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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

A micromachined electrostatically suspended six-axis accelerometer, with a square plate as proof mass housed by a top stator and bottom stator, is presented. The device structure and related techniques concerning its operating principles, such as calculation of capacitances and electrostatic forces/moments, detection and levitation control of the proof mass, acceleration measurement, and structural parameters design, are described. Hybrid MEMS manufacturing techniques, including surface micromachining fabrication of thin film electrodes and interconnections, integration fabrication of thick nickel structures about 500 μm using UV-LIGA by successful removal of SU-8 photoresist mold, DRIE of silicon proof mass in thickness of 450 μm, microassembly and solder bonding, were employed to fabricate this prototype microdevice. A levitation experiment system for the fabricated microaccelerometer chip is introduced, and levitation results show that fast initial levitation within 10 ms and stable full suspension of the proof mass have been successfully demonstrated.

An electrostatically suspended accelerometer mainly comprises a mechanically free proof mass and stator electrodes, which maintain the mass suspension at its null position by capacitive position detection and electrostatic levitation control. The operation principle of this servo-controlled electrostatic accelerometer is based on the measurement of the electrostatic force necessary to maintain the proof-mass motionless with respect to the sensor cage [

Traditional electrostatic space accelerometers, whose proof-mass is usually made of platinum-rhodium alloy or gold coated titanium alloy, are mainly manufactured by accurate machining and grinding, and thus suffer from the problems associated with complicated machining processes, large size and high cost, which limits their potential applications for micro platforms such as micro spacecraft, micro aerial vehicles, unmanned underwater vehicles, small long-range munitions,

Although the ESMA has the potential to deliver navigation-grade performance, relatively little work has been done to realize it, due to the difficult techniques required, such as microfabrication, high vacuum packaging, detection and control of six degrees of freedom (6-DoF) of the proof mass. Among these techniques, one of the most challenging is microfabrication, because very small gap spacing, for electrostatic forces or torques generation and capacitive detection, should be formed between the proof mass and the stators. Using proprietary Ball semiconductor technology [^{1/2} level. For a flat disc-like proof mass mostly employed, the micromachined electrostatically suspended accelerometer, as well as rotational microgyroscope by rotating the disc proof mass at high speed, have been reported mainly in [^{1/2} in 10 Hz bandwidth [

Two main popular micromachining techniques have been investigated to develop the levitated microaccelerometer, that is, high-aspect-ratio electroplating [

To improve the operational stability and capacitive detection sensitivity of the ESMA in all directions, especially in lateral orientation, thick flat proof mass and high aspect ratio lateral gaps between the proof mass and sidewall electrodes are required. High aspect ratio micromachining methods, like the well-developed bulk silicon DRIE process and non-silicon X-ray LIGA or UV-LIGA process, can be used to fabricate such thick proof mass and sidewall electrodes. Although the X-ray LIGA and DRIE techniques are better processes for high aspect ratio microstructures, advanced equipment as well as complicated fabrication procedures are an evident burden to production cost. The UV-LIGA technique, utilizing ultraviolent (UV) light as exposure energy, can be performed with the standard lithography equipment. As for the ultrathick, high-aspect-ratio MEMS-type applications, the most promising UV-LIGA technology is based on the SU-8 photoresist [

In this paper, the structural design and detailed operation principles of a six-axis electrostatically suspended microaccelerometer, with a square plate as proof mass housed by a top stator and bottom stator, are presented. Hybrid MEMS manufacturing techniques, including surface micromachining fabrication of thin film electrodes and interconnections, integration fabrication of about 500 μm thick nickel structures using SU-8 UV-LIGA, DRIE of silicon proof mass in thickness of 450 μm, microassembly and solder bonding, were employed to fabricate this sensor prototype. In addition initial levitation and stable suspension of the proof mass for the fabricated ESMA chip were successfully demonstrated. Section 2 of this paper describes the design of the ESMA device and its detailed operating principles. The microfabrication processes of the designed ESMA, including processes of the top stator, bottom stator, silicon proof-mass and reflow bonding, are described in Section 3. Section 4 is the fabrication results and discussion. Levitation experiment system and results for the fabricated ESMA chip are presented in Section 5. Section 6 gives the conclusions of this paper.

In the following, the designed structure and detailed techniques about the operation principles for the micromachined electrostatically suspended six-axis accelerometer are described.

