 |
| |
 |
| |
 |
| |
 |
|
CHAPTER 3
LASER
MICROMACHINING OVERVIEW
3.2 MICROMACHINING WITH EXCIMER LASERS
Excimer laser micromachining is normally performed by illuminating an aperture (mask) with the laser beam and forming a demagnified image of the aperture on the workpiece surface, in order to achieve the desired feature size and laser fluence needed to ablate the material. This projection technique is based upon the same principles of a slide or movie projector. The laser functions as the light source, the mask functions similarly to the slide or film, and the sample surface is the projection screen. Both systems typically use an imaging lens (i.e, objective). The slide projector “magnifies” or enlarges the image of the slide, while in excimer laser micromachining the mask image is usually “demagnified” (Figure 3.2.1). This is advantageous if small features are to be produced, since the mask dimensions may be typically 5 to 30 times larger than those to be machined, simplifying the manufacturing of masks.
|
|
The magnification or demagnification is a function of the distances between the mask, the objective lens, and the projection surface. The geometry of any imaging system can be analysed using the following equations:
|
|
(3.2.1) (3.2.2) |
where i is the image distance, o the object distance, f the focal length of the objective at the specified wavelength, and M the magnification of the image. Using the above equations, the following relationships can be derived
|
|
(3.2.3) (3.2.4) |
For micromachining applications the laser fluence F (J/cm2) at the image plane (target surface) is typically increased due to the demagnification factor of the system (M’=1/M=o/i), and is given by
|
|
(3.2.5) |
where Fmask is the fluence at the object plane (mask) and Lloss is the optical loss factor. For example, if the fluence at the mask is 0.15 J/cm2, the system demagnification M’ is 30´, and the optical losses are 20%, then the fluence on target will be 0.15´900´0.8=108 J/cm2. When high fluence is required, the beam cross section may be optically reduced by suitable condensing optics to produce higher fluences at the mask. Even when sufficient laser fluence is available at the output beam, there may be good reasons for using beam compression techniques because it allows the excimer laser to run at lower pulse energy, and hence at lower discharge voltage, leading to increased operating time between gas fills. On the other hand, excessive fluences can damage or cause thermal distortion of the mask.
Excimer laser micromachining may be performed using other techniques. The mask that defines the pattern may be in direct contact with the substrate (Figure 3.2.2). This contact mask technique is commonly used for processing photopolymers, but because the masks are typically made of aluminium, molybdenum or stainless steel they are not suitable for applications requiring high fluence, since the masks undergo thermal ablation. In the direct writing technique, the pattern is machined by moving the substrate under the focused laser beam (Figure 3.2.3). To achieve uniform machining a system that synchronises motor step increments with laser triggering is required. Typically, a CAD drawing is converted via a post processor into machine instructions. The possibilities are endless but frequently only a small fraction of the laser beam is used. In addition, the large beam divergence of excimer lasers limits accuracy of the features produced with focussed beams. In a projection technique the beam divergence has less importance, providing the finite aperture of the projection lens captures the laser energy.
|
Figure 3.2.2: Contact masking technique (adapted from [7]). |
Figure 3.2.3: Direct writing technique (adapted from [7]). |
3.2.1 Mask projection technique
Throughout this work a mask projection technique was used, and consequently in what follows this technique is discussed in further detail. Figure 3.2.4 shows the typical set-up of an excimer laser micromachining system using a projection technique. The illumination optics typically include a beam forming telescope, homogeniser optics and a field lens. The telescope is normally arranged as a Galilean configuration using two positive lenses. The conjugate planes are arranged close to the laser exit (<1 m) and at the homogeniser entrance aperture. Alternatively, a Kepler telescope (constituted by a positive and negative lens) can also be used to reduce the optical path. The disadvantage of this telescope set-up is that the conjugate planes are at the infinity and that no aperture (in rigour, near field profile at the exit or close to the excimer laser resonator) is imaged. Because the intensity distribution within the illuminated part of the mask has to be uniform for most applications, a beam homogeniser is often required. Most homogeniser optics divide the beam cross section in several parts and mix them to obtain a homogeneous energy distribution. However, homogenisers typically increase the raw beam divergence, and this has to be taken into account when designing the illuminating system because the high divergence leads to poor beam quality and high numerical apertures at the target especially for very high system demagnifications. In this case, working without the homogeniser may increase resolution, but only the inner part of the raw laser beam (approximately 30-50% of the total emitted laser light) can be used to get a homogeneous illumination within 90%. The homogeniser is followed by a field lens that is frequently located close to the mask where the homogenised field is produced. The field lens images the light source into the aperture of the projection lens, thus coupling the illumination optics to the imaging optics.
