Rapid Prototyping of Diffractive Optical Elements (DOEs) for Packaging Applications
M. Flury, J. Fontaine, P. Gérard, T. Engel, Laboratoire des Systèmes Photoniques / ENSPS, Boulevard Sébastien Brant, 67400 ILLKIRCH — FRANCE; JP. Schunck, Laboratoire PHASE / CNRS, 23 rue du Loess, 67037 STRASBOURG — FRANCE. joel.fontaine@ensais.u-strasbg.fr Tel: 00 33 3 88 14 47 47 Fax: 00 33 3 90 24 46 19
Abstract:
Our purpose is to describe prototyping Diffractive Optical Elements (DOEs) with low cost techniques that can be used for light beam shaping. In this paper, we first present the design of DOEs which require iterative algorithm. After, we discuss and compare different technologies of manufacture. We propose to use indirect etching with laser ablation, direct etching in polymers or photolithography with flexible mask to obtain the diffractive relief. The two first methods use micro-machining with an excimer laser. In the case of DOEs used with infrared high power laser, we also present possible applications in the packaging field like pattern marking or multi-point brazing.
1. Introduction.
Diffractive Optical Elements (DOEs) are optical processors that permit change to the characteristics of an incident light beam [1]. Generally, they are of the Fourier or Fresnel type. The Fourier element focuses its image at infinity, which is why an intermediate lens is needed to observe the image. The Fresnel element includes the focusing function in the component, with the disadvantage of being sensitive to the profile of the incident wave. They entail greater manufacturing constraints than Fourier elements. There are several classes of elements, and the ones of main interest to us are the kinoforms that contain a relief modulation [2, 3]. The phase profile of the diffractive element is usually sampled at two or more levels.
DOEs are not equivalent to classical optics; they can be applied a new set of functions. They enable many applications such as imaging, optical interconnects, optical read heads, beam shaping and beam splitting [1, 3]. This document is mainly concerned with DOEs with infrared laser sources: these introduce functions of great advantage for applications in the field of laser machining with CO2 or YAG lasers [4, 5]. For packaging applications, they can be used for multi-points brazing, marking, thermal treatments and components assembling.
Technology for DOEs manufacturing is relatively new. Advances in microelectronics (photolithography and ion beam etching) have allowed realization of DOEs with very good quality and high diffraction efficiency in the early nineties [2]. These techniques are complex and costly, however, prototyping is interesting for more and more manufacturers today. We propose a method of Laser Ablation Lithography (LAL) with an excimer laser for rapid DOEs prototyping. The first part of our paper is a survey of DOEs design methods. We present then different manufacturing techniques. The two first are based on laser ablation process. The last is the classical photolithography with flexible masks. Finally, we summarise briefly applications in packaging.
2. DOEs Design.
Our elements are designed essentially according to the scalar theory of light diffraction. For a Fourier element, a diffracting plane can be reconstructed simply by a discrete Fourier transform, or numerically by a Fast Fourier Transform (FFT). The formula is more complex for a Fresnel element, because two quadratic phases appear. Numerically, the distribution of the U (x’, y’, z) field in the reconstruction plane is related to the incident field U(x, y, z = 0) by:
(1)where l is the wavelength of the laser beam and z is the distance between the DOE plane and the observation plane. (O, x, y, z) is the Cartesian coordinate system centered on the DOE, and (O, x’,y’,z’) is the Cartesian system centered on the reconstruction plane. Figure 1 represents the coordinate system. Our DOEs are similar to a transmittance sampled in both transverse directions, but also dependent on the number of phase levels.
For design purposes, we use an iterative algorithm developed by Gerchberg-Saxton [6] and later enhanced to generate more accurate phase profiles [7]. The iterative algorithm contains the idea of constraints that influence the resulting phase profile and reconstructed field at each iteration. These constraints can be the amplitude and phase profiles of the incident beam, or the number of phase level. It is important to note that the constraints strongly influence the design. Consequently, DOEs may fail to operate if the real constraints of the incident beam are not introduced. In figure 2, we show an example of a Fourier element with the encoded object.
Up to this point, we have been considering that the incident wave is normal to the plane of the DOEs. In our particular case, we want to make DOEs that operate in reflection mode, so the phase profile will be modified. A number of numerical methods can be found in the literature for approximating the real configuration [8].
We test numerically our DOEs according to four parameters: diffraction efficiency, RMS error, correlation coefficient, and uniformity. Only the diffraction efficiency is easily measurable on a real reconstruction. It is recalled that the theoretical order-one diffraction efficiency for a binary element is 40.5%. For a four-level element, the efficiency is 80.1%. After ten iterations, the efficiency achieved for a binary element is about 38%, and 79% for a four-level element.
2. Manufacture by Laser Ablation Lithography (LAL).
Here we present our rapid computer-designed DOEs prototyping method based on the laser ablation of polymers [5]. This technique has the advantage of manufacturing elements very quickly, requiring no complex room facility.
The purpose of our study is to manufacture DOEs with lasers emitting in the infrared. Reflection mode was chosen because of the type of available material with high reflection coefficients in the infrared. Metals with very good heat conductivity are good candidates, and the Fresnel losses are also very low.
2.1. Installation.
Our technology uses a micromachining station with an excimer laser emitting a power of 20W at 248nm. The average pulse duration is about 20ns, and the maximum pulse repetition frequency is 200Hz. The necessary fluence is well controlled by attenuator directly at the laser beam output. Also, the station uses a projection system with an Optec three-lenses optic. The reduction factor is about ten.
A mask containing the basic DOE shape is reduced through the optics and transferred to the sample. Our basic shape is a square, but other shapes are of course possible, depending on the initial design. The masks are cut in stainless steel by an ordinary laser micromachining technique. We use only one mask to produce the diffractive pattern. The DOEs are manufactured very quickly with a micro-positioning table floating on air bearings. High accuracy can also be obtained using interferometric measurement. The entire station is automatically controlled by computer, with synchronized computer shots and table motions.
2.2. Direct Laser Ablation Etching.
This technique sketched in figure 3 has already demonstrated its advantage for rapid DOEs manufacture in the visible wavelengths: no problem with squares of large dimensions (10 to 50 microns). Multilevel systems can also be created with profiles compatible with diffraction optics requirements, which provide a way of prototyping diffractive elements [9, 10]. We have mainly tested polycarbonate and polyimide. Figure 4 presents a detail of a DOE produced in polycarbonate. The ablation rate is 0.6µm per pulse for polycarbonate, for a fluence of 2J/cm, and 0.5µm for polyimide.
For a transmissive element, the optimal etching depth is [1, 2]:
in which N is the level number and n the optical index of the material at the wavelength l.
This type of material is too absorbent for lasers emitting in the infrared. A film of gold can then be deposited by evaporation to carry out a reflective element. The optimal etching depth for the reflection configuration is given by [5]:
where N is the level number and q the angle of reflection at wavelength l. However, this solution is still poorly suited to high-power laser beams.
Manufacturing diffraction optics directly by laser ablation in a metal like gold or copper is impractical for reasons of machining quality and time. The ablation rate in metals, for example, is very low. The ablation rate for copper is about 0.07µm per pulse for a fluence of 20J/cm. The etching depth needed for a reflection DOEs is 2.79µm with the beam at an incidence of 15° and a wavelength of 10.6µm. So it would take more than four hours to make DOEs of 256´256 squares in copper at a pulse frequency of 100Hz. Moreover, ordinary profilometric measurements have shown a roughness of nearly 1.5µm at the bottom of the squares, so the squares are more like craters [11].
These considerations prompted us to develop a new indirect manufacturing method.
2.3. Indirect Laser Ablation Process.
First we deposit a thin film of photosensitive resist on a substrate of metal, silicon, or quartz, by ordinary centrifugation. To produce a two-level DOE, we then proceed as follows. We transfer the pattern to the resist by table motions. The substrate surface state is modified very little because the fluence used for ablation is low. We tested two resists: Shipley 1800 and Az 5218. It takes about five pulses to ablate a 2-µm thickness of resist with a fluence of 1J/cm on a silicon substrate. The resist is used as a mask in a later step in the process. The indirect etching is sketched in the figure 5.
Naturally, the resist could be exposed to the excimer laser and developed, as it is done in the direct laser writing method. Much less laser fluence is then required. But in LAL, the various profiles can be inscribed in the resist in one step. For a DOE of 128 ´ 128 squares, it takes about 18 minutes of machining with a pulse frequency of 150 Hz.
After the transfer, we use the most appropriate method for creating the diffractive relief in the substrate. The etching depth is always 2.79µm for a binary element with an incidence of 15°, and is greater for several phase levels. Electrolytic deposit on metallic substrate has the advantage of producing binary diffraction optics very quickly and at lesser cost. Therefore, the roughness can turn out to be too large for valid reconstruction. Figure 6 presents different methods to create the diffractive relief in the substrate.
Ion beam etching with SF6 is very well suited to the silicon substrate, and yields excellent results considering the initial constraints. We are currently testing this indirect method for the cases of several phase levels. We stabilize the laser power and then a profile can be achieved in the resist by modulating the number of pulses. Figure 7 is a three-dimensional view of pixels produced with LAL and indirect etching in silicon. This figure is obtained with scanning interference microscopy.
With controlled etching rate, the profile of several levels can be transferred into the substrate. Other solutions are possible to create systems with several phase levels. After etching, a film of gold can be deposited on the substrate. A 30-nm thickness is the optimum choice with CO2 laser.
3. Manufacture with classical contact photolithography.
We have also tested a classical method to obtain DOEs for infrared high power laser: we chose the photolithography by contact with flexible mask. Masks were achieved with phototracing systems or photoreduction on polymer film. Pixels with 50 microns size are easily produced. Thus, the technique is adapted for prototyping cheap DOEs. The obtained quality meet the constraints of the scalar theory. Figure 8 shows an example of Fourier element after the photolithographic transfer. The same techniques as those described above (ionic etching, electrolytic process) enables relief modulation.
4. Applications in packaging.
Our aim in this project is to propose real prototyping with a control of all steps in producing DOEs: design, manufacture and test. The concept of DOE is very well adapted to operate multi-operation at the same time. The encoded image in the DOE is not limited and we can imagine many applications with this wavefront processor. Multi-points brazing or laser marking can be considered with various pattern in integrated circuits production. Among other possible applications, we propose: multipoint thermal treatment, multi-components simultaneous assembling. We show in figure 9 some possibilities of pattern recorded in DOEs.
Today, DOEs are static. The future is dynamic diffractive structures in real time to form different images. Technological advances in micromachining supply new devices which are available for this application.
5. Conclusion.
Our purpose was to prove the possibility of using low cost processes for the manufacturing of DOEs for infrared laser beam. We discussed our different technologies and their advantages. We studied three processes: direct etching, indirect etching with ablation and photolithography with flexible masks. The two first technologies do not require high cost masks for the transfer of diffractive patterns. In the second process, the diffractive relief is obtained by indirect transfer using an intermediate resist layer. This layer is used as a mask for a further etching or deposition step. The processing time and the roughness had to be considered in each case to evaluate limitations. The potential application of DOEs for CO2 laser in the electronics industry are fast assembling of components, multi point brazing or marking.
Acknowledgements:
The excimer laser and the micro-machining system is from IREPA Laser, Pôle API, 67400 ILLKIRCH — France. The authors would like to thank A. Benatmane and P. Montgomery of the PHASE Laboratory for the measurements made on the interference microscope.
References:
1. H. P. Herzig and J. Turunen, Micro-optics: elements, systems and applications. Taylor & Francis Ltd ed. 1997.
2. P. Meyrueis and B. Kress, Digital diffractive optics. Wiley & Sons Ltd ed. 2000.
3. J. Turunen and F. Wirowski, Diffractive optics for industrial and commercial applications. Akademie Verlag ed. 1997.
4. C. Haupt, et al., Computer generated microcooled reflection holograms in silicon for material processing with CO2 laser. Appl. Opt., 1997, 36(19), p. 4411-4418.
5. M. Flury, et al. Excimer laser fabrication of low cost diffractive optical prototypes for CO2 laser marking applications, in Laser 99. 1999, Munich, Proceedings of SPIE, Vol. 3822, p. 155-162.
6. R. W. Gerchberg and W. O. Saxton, A practical algorithm for the determination of phase from image and diffraction planes pictures. Optik, 1972, 35(2), p. 237-246.
7. G. Yang, et al., Iterative optimization approach for the design of diffractive phase elements simultaneously implementing several optical functions. J. Opt. Soc. Am. A., 1994, 11(5), p. 1632-1640.
8. C. Frère and D. Leserberg, Large objects reconstructed from computer generated holograms. Appl. Opt., 1989, 28(12), p. 2422-2425.
9. M. Haruna, et al., Laser beam lithographed micro-fresnel lenses. Appl. Opt., 1990, 29(34), p. 5120-5126.
10. G. P. Behrmann and M. T. Duignan, Excimer laser micromachining for rapid fabrication of diffractive optical elements. Appl. Opt., 1990, 36(20), p. 4666-4674.
11. N. A. Vainos, et al., Excimer laser for microetching computer generated holographic structures. Appl. Opt., 1996, 35(32), p. 6304-6319.
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