To realize stable electrostatic levitation, the electrostatic forces should be generated in such a way that the net potential of the proof mass is held a constant, usually virtual ground, so that by changing potentials of the control electrodes, magnitude and direction of the resultant forces or moments exerting on the proof mass can be precisely adjusted. Since there is no mechanical connection to the substrate, to maintain the proof mass at zero potential and to measure its suspended position, two special approaches are often taken. One approach, employed in the macro electrostatically suspended accelerometer [

The structure of an electrostatically suspended six-axis microaccelerometer with a square plate proof mass housed by stator electrodes, assembled using hybrid microfabrication technology, is illustrated in

A square plate, which may have a certain number of through holes (not shown in this figure) to reduce air damping when working at low vacuum environment, is employed as the suspended proof mass. It can be made of silicon fabricated by DRIE or metal employing LIGA-like micromaching methods or conventional ultra-precision machining methods.

A top glass plate and a bottom glass plate are used as substrates for carrying electrodes and interconnections. The stator electrodes, composed of axial levitation electrodes, lateral levitation electrodes and common electrodes, are symmetrically arranged around the proof mass to form capacitors for 6-DoF electrostatic levitation and capacitive detection. Here levitation control electrodes are used for electrostatic forces generation and proof-mass position sensing simultaneously. On the bottom stator and top stator, thin film electrodes, composed of axial levitation electrodes and common electrodes, are formed. Four pairs of axial levitation electrodes at each stator plate, illustrated as Z1∼Z4 at the top stator and Z5∼Z8 at the bottom stator, are symmetrically disposed along the X-axis and Y-axis. These axial levitation electrodes are used to detect and control the proof-mass displacement in three degrees of freedom, which are the translation along the Z-axis and the rotations around the X and Y axes. There is a common electrode located in the center of each plate. Both the common electrodes, used for signal pick-off or exciting, are connected together to form a large common electrode. On the bottom stator, eight pairs of lateral levitation electrodes made of thick electroplating metal, labeled X1∼X4 in the X-axis direction and Y1∼Y4 in the Y-axis direction, are symmetrically distributed around the axial levitation electrodes. The lateral levitation electrodes are detection and control electrodes used for 3-DoF in-plane motion control of the proof mass, that is, the translations along the X and Y axes, and the rotation around the Z-axis.

To prevent large axial and lateral displacements of the proof mass from contacting the stator electrodes, five axial stoppers and eight lateral stoppers, as shown in

To develop a six-axis microaccelerometer, the levitated proof mass must be stably maintained at its null working position relative to the stator. The levitation of the proof mass, in 6-DoF of three translations and three rotations, is servo-controlled by capacitive position detecting and electrostatic actuating through control electrode pairs.

The closed-loop electronic control is used to generate electrostatic forces maintaining the proof mass at the center of the cavity. In the presence of external forces and/or moments, the proof mass displaces away from its nominal position, resulting in a change in the capacitance formed between the levitation control electrodes and the proof mass. The control electronics then detect these capacitance changes and, in turn, apply voltages to the levitation electrodes, generating electrostatic forces in order to rebalance the position of the proof mass. The voltages that are required to generate these balancing forces and/or moments can be used to measure the input accelerations.

To describe the operation of the designed 6-axis ESMA in detail, in the following, the capacitances and electrostatic forces/moments between levitation electrodes and the proof mass, position detection of the proof mass, acceleration measurement and main performance indexes, structural parameters design based on hybrid microfabrication method, are presented.

Here only the lateral levitation electrode pairs X_{ip}, X_{in}) (_{ip}, X_{in}) (_{ip} is set to application of positive levitation voltage while the other electrode X_{in} will be applied negative levitation voltage of the same magnitude. The derivative process of capacitances, electrostatic forces/moments for other levitation electrode pairs of Z-axial and Y-axial directions are similar and omitted here.

In _{x}_{1p}, is:
_{s0}_{0}_{1}_{1p}.

The capacitance between the lateral levitation electrode X_{1p} and the proof mass is:
_{1}_{0}

In the same way, the capacitance between the lateral levitation electrode X_{1n} and the proof mass is:
_{2}_{3}_{1n}.