|
|
|
Figure 3.2.4: Experimental set-up of an excimer laser micromachining system using a projection technique (adapted from [8]). |
The selection of the mask to be used depends on the shape of the features to be machined and on the process specifications and costs. The simplest photomasks for excimer laser projection are apertures in a metal sheet, which are larger than the pattern to machine by the demagnification factor. Masks of molybdenum, brass or copper foil produced by laser drilling are frequently used. These masks will undergo transient thermal distortion as they heat up and therefore must be allowed to thermally equilibrate before being used for precision work. Chemically etched masks are produced by imaging a photoplot onto the surface of a thin metal sheet that is coated with photoresist. Once the resist is exposed, the unexposed areas are etched with acid to form the mask. Applications using this mask technology include hole drilling, bulk material removal and pattern generation demanding low resolution and edge quality. Coated optical substrates patterned using standard photolithographic techniques have been used in the microelectronics industry for years, allowing consistent quality and high resolution. Due to the natural tendency of most materials to absorb strongly in the ultraviolet, the coating used must be highly reflective. High accuracy masks can be fabricated by photolitography processing of dielectric mirror structures on transparent substrates. These dielectric masks are extremely expensive, but their use may be justified in high accuracy, and high volume production applications. Chrome on fused quartz masks are readily available from photomask vendors, but are only suitable for use at low fluences. Phase masks are also fabricated as dielectric structures on ultraviolet transparent substrates for demanding applications.
The most simple projection lens is a plano-convex single element lens, which inevitably presents significant aberrations. In order to compensate for lens aberrations several single element lenses may be used together. Projection objectives are designed to compensate the aberrations within a defined field of view and to minimise field curvature and distortion. Further, objectives are designed for specific performance needs, namely
- demagnification and energy density at the image plane;
- focal length/working distance;
- Numerical aperture/resolution;
- Telecentricity.
The practical resolution capability of projection lenses depends on several factors [9]: wavelength, numerical aperture, lens aberrations, spatial frequency of the pattern to machine and coherence of illumination. The degree of spatial coherency and the spatial frequency of the pattern can be used to estimate the modulation transfer function (MTF) of the lens, a number used to specify the achievable resolution. The MTF allows quantification of the contrast in the mask that can be imaged onto the sample surface. However, contrasts different from the ones given by the MTF may nevertheless result in the machined sample due to nonoptical factors, i.e. the physics and chemistry involved in the material removal process. For example, a nonlinear photoablation process could actually increase the contrast, whereas heat conduction could possibly lower the contrast. The radiation wavelength is, however, by far the limiting factor in determining the resolution. The Rayleigh criterion [equation (3.2.6)] gives a limit for the resolution (Rop), although it merely represents the high-spatial frequency cut-off of the lens and is not directly related to the achievable resolution,
|
|
where NA is the lens numerical aperture (optical element diameter /2´focal length) and l the radiation wavelength. The short wavelength of excimer lasers as compared to other industrial lasers (i.e., CO2 and Nd:YAG) allows for sub-micron resolution, and explains their huge market in the semiconductor industry [2]. Singlet lenses have the versatility of being usable over a wide range of demagnifications, but their resolution is limited. If the application requires a large field and high resolution, the use of a doublet or multi element lens should be considered. Although these lenses correct for aberrations they are optimised for a certain demagnification, and are not nearly as versatile as singlet lenses. Multi element lenses frequently have high numerical aperture (NA), which leads to reduced depth of focus
|
|
(3.2.7) |
This may become a problem since the projection lens has to provide a depth of focus Z of the same range of the machining depth. In a projection technique, the field lens can be used for telecentric or non-telecentric imaging (Figure 3.2.5). In a telecentric arrangement the beam in the image plane is nearly parallel to the optical axis behind the imaging optic. For a given NA of the projection lens, the depth of focus increases significantly as compared to a non-telecentric arrangement. In lithographic methods, Schwartzchild reflecting optics are commonly used instead of refracting optics to avoid absorption in the optical elements that may lead to image distortion and energy loss.
|
|
|
Figure 3.2.5: Optical design of telecentric (grey shadowed) and a non-telecentric (dashed lines) imaging in a projection technique using variable field lenses. |
In order to produce complex and/or large features the mask and/or the workpiece must be moved during machining, as briefly discussed in what follows. Laser micromachining systems are usually provided with numerical control of a beam forming mask, beam energy, laser triggering, and positioning of the workpiece. Part tolerances determine the motion system(s) specifications (i.e., resolution, accuracy and repeatability), as well as the characteristics of the illumination and projection optics.
