Low-temperature anodic bonding of silicon to silicon wafers by means of intermediate glass layers

Silicon wafers have been anodically bonded to sputtered lithium borosilicate glass layers (Itb 1060) at temperatures as low as 150–180 °C and to sputtered Corning 7740 glass layers at 400 °C. Dependent on the thickness of the glass layer and the sputtering rate, the sputtered glass layers incorporate compressive stresses which cause the wafer to bow. As a result of this bowing, no anodic bond can be established especially along the edges of the silicon wafer. Successful anodic bonding not only requires plane surfaces, but also is determined very much by the alkali concentration in the glass layer. The concentration of alkali ions as measured by EDX and SNMS depends on both the sputtering rate and the oxygen fraction in the argon process gas. In Itb 1060 layers produced at a sputtering rate of 0.2 nm/s, and in Corning 7740 layers produced at sputtering rates of 0.03 and 0.5 nm/s, respectively, the concentration of alkali ions in the glass layers was sufficiently high, at oxygen partial pressures below 10^-4 Pa, to achieve anodic bonding. High-frequency ultrasonic microanalysis allowed the bonding area to be examined non-destructively. Tensile strengths between 4 and 14 MPa were measured in subsequent destructive tensile tests of single-bonded specimens.     

The fabrication of LIGA mask using photolithography and synchrotron radiation lithography

 The fabrication of LIGA mask is a very important step in LIGA process. Usually an intermediate mask with gold absorber pattern of 2 μm thickness is fabricated firstly using gold electroplating for absorber pattern in the resist structure written by Electron Beam (e-Beam), then the LIGA mask can be copied from the intermediate mask using synchrotron radiation lithography and gold electroplating. Recently, we use photolithography (instead of e-beam) to make the primary structure, and produce the intermediate mask with gold absorber pattern of 1.5 μm thickness produced by etching gold film with 1.5 μm thickness under the photoresist structure using Ar⁺. The LIGA mask with absorber pattern of 13 μm thickness is copied from the intermediate mask using synchrotron radiation lithography and gold electroplating. Received: 30 October 1995/Accepted: 9 December 1995     

The study of LIGA technique at NSRL

 The activity of LIGA technique was started in the fall of 1993 at National Synchrotron Radiation Laboratory (NSRL), some primary results have been obtained. Micro gearwheels were produced by deep-etch lithography using Hefei light source. The largest diameter of gearwheel is 200 μm, the smallest one only 35 μm, both with thickness of 50 μm. The first metallic structure of nickel was fabricated recently. The mask used in experiments was made by ourselves, it is a composite of kapton membrane as a mask blank, and gold as an absorber. Received 30 October 1995/Accepted: 25 January 1996     

Silicon IC compatible LIGA mask-fabrication process

 This paper describes a technique for fabricating a LIGA mask that offers good compatibility with the silicon IC process. X-ray exposure can be eliminated from the LIGA mask-fabrication process, so that LIGA masks can be fabricated with existing silicon IC process equipment. A gold absorber pattern, 2 μm wide and 10 μm thick, has been successfully fabricated by combining a three-layer resist-patterning process with the electroplating process. Improvements in both the mask structure and fabrication process alleviate the problems of dust in a cleanroom and contamination in the etching chamber. Received: 25 August 1997/Accepted: 23 September 1997     

Manufacturing pinhole arrays for X-ray source diagnostics using LIGA

 The manufacturing of pinhole arrays with diameters 10 μm up to 60 μm in 100–300 μm gold foils is described using LIGA. These structures are used to characterize the electron beam in a storage ring by means of the emitted synchrotron radiation. The source inside the bending magnet 13 at BESSY I (single bunch mode, 70 mA ring current, pinholes: ∅60 μm) is (448±40) μm in height and (768±40)μm in horizontal width. For BESSY II pinholes with ∅ 10–20 μm will be used utilizing bending magnet radiation (resolution ∼10 μm), single pinholes with 1–5 μm in diameter are dedicated to insertion device characterization (coherence). Received: 25 August 1997/Accepted: 23 October 1997     

