This paper reports the complete fabrication process flow based on UV-LIGA technology for realization of metal based MEMS inertial sensor. In this process, nickel is used as the structural layer and copper as the sacrificial layer. The economical three mask process has been optimized and the detailed step by step procedure for carrying out the fabrication is presented. The optimized process parameters to achieve void free copper and nickel electroplated layers with extremely low roughness have been reported. Footing problem associated with lithography process has been analysed and its solution discussed. The fabrication results after each process step have been presented and discussed. Scanning electron micrograph images of the released prototype inertial sensor devices have been presented to demonstrate the successful fabrication of the prototypes using the economical UV-LIGA process.
LIGA is a micro fabrication technique used to fabricate micro structures with high
aspect ratio, from a variety of materials (plastics, metals and ceramics). LIGA is the
German acronym for Lithographie, Galvanoformung (electro deposition), Abformung
(molding). It was developed in the early 1980s at the Institute for Nuclear Process
Engineering at the Karlsruhe Nuclear Research Center [
LIGA finds application in the MEMS industry due its capability of forming molds from various materials with complex shapes and with high aspect ratio and reasonably good absolute tolerances, which is essential for the realization of high aspect ratio MEMS devices. The advantage of LIGA over other microfabrication techniques such as bulk and surface micromachining is its capability of forming structures with comparable dimensions not just in the lateral direction but also in the z-direction defining the thickness of the device.
There are two major variations of LIGA namely, X-ray LIGA and UV-LIGA. X-ray
LIGA is used for fabrication of microstructures with aspect ratio as high as 500:1 with
lateral precision below one micrometer and parallel, smooth side walls. But the
synchrotron source used to generate X-rays is expensive, hence rendering it out of reach
for low cost production [
The schematic representation of the proposed inertial sensor structure is shown in Fig. 1. In this design, the structure is comprised of two masses, 1 and 2, supported by flexures as shown in the figure. There is an intermediate frame mass, which acts as a decoupling mass. The spring, in the x-direction, connects the decoupling frame mass, , to an inner anchor.This structure is based on anchoring of the frame mass, which acts as a decoupling mass between the drive and sense masses. This configuration provides a reduced bandwidth and decoupled motion of the sense mass. The device has been designed considering fabrication compatibility with UV-LIGA process. The minimum feature size in the structure is 4µm gap between the comb fingers. The structure is designed with 8 µm x 8 µm perforations to aid in the sacrificial release process. 3
UV-LIGA process mainly consists of the well-established UV- Lithography process and Electroplating process. The process flow based on UV-LIGA implemented for fabrication of the MEMS inertial sensor is shown in Fig. 2. 3.1 Steps in UV-LIGA a) Silicon wafer is thermally oxidized in a furnace to form silicon-di-oxide layer of 1 µm thickness, for electrical isolation of the device from the substrate. b) 200 Å- 2000 Å Ti-Au is deposited, patterned and etched to form the metal interconnects of the device. c) Again 200 Å- 2000 Å Ti-Au is deposited to form a seed layer for copper electroplating (sacrificial layer). d) 8 µm SU-8 2010 photoresist mold is formed using optimized Lithography process for selective copper electroplating. e) Copper is electroplated to a thickness of 6 µm in the SU-8 resist mold using copper sulphamate bath with optimized process parameters to achieve non-porous copper layer. f) SU-8 resist is stripped using PG-remover and Plasma Asher. g) 11 µm SU-8 2010 photoresist mold is formed using Lithography for electroplating
Nickel structural layer. h) 9 µm nickel electroplating is carried out in the SU-8 resist mold to form the structural layer. i) SU-8 resist is stripped using PG-remover & Plasma Asher. This is followed by dicing operation to singulate the dies from the wafer. Sacrificial etching of copper is carried out followed by etching of seed layer of Ti-Au (step c), leaving the nickel structural layer freely suspended.