3.2.1.4 Step and repeat method
For a single mask pattern projected onto the workpiece a single structure of limited area is machined. This pattern may be repeated by stepping the workpiece as often as required to fabricate large numbers of identical features. By using various masks as laser processing proceeds, complex 3D shapes may be formed. All the sequential mask patterns can be contained on a single mask plate, which is positioned in the beam on X-Y mask stages. Alternatively, for structures that are based on square or rectangular patterns a dynamic aperture with programmable adjustable blades can be used, whereas for circular structures a variable radius circular iris is suitable as a mask. Figure 3.2.6 shows a nozzle drilled in polymide for an ink jet printer manufactured by this process. Such features can be repeated by stepping the workpiece to machine an array, or a complete linear array of nozzles can be drilled simultaneously by configuring the beam in a line and using a projection lens of appropriate large field. In a variation of this technique, both mask and workpiece may be stepped sequentially to generate structures over relatively large areas. The overlap and position of machined spots are controlled so that specific areas receive more laser pulses than others to machine a pattern of variable depth. The sequence may continue with the aperture being reduced in size and changed in shape sequentially until the smallest required features are machined. This technique allows highly complex 3D structures to be machined to unlimited depth over large areas, but has the disadvantage that the process time is extremely long.
A mask pattern may be projected onto the surface of the workpiece which is moved at constant velocity to form a structure of constant profile. If the workpiece is accelerated or desacelerated curved structures are then formed. By using different masks different profiles can be created, while by rotating the workpiece and repeating the process, crossing of linear patterns generates periodic regular 3D structures (Figure 3.2.7). Instead of simply moving the workpiece, both the mask and the workpiece may move in synchronism through a stationary laser beam. In this case, the position and velocity of the mask are at all times exactly related to the workpiece position and velocity by the lens demagnification factor. For a typical demagnification of 4´, areas up to 30´30 mm2 can be patterned with a resolution that depends on the accuracy, resolution and repeatability of the workpiece stages (and to a lesser extent, on the mask stages) rather than on the optical resolution of the projection lens. The merit of mask and workpiece scanning techniques is that high accuracy shapes can be machined at high speed over a large area, limited only by the mask size and stage limits.
REFERENCES
1. D. Bäuerle: Laser Processing and Chemistry,
(Springer-Verlag, Berlin Heidelberg 2000)
2. D. Basting: Excimer laser technology: laser sources, optics, systems and applications, (Lambda Physik AG, Göttingen, 2001)
3. O. Svelto: Principles
of lasers, (Plenum Press, New York
1989)
4. J. P. Sercel and P. J. Sercel: Putting Industrial Excimer Lasers to Work, Photonics Spectra (May 1990)
5. J. P. Sercel: Production
Excimer Laser Equipment Overview, SPIE 1835,
172-183 (1992)
6. H. Gerhardt: Fundamentals
of Laser Physics and Laser Technology,
(Lambda Physik)
7. T. R. O'Keeffe and T. E. Lizotte: Excimer laser as a manufacturing tool,
Proc. SPIE 2062, (1994)