Inexpensive, quickly producable X-ray mask for LIGA

 The capability to produce X-ray masks inexpensively and rapidly is expected to greatly enhance the commercial appeal of the LIGA process. This paper presents a process to fabricate X-ray masks both inexpensively (under $1000) and rapidly (within a few days). The process involves one UV lithography step and eliminates the need for an intermediate X-ray mask. The X-ray mask produced by this process consists of a 125 μm thick graphite membrane that supports a gold-on-nickel absorber pattern. The thickness of the absorber structures is great enough to supply sufficient contrast even when radiation sources with high characteristic photon energies up to 40 keV are utilized and/or when deep exposures are desired. The mask fabrication process is initiated by spin coating 30–50 μm of SU-8 directly on a graphite membrane. The SU-8 is then patterned using a UV mask. Gold-on-nickel absorber structures are electroplated directly onto the SU-8 covered graphite. Once the remaining SU-8 is removed, attaching the graphite membrane to a frame completes the mask. To test the performance of the mask, a nickel mold insert was fabricated. A sheet of PMMA 500 μm in thickness was bonded to a nickel substrate, then exposed to X-rays through the mask, and developed. Electroplating nickel into the patterned PMMA sheet produced a mold insert. SEM pictures taken of the SU-8, the X-ray mask, and the mold insert are shown. This method of rapidly producing an inexpensive X-ray mask for LIGA resulted in a mold insert with smooth, vertical sidewalls whose dimensions were within two micrometers of the UV mask dimensions. Received: 12 December 1998/Accepted: 2 February 1999     

Bond-quality characterization of silicon-glass anodic bonding

A simple testing method is presented that allows the comparison of the bond quality for anodically bonded wafers. An array of parallel metal lines of predetermined thickness is formed on a glass wafer. The estimation of the bond quality can be performed by visual inspection after the bonding. This method enables comparison of the anodic-bonding process performance for different glasses, for intermediate layers and various bonding conditions. The optimization of silicon-glass anodic bonding with an intermediate phosphosilicate glass (PSG) layer is shown using this technique.     

Fabrication method of spacers with high aspect ratio – Used in a field emission display (FED)

Glass to glass anodic bonding using a metal interlayer was used to develop a fabrication method of spacer for field emission display (FED). In this paper, spacers with width 100 μm and height 1000 μm and a 3.54 inch mono color anode plate patterned with Al/Cr film as an interlayer were bonded by the anodic bonding. To bond the spacers on the anode plate vertically, two types of spacer holders were designed and fabricated with photoetchable glass and n(110) Si wafer. The spacer holder using Si wafer was used to fabricate for evacuated FED panel. Received: 22 November 1999/Accepted: 27 January 2000     

Movable microstructures made by a sub-micron LIGA process

 Movable microstructures with high aspect ratios were made with lateral dimensions down to the sub-micron domain by the LIGA process and a sacrificial layer technique. Compared to microstructures usually made by LIGA, all dimensions were reduced approximately by a factor of 10, while the aspect ratio was kept constant. The smaller resist thickness in the range of some ten micrometers allowed much lower X-ray doses and energies to be used for exposure and the absorbers with a thickness of only 3 μm to be employed. As the lateral dimensions are smaller, a much larger number of devices can be fabricated in one batch. Therefore, the production costs of deep etch X-ray lithography are reduced dramatically. Electrostatic linear actuators with lateral dimensions as small as 500 nm were manufactured to demonstrate the advantages of LIGA in sub-micron dimensions. An X-ray mask with absorbers 2.8 μm high was produced by a three-level technique. The linear actuators consisted of several arrays of capacitor plates combined into an electrostatic driving unit with an active area, typically, 0.3 mm² and less. The driving unit was supported by folded beam flexures to avoid frictional forces. They also guaranteed parallel movement of the capacitor plates. The functionality of these devices was demonstrated by measuring displacement as a function of the voltage applied. Received: 30 October 1995 / Accepted: 17 January 1996     

LIGA-fabricated compact mm-wave linear accelerator cavities

 Millimeter-wave rf cavities for use in linear accelerators, free-electron lasers, and mm-wave undulators are under development at Argonne National Laboratory. Typical cavity dimensions are in the 1000 μm range, and the overall length of the accelerator structure, which consists of 30–100 cavities, is about 50–100 mm. An accuracy of 0.2% in the cavity dimensions is necessary in order to achieve a high Q-factor of the cavity. To achieve this, these structures are being fabricated using deep X-ray lithography, electroforming, and assembly (LIGA). The first prototype cavity structures are designed for 108 GHz and 2π/3-mode operation. Input and output couplers are integrated with the cavity structures. The cavities are fabricated on copper substrates by electroforming copper into 1 mm-thick PMMA resists patterned by deep x-ray lithography and polishing the copper down to the desired thickness. These are fabricated separately and subsequently assembled with precision spacing and alignment using microspheres, optical fibers, or microfabricated spacers/alignment pieces. Details of the fabrication process, alignment, and assembly work are presented in here. Received: 25 August 1997/Accepted: 20 October 1997     

Transfer of small structures by bonding

The work presented in this paper deals with the bonding of small structures, down to 1 μm. Its aim is to evaluate the dimensional limits of anodic bonding between silicon and pyrex 7740 glass. Test structures consisting in silicon pillars with controlled radii have been developed. The silicon pillars have been fabricated by deep reactive ion etching to allow a good geometry control of the structures. A collection of matrices of 3×3 identical silicon test structures with dimensions from 200 to 1 μm has been fabricated to determine the smallest area that can bond anodically. The test results have been applied to the transfer of small structures from one wafer to another wafer by bonding, with the final objective of transferring tips for AFM probes. From the test results, a new test for bonding has been defined, based on the pull test of small structures with controlled dimensions. Preliminary simulations by FEM of the pull test of the test structures are in agreement with the experimental results. The test has been used to determine the effect of the voltage and temperature conditions during the anodic bonding on the bond strength.     

Pneumatically bidirectional microfluidic regulation using Venturi pumps by deep RIE and bonding technology

 A novel design for bidirectional fluidic motion has been demonstrated which is widely used in the biochip or microfluidic component. Two miniaturized Venturi pumps as well as pneumatic servo system are designed to easily control the bidirectional fluidic motion by simple fabrication. The pumping velocity is 0.86 μl/min at a 2.75 slpm (standard liter per minute) air flow read from mass flow controller (MFC) for totally 4.3 μl blue ink in a 300 μm wide by 300 μm deep channel. The higher airflow, the faster fluidic pumping speed. Numerical simulation is performed to correlate the experimental data of fluidic speed and air flow in microchannel. The test chip with two Venturi pumps and channel was batchedly fabricated by silicon deep reactive ion etching (RIE) and glass anodic bonding. The ICP LIGA process is also investigated after deep RIE followed the electroforming and hot embossing. Received: 10 August 2001/Accepted: 24 September 2001     

Direct, high throughput LIGA for commercial applications: a progress report

Direct LIGA; LIGA without injection molding; has the potential to become a cost effective, high throughput form of LIGA. The process requires high energy photons; near 20,000 eV; which are best produced in facilities such as the X-ray ring at Brookhaven National Laboratory. The increased absorption lengths over lower energy photons eliminates the need for a membrane type X-ray mask. This in turn facilitates very large area X-ray masks fabricated from standard silicon wafers with 20 μm gold absorbers. The absorption length increase in PMMA to 2 cm is used to implement stacked PMMA exposures in which 1 mm thick PMMA layers are used to produce exposed PMMA sheets. These sheets are eventually solvent bonded to working substrates with plating bases. New high energy X-ray masks have been developed. Two exposure stations at Brookhaven are operational. The recently commissioned manufacturing exposure station which uses a 22 inch scanner which can expose four separate PMMA-mask combination is in the testing phase. Received: 7 July 1999/Accepted: 30 July 1999     

Fabrication of intermediate mask for deep x-ray lithography

 This paper presents the fabrication of intermediate x-ray mask for deep x-ray lithography. In order to have working mask with absorbers thickness larger than 10 μm, the intermediate mask should have absorbers of 0.7 μm in thickness. To demonstrate intermediate mask fabrication, x-ray zone plates are fabricated on the 1.2 μm low-stress silicon-rich silicon nitride (SiN_x) membrane with the tri-layer Chromium-Tungsten-Chromium (Cr–W–Cr) as the x-ray absorbers. The chromium layers both 200 angstroms are used as adhesion and for stress relief. The SiN_x film is deposited with low pressure chemical vapor deposition (LPCVD) and the free standing membrane are formed by KOH silicon backside etching. With the e-beam lithography and reactive ion etching, width of 0.8 μm of outmost zone of the x-ray zone plates has been achieved on the membrane. The scanning electron microscopy (SEM) images of the x-ray zone plates and pictures of intermediate masks are demonstrated. Received: 25 August 1997/Accepted: 3 September 1997     

Progress in magnetic microactuators

 Magnetic microactuator construction has benefited from two processing extensions: stacking in the vertical and horizontal direction and multiple X-ray mask sacrificial layer LIGA. Vertical stacking requires height control of electroplated structures. This has been achieved by material insensitive lapping and polishing techniques. Structural heights of more than 1 mm have been achieved with 300 μm low-energy exposures. This has resulted in actuators with output forces to 100 mN and throws to 2 mm. Re-planarization after electroplating without photoresist damage enables second layer photoresist application via solvent bonding and fly cutting. Exposure of the substrate with a second X-ray mask becomes useful if the second mask can be aligned to the substrate. This has been accomplished with sufficient accuracy via mechanical techniques. A variety of magnetic actuators have been constructed. All of them use assembled rather than integrated coils. The performance of the assembled coils is adequate for position sensing in linear actuators and has resulted in a closed loop control device. Received: 25 August 1997 / Accepted: 3 November 1997     

The formation of moulds for 3D microstructures using excimer laser ablation

 Fabrication by the ‘Laser-LIGA’ process of three-dimensional structures for micro-fluidic devices is investigated. Polymer moulds are formed by projection ablation using an excimer laser operating at 248 nm wavelength. The moulds are replicated by electroforming –as in conventional X-ray LIGA – to produce fixed and freely moving nickel microturbine parts with heights of 150 μm, wall angles of 1.5° from vertical and surface roughnesses below 100 nm. Received: 30 October 1995/Accepted: 4 March 1996     

The study of LIGA technique at NSRL

The activity of LIGA technique was started in the fall of 1993 at National Synchrotron Radiation Laboratory (NSRL), some primary results have been obtained. Micro gearwheels were produced by deep-etch lithography using Hefei light source. The largest diameter of gearwheel is 200 μm, the smallest one only 35 μm, both with thickness of 50 μm. The first metallic structure of nickel was fabricated recently. The mask used in experiments was made by ourselves, it is a composite of kapton membrane as a mask blank, and gold as an absorber. This work was support by National Nature Science Foundation of China     

Study on triple-stack anodic bonding using two electrodes

The principle of two-step triple-stack anodic bonding using two electrodes is presented. Experimental study on anodically bonding pyrex glasses and silicon stacks of glass–silicon–glass structure is performed. Current character during this bonding process especially during the second bonding process is studied. Sodium clustering as yellow-brown point in glass is studied using SEM and EDAX apparatus. De-bonding effect at first bonded interface during second bonding process is studied. And strength testing of bonded triple stacks is performed, the results shows that this bonding method is suitable for most of MEMS packaging.     

A variable reluctance stepping microdynamometer

 A magnetic variable reluctance stepping micromotor fabricated using LIGA processing is described which is integrated with an external planar coupled gear train and electromagnetic brake. Variable mechanical loading of the micromotor through the electrically controlled brake allows dynamometry measurements to be performed. All components are fabricated from electroplated 78 Permalloy. A planar gear coupling is achieved by using involute gear teeth as the salient elements in the rotor. The rotor used for testing has a pitch radius near 500 μm with 180 μm thickness and is complemented with a stator which provides for a step rotation of 2.4°. Coil windings are implemented with assembled spring clip coil forms with 400 turns of 50 AWG magnet wire. Improved coil efficiency is responsible for decreasing the minimum power required for rotor rotation to 52 microWatts. Maximum speeds near 8000 rpm are achieved although stepping action is affected by stepping instabilities which are a function of excitation magnitude and frequency. Calibration of the electromagnetic brake is accomplished with static beam bending measurements and indicates a maximum peripheral rotor output force of 0.6 milliNewton corresponding to an output torque near 0.3 microNewton-meter. Dynamometry measurements indicate a maximum mechanical power output of 20 microWatts. Received: 30 October 1995 / Accepted: 25 January 1996     

Versatile low temperature wafer bonding and bond strength measurement by a blister test method

We present a low temperature plasma assisted bonding process that enables the bonding of silicon, silicon oxide and silicon nitride wafers among each other at annealing temperatures as low as room temperature. The process can be applied using standard clean room equipment. Surface energies of differently treated bonded samples are determined by a blister test method for square shaped cavities. For this reason, we extend the well-known blister test method for round shaped cavities to the square shaped case by a combined analytical and numerical approach. Accordingly, the energetic favored crack front propagation in the bond interface is determined by numerical simulations. The surface energies of the tested samples are calculated and compared to anodic silicon-to-Pyrex^® bonds. Surface energies of up to 2.6 J/m² can be achieved between silicon and silicon oxide wafer pairs at low annealing temperatures. Room temperature bonded samples show a surface energy of 1.9 J/m². The surface energy of silicon-to-Pyrex glass bonds yields 1.3 J/m². Small structures, e.g., bridges down to 5 μm can be bonded using the discussed bonding process. Selective bonding of silicon-to-silicon oxide wafer pairs is performed by structuring the oxide layer. The successful integration of the bonding process into the fabrication of micropumps is highlighted.