Substrate (a) Thermally oxidized Silicon wafer
pad metals (b) Pattern Ti/Au metallization pad on
Silicon substrate (c) Sputter Ti/Au on the whole surface as a seed layer (d) Coat and pattern SU-8 photoresist as a mold for electroforming sacrificial copper (e) Electroplate copper as the sacrificial layer (f) Remove the photoresist mold used
for copper electroplating (g) Coat and pattern thick photoresist mold for nickel electroplating (h) Electroplate nickel into the photoresist
mold (i) Release the structures by removing structure mold and sacrificial Cu, Au and Ti The prototype device has been fabricated using the process flow presented in this paper. Images of the fabricated device have been captured using Zeta optical profiler after each process step during fabrication. Fig. 3 (a) shows the image of the device after first lithography of the metal pattern and Fig. 3(b) shows the device after metal etch process. The formation of SU-8 2010 photoresist mold after UV exposure is shown in Fig. 3(c). Fig. 3(d) shows electroplated copper layer (captured after SU-8 2010 resist stripping). The bright portion is the Ti-Au layer while the rest of the area is plated with copper. The Ti-Au layer forms the anchors for the structural layer. The structural layer mold formation using SU-8 resist is shown in Fig. 3(e). The device after Nickel electroplating and stripping of the SU-8 2010 resist is shown in Fig 3(f). The SEM image of the fabricated structure after sacrificial release is shown in Fig. 4.
Copper electroplating is done at room temperature using copper sulphate solution to
achieve ~6 µm thick sacrificial layer (Fig. 3(d)). Table 1 summarizes the operating
conditions for copper electroplating process.
The above process parameters have been optimized after several electroplating trials
by varying temperature, current density etc., to achieve a void free copper layer with
fine grain size which is a very challenging art [
Similarly, the process parameters such as applied current and temperature have been optimized in order to achieve a shiny and uniform nickel layer using nickel sulfamate bath. The optimized parameters are summarized in Table 1.
Figure 6 shows the surface roughness of the electroplated nickel layer during optimization trials, profiled using Zeta optical profiler.
6
After selective plating, SU-8 photoresist is removed. During stripping, a major
concern is to ensure that the electroplated structure does not get damaged. This is
arguably the most challenging step, because no solvent has been found that can simply
dissolve SU-8 after hard bake. Several methods of striping SU-8 molds have been
suggested and employed such as burning at 450-600 oC [
(a) (b)
The selectively Cu plated wafers are coated with primer (Omnicoat) in order to improve the adhesion between the electroplated copper and SU-8 photoresist. This is followed by drying the Omnicoat and coating of SU-8 photoresist at 1500 rpm and subsequent soft bake process which is done at 65 oC for a period of 7 min and then at a ramped up temperature of 95 oC for 14 min. The wafer and mask plate are held inhard contact mode and exposed to UV light with wavelength of 365 nm. To ensure development at the bottom of the pattern, double exposure is given with each exposure lasting for 1.4 s with a brief interval of 10 s between the exposures. This is followed by a post exposure bake which is done in exactly the same conditions as soft bake explained above. After the post exposure bake, the resist is developed in SU-8 Developer [http://www.microchem.com] for 30 s in stable condition and then stirring the solution for 30 s. The treated wafers are then immersed in Isopropanol for 30 s in stable condition and then stirred for 30 s. During this process, footing problem is encountered in comb fingers and beams (Fig. 8(a)). After some experiments, it is found that insufficient soft bake and post bake are the reason, which caused the resist to stick to copper. The baking processes have been optimised by increasing the baking time up to 14 min for both 65 oC and 95 oC in soft bake and post bake respectively. This has resolved the issue (Fig. 8(b)).
Fabrication process based on UV-LIGA technique has been presented for realization of metal based MEMS inertial sensor. The process flow has been optimized after several trials and the same is presented in this paper. The structural nickel layer thickness implemented in the process is 9 µm and the minimum feature size is 4 µm, which is between the comb fingers. The process parameters optimized to achieve Copper and Nickel electroplated layers with surface roughness less than 0.2 µm and 0.02 µm respectively, have been presented. Footing problem associated with lithography process has been analysed and its solution discussed. The results of unit process steps and finally the SEM images of the fabricated MEMS inertial sensor are presented, demonstrating the successful fabrication of the prototype using UV-LIGA process.