Therefore, the linear expression for the total capacitance between the lateral electrode pair X1(X_{1p},X_{1n}) and the proof mass is:

Here, nominal capacitance between one lateral electrode pair and the proof mass at the null position is:
_{xN}_{ϕN}

In the same way, the linear capacitance expressions between the other three lateral electrode pairs and the proof mass are:

So the differential capacitances caused by the translation displacement

Similarly, the linear capacitance expressions between the four lateral electrode pairs Y_{ip}, Y_{in}) (_{yN}

Using the same derivative process, the linear capacitance expressions between the eight axial levitation electrode pairs Z_{ip}, Z_{in}) (_{Z0}_{zN}_{ψN}_{θN}

The electrostatic force applied to the proof mass generated by the lateral electrode X_{1p} with actuation voltage of _{x1}

As well, the electrostatic force generated by the electrode X_{1n} is:

Therefore the electrostatic force generated by the lateral electrode pair (X_{1p}, X_{1n}) is:

Here _{s0}

In the same way, the electrostatic forces generated by the other three lateral electrode pairs are:

For the voltage applied to the levitation electrode, a preload bias voltage superimposed feedback voltage, which is generated by a controller according to displacements of the proof mass, is employed. Assuming the feedback voltage that makes the proof mass have positive linear or angular displacement, in right-hand rule, is positive, the voltages applied to the lateral levitation electrode pairs X_{ip}, X_{in}) (_{x}_{ϕ}

So, the resultant electrostatic force along the X axis generated by the lateral levitation electrode pairs (X_{ip}, X_{in}) (

This is a one order model about electrostatic force and displacements of the proof mass, where axis-coupling between ^{X}^{X}^{X}

When the active electrostatic bearing system of the micro-accelerometer is working at the null place, the _{xN}_{ϕN}_{ϕ}_{ϕ}

This is a linear and decoupled model of electrostatic force and displacements of the proof mass, which is often used as control model of the electrostatic bearing system and by combining PID controller using different control algorithms, the performance parameters of the system such as frequency characteristics, bandwidth et al could be designed [

According to the formula (2), the electrostatic moment applied to the proof mass, generated by the lateral electrode X_{1p} with actuation voltage of _{x1}

As well, the electrostatic moment generated by the electrode X_{1n} is:

Therefore the electrostatic moment generated by the lateral electrode pair X1 (X_{1p}, X_{1n}) is:

Here:

In the same way, the electrostatic moments generated by the other three lateral electrode pairs are:

The resultant electrostatic moment around the Z axis generated by the lateral levitation electrode pairs X_{ip}, X_{in}) (

This is a one order model with respect to the electrostatic moment and axis-coupling displacements of the proof mass, which is used to accurately describe angular movement of the proof mass. Like the electrostatic force ^{X}

When the micro-accelerometer is working at the null place, the small second order quantities of the _{xN}_{ϕN}_{ϕ}_{ϕ}

According to geometry symmetry and the same derivative process as the X-axial lateral electrode pairs, we can get similar linear and decoupled model as (31) about electrostatic moment

Therefore the total electrostatic torque ^{ϕ}

The electrostatic forces along Y-axis and Z-axis, and total electrostatic moments around X-axis and Y-axis can be derived in the same way.

To detect displacement signals of the ESMA, two capacitive detecting schemes can be applied to the same accelerometer structure. One detecting scheme is the frequency multiplex method with common electrodes used for signal pickoff [

In this paper, the frequency multiplex detecting method is employed. For the electrodes structure of the microaccelerometer shown in _{1p}, X_{1n}) for example, the actuation voltages +_{x1}_{x1}_{1p} and X_{1n} respectively, meanwhile a sensing voltage _{x1}_{x1}_{x1}

The top and bottom common electrodes are electrically connected together, so a total common capacitance between a common pick-up electrode and the proof mass is _{c}_{T}_{B}_{p}_{m}_{m}

Set Laplacian operator _{c}

Assuming (_{x}_{y}_{z}_{ψ}_{θ}_{ϕ})

Here they have six different frequencies, and the carrier signals applied to differential electrode pairs are of the same magnitude but phase-inverted. Then, the hybrid-frequency sensing signal applied to each control electrode pair is expressed as:

When the proof mass is suspended at its null position by electrostatic bearing system, its 6-DoF displacements (_{p}_{c0} are the nominal capacitances of common electrodes, for small displacements of the proof mass, _{T}_{B}_{c0}_{p}_{c}C_{z0}_{c}C_{r0}