8. K. H. Gerlach, J. Jersch, K. Dickmann, and L. J.
Hildenhagen: Design and performance of an
excimer-laser based optical system for high precision microstructuring,
Optics and Laser Technology 29 (8),
439-447 (1997)
9. J. H. Brannon: Micropatterning
of surfaces by excimer laser projection, J. Vac. Sci. Technol. B 7 (5), 1064-1071 (1989)
10. P. T. Rumsby, E. C. Harvey, and D. W. Thomas: Laser Microprojection for Micromechanical
Device Fabrication, Proc. Experimental Mechanics: Advances and Applications
2921, 684-692 (1996)
11. I. Watt: The
principles and practice of electron microscopy, (University Press, Cambridge 1985)
12. JEOL report: Invitation to the SEM world
13. D. Instruments: Multimode SPM Instruction Manual, Version 4.31ce, (1996-97)
14. M. Sander: A
practical guide to the assessment of surface texture, (Feinprüf GmbH, Göttingen 1991)
15. J. Yvon: http://www.jobinyvon.com (2001)
16. J. A. B. Faria: Óptica - fundamentos e aplicaçőes, (Editorial Presença, Lisboa 1995)
17. M. Born and E. Wolf: Principles of Optics,
(Pergamon Press, Oxford 1970)
18. S. J. Burden: Comparison
of Hot-Pressed Alumina-Titanium Carbide Cutting Tools, Am. Ceram. Soc. Bull. 67 (6), 1003-1005 (1988)
19. Y. Choi and S. Rhee: Effect of Precursors on the Combustion Synthesis of TiC-Al2O3
Composite, J. Mater. Res. 9 (7),
1761-66 (1994)
20. R. Cutler, A. Hurford, and A. Virkar: Presureless-sintered Al2O3-TiC
composites, Mater. Sci. Eng. A195/106,
182-192 (1988)
21. R. P. Wahi and B. Ilschner: Fracture behaviour of composites based on Al2O3-TiC,
J. Mater. Sci. 15, 875-885 (1980)
22. T. Xia, Z. Munir, Y. Tang, W. Zhao, and T. Wang: Structure formation in the combustion synthesis
of Al2O3-TiC composites, J. Am. Ceram. Soc. 83 (3), 507-512 (2000)
23. L. Wang, Z. Munir, and Y. Maximov: Thermite reactions: their utilisation in the
synthesis and processing of materials, J. Mater. Sci. 28, 3693-3708 (1993)
24. 3M Ceramic
Materials Department Product bulletin
25. M. V. Swain: Structure
and properties of ceramics, R.W. Cahn, P. Haasen, and E.J. Kramer (VCH, New
York 1994)
26. H. Gatzen: Rigid
disk slider micromachining challenges to meet microtribology needs, Trib.
Int. 33, 337-342 (2000)
27. C. Lacey, C. Durán, K. Womack, and R. Simmons: Optical measurements of flying height,
Proc. Future Dimensions in Storage Symposium
81-88 (1997)
28. P. d. Groot: Optical
properties of alumina titanium carbide sliders used in rigid disk drives,
Appl. Opt. 37 (28), 6654-6636 (1998)
29. K. Komeya: In Structure
and properties of ceramics, ed. by M. Swain (VCH, New York 1994), Vol. 11,
pp. 517
30. W. J. Tropf and M. E. Thomas: Aluminum Oxide (Al2O3)
Revisited, (Academic Press, 1991), pp.
653-682
31. F. Gervais: In Handbook
of Optical Constants of Solids II (Academic Press, 1991), pp. 761-775
32. J. D. Cawley and W. E. Lee: In Structure and properties of ceramics,
ed. by M. Swain (VCH, New York 1994), Vol. 11, pp. 47
33. P. Liu, W. L. Smith, H. Lotem, J. H. Bechtel, N.
Bloembergen, and R. S. Adhav: Absolute
two-photon absorption coefficients at 355 and 266 nm, Phys. Rev. B 17 (12), 4620-4632 (1978)
34. M. J. Weber: Handbook of Laser Science and
Technology, (CRC, Baton Rouge, 1986),
Vol. IV
35. W. W. Duley: UV
lasers: Effects and Applications in Materials Science, (Cambridge University Press, 1996)
36. H. O. Pierson: Handbook
of Refractory Carbides and Nitrides: properties, characteristics, processing,
and applications, (Noyes
Publications, 1996)
37. Z. A. Munir and J. B. Holt: Combustion synthesis of titanium carbide: theory and experiment, J.
Mater. Sci. 21, 251-259 (1986)
38. A. G. Akopyan, S. K. Dolukhanyan, and I. P.
Borovinskaya: Com. Explos. Shock Wave 14,
327 (1978)
39. E. K. Storms: The
refractory carbides, (Academic
Press, New York 1967)
40. W. S. Williams: Phys. Rev. 135, 505-510 (1964)
41. W. S. Williams: Electrical properties of hard materials, Int. J. Refractory Metals
& Hard Materials 17, 21-26
(1999)
42. S. Méçabih, N. Amrane, Z. Habi, B. Abbar, and H.
Aourag: Description of structural and
electronic properties of TiC and ZrC by generalized gradient approximation,
Physica A 285, 392-396 (2000)
43. J. Pflüger and J. Fink: In Handbook of Optical Constants of Solids II (Academic Press, 1991),
pp. 303-311
44. D. R. Lide: CRC
Handbook of Chemistry and Physics,
(CRC Press, 1996)
45. TAPP: Thermochemical
and Physical Properties (ES Microware, 1991)
46. V. N. Tokarev, W. Marine, C. Prat, and M. Sentis: Clean processing of polymers and smoothing of ceramics by pulsed laser melting, J. Appl. Phys. 77 (9), 4714-4723 (1995)
| Copyright 2010 - MMendes Tech - All rights reserved |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |