In the case of conventional silicon micromachining, the aspect ratio cannot be large because of limitations inherent in etching. Generally, the depth to which machining can be done is limited to a few tens of microns only. This imposes severe constraints for developing three-dimensional microsystems. To produce three-dimensional micro devices with high aspect ratio components, deep etch lithography is necessary.
Such a process using high energy X-ray (wavelength 0.2-0.6 nm) lithography followed by electroplating was developed towards the end of the 1970s at Karlsruhe in Germany. Later, similar processes were developed using deep etch UV lithography.
LIGA Process: Steps involved and X-Ray Mask Membrane
1. What is LIGA Process? (Steps Involved):
The acronym LIGA stands for Lithographie (lithography), Galvanoformung (electroplating), and Abformung (moulding). The comparison of the characteristic lateral and vertical depth dimensions in Si microlithography with those in LIGA is presented in Fig. 7.33. It is clear from the figure that for LIGA the key consideration is high aspect ratio. It can be as high as 50.
The LIGA process is based on deep X-ray lithography to achieve such high aspect ratios. Since the basic technique depends on shadow printing of the mask pattern and X-ray beams cannot be collimated by optics, high energy X-ray synchrotron radiation is used. The most commonly used X-ray sensitive photoresist is Poly Methyl Methacrylate (PMMA). In general, PMMA is attached to a substrate that can be used as an electroplating base. The steps in LIGA are shown in Fig. 7.34.
After the PMMA layer is attached to the substrate and the X-ray mask is ready, the photoresist is exposed to synchrotron radiation through the mask. The regions of PMMA irradiated by X-ray experience bond scission as indicated in Fig. 7.35a. The X-ray induced main chain scission in PMMA results in a reduction in the molecular weight as the molecules become smaller. This phenomenon is indicated in Fig. 7.35b as a reduction in molecular weight with irradiation dose.
When subjected to chemical development, the exposed portions selectively dissolve. After the development of PMMA, the cavities are filled up by some suitable metal through electroplating from the conductive base. Next, the unexposed PMMA is dissolved and the remaining patterned metal part can be used either as a mould for further mass production of micro parts (of some polymer) or the resulting metallic object can be directly used as a micro part. Figure 7.34 shows these two end usages of the micro part obtained by the LIGA process. Figure 7.36 shows the process through simple diagrams.
2. LIGA Masks:
The deep X-ray lithography for the LIGA process requires special X-ray masks.
These masks have primarily three components, viz. (see Fig. 7.36b), – (i) mask membrane for supporting the patterned absorber, (ii) absorber, and (iii) holding ring.
The mask membrane material should be acceptably transparent to X-rays and should possess sufficient mechanical stiffness. The most commonly used materials are Si based, e.g., Si, SiC, and Si3N4, and such others as Ti, Be, and C. The thickness of the mask membrane can be in the range 5-200μm. The holding ring supports the whole subassembly and materials used can be pyrex, glass, or any suitable material. The pattern is created by depositing a suitable metal to absorb the incident X-rays.
The most commonly used absorber materials are Au, W, Tn, etc. The thickness of the absorber can vary from 1μm to 50μm. The CAD pattern of the desired shape to be created is transferred to a thin resist using an optical pattern generator. This pattern is copied on to an X-ray mask membrane. Then, Au (or other absorber metal) is deposited according to the pattern and the X-ray mask is achieved. Figure 7.37 shows the steps for preparation of the X-ray mask.
3. X-Ray Energy Absorption:
When the dose of X-ray is beyond a certain value, PMMA is damaged by swelling and bubbling. Therefore, the X-ray beam coming out of a synchrotron needs to be absorbed by a layer of a pre absorber material (usually Kapton). This beam is then used for lithography where the mask absorber further reduces the energy of the X-ray to bring it below the threshold level. At this exposure, PMMA remains intact while developing.
Figure 7.38 shows how the X-ray energy reduces at the time of irradiation. The absorbing characteristics of various materials commonly used in X-ray mask preparation are indicated in Fig. 7.39.
The development rate of the polymer resist is usually specified in micron of layer thickness removed per unit time (i.e., μm/min). This rate depends on the type of the polymer (primarily the length of PMMA molecule chains) and the absorbed X-ray dose. For a typical case of PMMA, the development rate as a function of the absorbed dose is shown in Fig. 7.40.
An approximate fit of the data yields the empirical relation –
where ER is the development rate (i.e., the etching rate while developing) and J is the absorbed dose in kJ/cm3. As shown in Fig. 7.38, the absorbed dose also gradually decreases as one goes in the depth direction of PMMA resist. In a typical PMMA resist, the absorbed dose approximately decreases with depth as –
where J0 is the absorbed dose at the surface and x is the depth below the surface in μm.
4. Extended LIGA Process:
The parts of a microsystem can be produced by the conventional LIGA technology. When microstructures are produced by the LIGA process, these are generally fixed on a base plate. To obtain microsystems with parts possessing the capability of relative motion, the sacrificial layer technique along with LIGA is necessary.
To start with, a thin metallization layer is deposited onto the substrate; this consists of an adhesion layer of a good electrical conductor like Ag. This layer serves as a plating layer for the electroforming process. This layer is patterned as required by standard optical lithography and wet etching technique. Next, the sacrificial layer is deposited onto this prepared substrate.
In the case of extended LIGA process, Ti is a very suitable material for the sacrificial layer as it provides excellent adhesion to the PMMA resist, and a conductive layer; at the same time, it etches selectively to Ni, Cu, Ag, etc., used for electroplating. This sacrificial layer of Ti is next patterned by optical lithography and etching.
The typical thickness of this sacrificial layer is about 3-5μm. The X-ray mask is so designed (and aligned) that the movable parts of the microsystem are patterned onto the Ti covered areas and the fixed parts are positioned directly onto the conductive layer.
Al is also used as sacrificial layers in extended LIGA process. After the PMMA resist is irradiated and developed, electroforming of parts / components of the microsystem is done. Next, the PMMA resist (not removed during development) is removed as in the standard LIGA process. Finally, the Ti sacrificial layer is etched out using 0.5% HF. Thus, the movable parts are released while the fixed parts remain anchored to the conductive base.
After metallization (step 1), the base for the shaft is created by lithography and etching (steps 2 and 3). Next, the sacrificial layer is deposited (step 4) which is again patterned by lithography (steps 5 and 6). Then, PMMA is laid (step 7) and subjected to synchrotron irradiation (step 8). After removing the exposed PMMA (step 9) Cu or a suitable metal (e.g., Ni) is electroplated (step 10).
The unexposed PMMA is subsequently removed and in the final step the sacrificial layer is removed (step 11). Thus, a gear free to rotate on the micro shaft is obtained as shown in the figure. For a complex structure, more number of steps are required.