is made, the potential of the vertical clamping electrodes is set to −60 V, pulling the pinhole array to the clamping chip (e). Then the central frame releases the pinhole array. Finally, all potentials of the actuator chip equal that of the pinhole array, while the pinhole stays locked in its new position using the clamping electrodes of the clamping chip (f). The complete cycle for a single step is carried out in 180 ms.
Figure 11. Voltage courses of the actuator electrodes for one stepwise pinhole movement.
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Standard image High-resolution image5. Measurement results and discussion
The stepper drive motion is investigated using a high speed camera. The actuation plane of the actuator chip with the pinhole array underneath is oriented perpendicular to the gravitational force. In figure 12, one step in the (–y)-direction is shown with single frames of a high speed video.
Figure 12. Frames of the high speed video; showing one step in the (–y)-direction. The red overlay in (b) is highlighting the position difference of the pinhole array between (a) and (b).
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Standard image High-resolution imageIn figure 11, characteristic phases of the stepping mechanism are showed. To present the functionality of the 2D stepping drive a part of the pinhole array was captured without (figure 2(a)) and with (figure 2(b)) actuation. Here the pinhole array (pinhole diameter 40 µm, pitch 180 µm) was moved within 160 × 160 µm2; hence the whole object plane was captured, enabling a high quality HSI with the system as presented in [1]. An exposure with the hyperspectral imaging system can be made throughout the control phases (a), (b) and (f) (see figure 11), where the pinhole is locked to the clamping chip.
The step sizes for each direction are measured with a sequence of 10 consecutive steps and repeated twice. The tracked position of the pinhole array is presented in figure 13. The step size is calculated using the relative position differences. The results are summarized in table 4.
Figure 13. Step size measurement. (a) 10 steps in the +x and −x directions. (b) 10 steps in the +y and −y directions.
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Standard image High-resolution imageTable 4. Measured step sizes, respective deviances and maximum error of the stepper drive in each direction.
| Driving direction | Average value (µm) | Standard deviance (µm) | Maximum error (µm) |
|---|---|---|---|
| +x | 12.83 | ±0.24 | 0.96 |
| −x | 14.6 | ±0.48 | 1.77 |
| +y | 16.12 | ±0.33 | 1.52 |
| −y | 18.11 | ±0.86 | 1.43 |
While driven in the x-direction the position deviation along the inactive y-direction is 1.03 µm with maximum error about 4.22 µm. While driven in the y-direction the position deviation along the inactive x-direction is 1 µm with maximum error about 4.8 µm.
The measured step size differences in the x- and y-directions are mainly caused by etch rate inhomogeneity of the DRIE process resulting in different gap sizes at the mechanical stops. The average step size is 3–8 µm larger than the designed 10 µm. It is caused by two effects: The device structures are slightly overetched during the last the isotropic silicon etch process. This effect had not been taken into account during design. Additionally, the pinhole array has a vertical load on the center frame of the actuator chip in substep (d) (see figure 2). Therefore the edge of the center frame comes in contact with the etched wall of the mechanical stop, where the mask undercut and negative walls further increase the step size. In the case of future applications demanding higher precision, an optimization of the DRIE process has to be carried out.
The displacement range defined by the spacer chips is measured. The pinhole array is driven to a contact with a spacer chip and along the borders of the displacement area, as presented in figure 14. Whereas the maximal measured displacement area is 202 µm in the x- and 197 µm in the y-direction, the upper left corner is limited to 170 µm. The maximal displacement range is defined by the alignment precision of the assembly process and imperfections along the sliced edge of the spacers for the sample investigated here.
Figure 14. Measurement of the displacement range.
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The dynamic properties of the HSI system with the presented 2D stepper microdrive are depending on the chosen pinhole array. With a given pinhole array specification and actuator step size, the path of the stepping sequence has to be adopted to reach short measurement times by a full coverage of the object plane. In the example of figure 1(b), the sequence consisted of 86 steps with 208 ms step time covering a 160 × 160 µm2 area, resulting in a full scan within 18 s. In case of 16 spectral bands a hyperspectral image would take 288 s. Due to the 40 µm pinhole diameter and step sizes between 12–18 µm, the overlapping areas between the steps are higher than is necessary for a full scan of the object plane. Scaling down the pitch and diameter of the pinhole array can greatly reduce the scanning time [9]. Further reduction of the sampling time can be expected with an optimization of the control signals.
Due to the macroscopic functionality of the microdrive, there is a compromise which has to be considered. The risk of mechanical damage in the chip due to vibrations or shocks is avoided when the pinhole array is fixed to the clamping chip. The power consumption for the electrostatic fixing force is near to zero. Nevertheless, external accelerations during the operation of the imaging system have to be avoided with respect to the application, as the CCD exposure time is relatively high.
6. Summary and outlook
A 2D stepping micro drive was successfully implemented for pinhole array actuation. The pinhole array is based on a glass plate so it can also carry other types of filter arrays if required. The compact 14 × 10.7 × 1.1 mm3 microsystem features step sizes between 12.8–18.1 µm in the four driving directions. It is suitable for direct mounting on a standard CCD device. The step sizes have a standard deviation below 1 µm, which ensures a homogeneous coverage of the object plane in the hyperspectral imaging system. A 170 µm displacement range in two directions is achieved. The large 5 × 7 mm2 optical aperture allows a 1:1 imaging on a standard (8 mm diagonal) CCD. The presented stepping microdrive concept enables new applications for dynamically positioning relatively high payloads up to tens of milligrams with a highly integrated and compact microsystem e.g. also for angular band-pass arrays [18].
Acknowledgment
This work has been funded by the German Federal Ministry of Education and Research (BMBF) within the project Optical Microsystems for Ultra-compact Hyperspectral Sensors (OpMiSen | 16SV5575K).