The above displacement-detection _{p}_{f}_{p}_{p}

It can be seen that the output voltage _{p}_{p}_{j}

According to the force balance condition:
_{x}_{x}_{x}_{z}_{z}_{y}_{y}

When the proof mass of the accelerometer is actively suspended at its working position, the moment balance condition for measuring applied angular acceleration _{z}^{4}h/6._{ϕ}_{ϕ}

Similar relationships between the feedback voltage _{ψ}_{ψ}_{θ}_{θ}

From expressions (43) and (45), we can see that the inputted accelerations _{x}_{ϕ}_{x}_{ϕ}

When the feedback voltage is equal to the bias voltage, the proof mass is subjected to the maximum of electrostatic force/moment. Thus, the maximum measurable acceleration of the accelerometer can be theoretically calculated as:

Thus, by increasing preload bias voltage, the maximum measurable acceleration can be improved, however, the upper bound of allowable actuation voltage is set by electric breakdown voltage between the proof mass and the associated electrodes. According to the modified Paschen curve for MEMS structures [_{x}_{ϕ}

It can be seen that the sensitivity _{x}_{ϕ}

By designing large sensing capacitive gaps and employing a large mass

Remarkably, the detection sensitivity varies inversely with the maximum measurable acceleration, that is, when operation range of the accelerometer is designed larger, the detection sensitivity or resolution is lower.

According to the above-mentioned analysis, for different parameters of the structure and levitation control system, performances of the electrostatically suspended microaccelerometer may vary depending on different applications. For instance, the ESMA may be used as following [

According to the above analysis, the mass and size of the proof-mass, the capacitive gaps, and the surface flatness of the electrodes and the proof-mass are directly linked to the measurement range, sensitivity and accuracy of the ESMA. A relatively large proof mass, with a thickness of several hundred microns, should be fabricated, which will reduce the Brownian displacement noise, consequently, making the device potentially suitable for the development of a high performance accelerometer, so the accurate microfabrication of large area, small gap spacing capacitors is one of the key techniques for this highly sensitive ESMA device.

Microassembly is a critical enabling technology for hybrid manufacturing of complex 3D MEMS devices that utilize non-semiconductor materials and incompatible fabrication processes [

As shown in _{p}_{z0}_{s0}_{c0}_{c}

In this paper, the main structural parameters illustrated in

As the structure shows in

In the following, the microfabrication processes of the two stators, the proof mass and the bonding are described, respectively, and the key fabrication steps are explained in detail.

To fabricate the top stator of the ESMA, special attention should be paid to the wet etching of the positioning pits, because by metallization of the pits, such as in electroplating, the top stator can be solder joined with the pillars of the bottom stator, which provides signal routes to the pads of the bottom stator. For metallization, the deep etched pits should have sloped walls with smooth surfaces and a small ratio of lateral undercutting to the etch depth. Borosilicate glass, such as Pyrex, can be wet etched with better pits or grooves profile, so a Pyrex 7740 wafer is used as the top substrate in this work. The fabrication flow of the top Pyrex wafer is simply illustrated in

The first part, as shown in

As shown in _{2}O_{2}-H_{2}SO_{4} (1:2 vol. parts), rinsed with deionized water and then dried. Secondly, as sputtered thin films were reported to have better performance as etch masks due to their high film density and better adhesion, in this work, magnetron sputtering was employed to deposit a 40 nm thick Cr adhesive layer and a 300 nm gold layer as metal etch mask [

The second part is a surface micromachining process, as shown in

For bottom stator fabrication of the gyroscope, the surface micromachining process was employed to form stoppers, thin film electrodes and the interconnections. Then, using SU-8 ultrathick photoresist as electroplating mold, the thick nickel structures, such as pillars, lateral electrodes and stoppers, were integrated with the fabricated circuitry on the same chip. The key fabrication technique of the bottom stator was the integration of thick nickel structures with the bottom structures by successful removal of SU-8 mold.

SU-8 has been used extensively in making high aspect ratio MEMS device structures [

A schematic diagram of the fabrication process of the bottom stator, using quartz glass as substrate, is shown in

For subsequent integration of thick nickel microstructures, a 0.5 μm alumina passivation layer was patterned with phosphoric acid to expose the surface of the metal foundations, on which thick Ni structures would be electroformed. To obtain good step coverage, a 30/200 nm Cr/Au layer, serving as a seed layer for electroplating, were deposited via sputtering [

On the exposed areas of the Cr/Au seed layer, the Ni structures were electroplated in a nickel-sulfamate based solution. The electroformed Ni structures, together with the SU-8 mold, were planarized by a grinding step [_{2}SO_{4}·xSO_{3}), with 40% free sulfur trioxide content, holding in airtight glass container. During etching and oxidizing process, the cross-linked SU-8 mold was broken into gel-like tiny pieces. With the penetration of the acid, the SU-8 mold was gradually etched from the surface to the depth. For complete etching removal of the about 500 μm thick SU-8 molds, the wafer was continuously immersed for about 4 hours. Finally, the etched residue was cleaned in acetone and DI water with ultrasonic vibration. After removing the SU-8 mold, the lapped Ni structures standing on the substrate were released [

The proof mass fabrication process was relatively simple because only one lithography step was needed. However, a flat proof mass with vertical sidewalls and smooth surfaces had to be carefully fabricated. In this paper, a monocrystalline silicon proof mass was micromachined by DRIE using an Alcatel 601E machine. Firstly, a double-side polished, n-type Si (100) wafer, in a designed thickness of the proof mass, was bonded with an accompanying wafer, using 4 μm benzocyclobutene (BCB, Dow Chemical) void-free coating as the adhesive material [

For the assembly, each released proof mass, with required sizes of side length and thickness, was put into each bottom stator which has been thinned by grinding and polishing with controlled height of the lateral electrodes; then, the bottom stator was aligned with the top stator through the transparent glass substrate. They were then clamped by a jig, which would provide a pre-tightening force for tight contacting between the top stator and top surface of the lateral electrodes after reflow bonding, consequently, the designed axial gaps would be ensured. Finally, the two clamped stators were bonded by reflow of SnAg solders at N_{2} atmosphere in a reflow oven, with a peak temperature of about 260 °C. This also provided an automatic self-alignment of the two stators due to the surface tension during the reflowing process. This bonding could be wafer-level or chip-level.

A microphotograph of one of the etched pits after removal of the photoresist is shown in ^{2}, was measured to be about 27 μm in depth and laterally, the undercutting was only 38 μm. In these etched pits, the nickel pads and SnAg solder were successfully electroplated. An optical photograph of one die of the fabricated top stator is seen in

For the fabrication of the bottom stator, as the SEM image in

Optical photographs of one of the silicon proof masses, made from the double-side polished wafer through DRIE then coated with sputtering deposited Cr/Au conductive layer, are shown in

After the three parts, namely top stator, bottom stator and the proof mass, were assembled and the two stators were bonded by solder reflow, an ESMA chip about 10 × 10 × 3 mm^{3} in size, forming lateral gap of 10 μm and axial gap of 4 μm, was obtained, as the optical image in

A schematic diagram of close-loop detection and control system for the ESMA sensor, which mainly consists of micro-displacement detection unit, DSP control unit and voltages combining unit, is illustrated in

Employing an AD9832 as direct digital synthesis (DDS) chip controlled by the 89C51 microcontroller, together with band pass filter (BPF) and phase shifter circuits, the signal generator can produce multi-frequency sine waves with changeable phases. These signals, with frequencies in the range of 100∼220 KHz, are used as 6-axis displacement detection carriers and reference waves for modulation and demodulation, respectively. By applying sensing signals to the control electrodes, the small AC current detecting signal _{p}_{p}_{1p}, X_{1n}) and X3(X_{3p}, X_{3n}), shown in _{x1}_{x3}_{x0}_{x}_{ϕ}_{x1}

At present, stable full levitation control of the proof mass has been demonstrated, as levitation displacement curves shown in

Since the Tektronix oscilloscope employed has only four channels, position response curves of the proof mass, in form of detection voltages, for channel Z1, Z2, Z3 and X positions are illustrated in

Here the proof mass was levitated against the force of gravity along the Z-axis direction from its seated axial stoppers to some given suspension position. Z1, Z2 and Z3 were the three differential capacitance positions of four differential capacitances formed between Z-axis levitation electrodes and the proof mass, as shown in

In this paper, a designed structure of a micromachined electrostatically suspended six-axis microaccelerometer, with a square plate as proof mass housed by top stator and bottom stator, was presented. Its detailed operation parameters such as the capacitances and electrostatic forces/moments, detection and levitation control of the proof mass position, acceleration measurement and main performance indexes, structural parameters design using hybrid microfabrication method, were presented. A precision ESMA prototype chip was achieved using hybrid microfabrication techniques, that is, the top stator and the bottom stator were fabricated by surface micromachining and UV-LIGA integration fabrication of thick nickel structures using SU-8 photoresist, the gold coated silicon proof mass of 450 μm in thickness was mainly obtained by DRIE, and then they were assembled and solder-reflow bonded. At the bottom stator, by successful removal of SU-8 mold using fuming sulfuric acid oxidizing method, the nickel structures about 500 μm thick were integrated with the surface micromachined structures such as axial stoppers, electrodes and interconnections. Levitation control experiments show that fast initial levitation within 10 ms and stable full suspension of the proof mass for the fabricated ESMA chip were successfully demonstrated. At present, linear and angular acceleration measurement and calibration of this microaccelerometer are ongoing. For higher microfabrication precision and smaller lateral gap spacing formation, the x-ray LIGA technique will be employed in further research. The hybrid microfabrication techniques described in this paper can be applied for fabrication of other complicated 3D MEMS devices.

The authors wish to thank Beijing Research Institute of Aerospace Control and Instrument for financial support. This work is partly supported by Aerospace Support Foundation (11GFZ-JJ02-005), Shanghai Municipal Natural Science Foundation (10ZR1415500), National Natural Science Foundation of China (51075278), Program for New Century Excellent Talents in University from Ministry of Education of China (NCET10-0583) and National Key Lab Foundation of Nano/Micro Fabrication Technology (9140C790405110C7904).

The exploded view of the microaccelerometer configuration.

Structural parameters of the accelerometer along X-axis direction.

Surface plots of ^{X}_{s0}_{s0}

Equvalent capacitive circuits of the micro-accelerometer when common electrodes are used for pickoff.

Schematic fabrication structure of the micro-accelerometer.

Process flow of the top stator.

Schematic diagram of the fabrication process of the bottom stator.

Fabrication results of the top stator:

Fabrication results of the bottom stator: ^{2}.

^{3}, where the pads are coated with solder bumps.

Schematic diagram of detection and control system for the ESMA, taking differential electrode pairs X1(X_{1p}, X_{1n}) and X3(X_{3p}, X_{3n}) for example.

Levitation experiment system for the fabricated micro-accelerometer.

Electrostatic levitation displacement curves of the proof mass:

Main design parameters of the first prototype of ESMA.

Basic side length of the proof mass, |
6 |

Basic thickness of the proof mass, |
450 |

Mass of the proof mass, |
43.74 (silicon), 144.18 (nickel) |

Main moment of inertia, _{z}^{−10} kg m^{2} |
2.624 (silicon), 8.651 (nickel) |

Moment of inertia around x- or y-axis, _{x}^{−10} kg m^{2} |
1.320 (silicon), 4.350 (nickel) |

| |

Nominal axial gap, dz0/μm | 3∼5 (vary with the lapped height of the lateral elctrodes) |

| |

Nominal lateral gap, ds0/μm | 3∼12 (vary with small changes of side length of the proof mass) |

| |

Area of each axial levitation electrode Az/mm^{2} |
2.288 |

| |

Area of each common electrode, AC/mm^{2} |
16.183 |

| |

Area each lateral levitation electrode, As/mm^{2} |
0.586 |

| |

Capacitance of all common electrodes,CC0/pF | 71.639(dz0 = 4 μm) |

| |

Capacitance of one axial electrode pair, Cz0/pF | 10.129(dz0 = 4 μm) |

| |

Capacitance of one lateral electrode pair, Cs0/pF | 1.020(ds0 = 10 μm for UV-LIGA) |

| |

2.550(ds0 = 4 μm for X-LIGA) | |

| |

Bias voltage for axial suspension, Vz0/V | 10 ∼ 40 |

| |

Bias voltage for lateral suspension, Vs0/V | 20 ∼ 50 |

Process conditions of SU-8 2100 (from MicroChem) with mold height of about 500 μm.

Spin-coating | Spread: 200 rpm ,15 s | Karl Suss RC8 |

Spin: 700 rpm, 20 s | ||

Soft bake | 65 °C, 30 min; 95 °C, 120 min | Hotplate |

Exposure | Dose 2.6 mW/cm^{2}, 250 s |
Karl Suss MA6 |

Post-expose bake | 65 °C, 30 min; 90 °C, 40 min | convection oven |

Development | 40 min immersion in PGMEA | |

RIE cleaning | O_{2} 35 sccm, pressure 5 Pa, 25 W, 300 s |
Alcatel Nextral 100 |