Excimer Laser Fabrication of Low Cost Diffractive Optical Elements
M. Flury, J. Fontaine, P. Gérard, T. Engel, Laboratoire des Systèmes Photoniques - E.N.S.P.S., Boulevard Sébastien Brant, 67400 ILLKIRCH, France, E. Fogarassy, J.P. Schunck, Laboratoire P.H.A.S.E./C.N.R.S., 23 rue du Loess, BP 20 CR, 67037 STRASBOURG Cedex 2, France, manuel.flury@ensps.u-strasbg.fr, tèl: +33 3 88 65 51 55
Different technologies are employed in Diffractive Optical Elements (D.O.E.) fabrication. This field has benefited from Very Large Scale Integration (V.L.S.I.) technologies used in microelectronics. In this paper, we first review several methods to produce such elements and compare their advantages and limitations. We describe the use of an excimer laser to realize D.O.E. for CO2 laser beam; the micro-machining process makes use of photoablation of polymer. There are mainly two solutions to record the relief modulation: the direct micro-machining and the indirect substrate etching. In this article, we also demonstrate the transfer of D.O.E. pattern on different substrates.
Introduction:
Interest in D.O.E. has grown up in the last ten years. Among other applications, diffractive optics are used for laser wavefront transformation. Different methods are available for the D.O.E. fabrication.
The first one is relative to the generation of amplitude elements on photographic films [1,2]. The second method, called diamond turning, produces continuous surfaces with cylindrical or axial symmetry [1]. The third solution makes use of dynamical devices such as Liquid Crystal Displays or Deformable Mirror Devices [2,3]. The last class, the microlithographic fabrication, is the most important one [1,2,3].
In this article, we discuss and compare different techniques from the last category. We describe also the micro-machining process to fabricate diffractive structures.
1. The microlithographic fabrication.
This technique is the most common one, although it remains expensive. It provides very accurate patterning and yields, therefore very good diffraction efficiencies. It also allows easy reproduction of the master element. There are mainly 3 steps involved in this technique: the mask or substrate patterning, the photolithographic transfer (UV exposure and resist development) and the substrate etching.
In the case of multi-levels relief elements, the operation has to be repeated M times, M being to the number of masks. The number of levels in the element is then equal to 2M. This difficulty can be avoided by using grey-tone masks which allow a multilevel pattern transfer within a single step.
Once the pattern has been transferred into photoresist over the final substrate, this pattern has to be etched down into the substrate. Several etching technologies are available and can be divided into wet and dry processes. Wet etching is a very simple technique, since it uses HF acid to etch the glass or quartz substrates. However, the isotropic etching may not give features with sharp edge.
Dry etching remains the only reliable etching technology for multilevel D.O.E fabrication. High energy collimated argon ion beams or plasma sources are used to etch the substrate with a vacuum chamber. For Reactive Ion Beam Etching or Chemically Assisted Ion Beam Etching, additional reactive gases are also interesting for fine tuning of proportional etching rates for different materials.
2. Direct writing methods.
2.1. Laser Beam Writing.
Laser beam Writers (L.B.W.) are a flexible tool for direct photoresist patterning [2,3,4]. This technique is well suited to realize any desired phase profile structure. The development of the resist film gives the required relief. Once a master micro-relief is fabricated, it can be reproduced by modern replication technology, which enables a large number of high quality samples to be reproduced by casting or embossing from a metal shim.
Micro-optical elements are fabricated as micro-relief structures by the programmable exposure of a photoresist film by raster scanning. The resist development characteristic is obtained by carrying out calibration runs with the given resist, coating procedures, development procedures and writing parameters. The resist coated substrate is mounted on a two axis translation stage, which glides on air bearings over a granite base by using linear motor drives and interferometer positioning controls. Vibrations due to floor movements can be minimized by passive vibration isolation elements. The beam intensity is computer controlled by means of an acousto-optical modulator, and the beam is routed to the focusing objective by an optical system. The choice of the objective depends on the micro-structure to be written, a 50x objective that produces a focused spot of 1,5 microns diameter is typical. An autofocus system with piezoelectric translators holds dynamically the focus over the scan area during the writing procedure. The translation stage and the modulator is driven by the table controller, itself monitored by a PC computer.
2.2 Electron Beam Writing.
Electron beam Writers (E.B.W.) consist of an electron beam gun in a computer controlled vacuum chamber that is focused on a special resist coated substrate and deflected by a set of electromagnetic lenses. Two axis translation stages are used to move the substrate, as in L.B.W. However, the writing being here performed by electromagnetic beam deflections, the E.B.W. produces a two dimensional pattern. In order to prevent electric charging of the substrate while the electron beam is patterning the resist coated substrate, an additional conducting layer is added to the resist coated substrate and linked to the ground.
3. Comparisons between L.B.W. and E.B.W.
The direct writing eliminates the transfer part of the classical lithography, which clears up the problem of alignment errors of the successive binary masks. Direct writing methods are also usually used for patterning masks. As by using greyñtone masks, multilevel structures can be obtained in one step.
In terms of diffraction efficiency, the cost per element is with L.B.W. much lower than with E.B.W. technology. Both L.B.W. and E.B.W. allow a large flexibility in the design and fabrication of the D.O.Es. The E.B.W. system has the advantage to be free of wavelength aberration; furthermore nearly diffraction limited spot size is easily obtained. However, E.B.W. requires a vacuum environment, which makes it more expansive than the L.B.W.
4. Laser Ablation Lithography.
We discuss next direct etching techniques for prototyping diffractive elements. This method, called here Laser Ablation Lithography (L.A.L.) [7], combines the advantages of the previously discussed technologies. L.A.L. is based on an intermediate process in term of quality, resolution and cost.
4.1 System description.
The process is specially suited to rapid prototyping. Here is its main advantage: there is no more need for high cost photolithographic mask at each change in the D.O.E. design. The scalar diffraction theory, in the easiest case of D.O.E. for CO2 laser beam shaping, leads to the 50-100 microns pixels width limitation. The first limitation of the process is the real resolution: the minimum feature size in this case is approximately 1 to 2 microns (it depends on the response of the material and the wavelength used).
In some specific cases like polys, the combination in one single step of the writing and etching functions together is possible. The micro-machining operating set-up does not require very special conditions like clean rooms or temperature controlled rooms. The mask image is projected on the substrate with a reduction optical system. The reduction ratio is 1:10. The mask defines the elementary pixel (50 µm width). The ablation of the material over the desired depth is made with a few KrF excimer laser pulses (l = 248 nm). The pulse duration is 20 ns. The whole micro-machining station is automatic: the laser shot numbers and the micropositioning stage movements are computer controlled. It is necessary to distinguish two cases depending on the nature of the substrate: metals and polymers.
4.2 Direct ablation process.
Ablation rates in metals being very small, the direct etching may be impossible for time considerations. In the case of copper, the measured ablation rate is 0.07 µm per pulse at the fluence of 20 J/cm2 [5]. The required etching depth is 2.79 microns for a reflection D.O.E. with a 15° incidence angle operating at the 10.6 microns wavelength. Consequently, 4 hours are necessary to fabricate a 256 x 256 pixels D.O.E. at 100 Hz pulses repetition rate. Besides, the surface roughness in metals is inadequate to create relief modulation [6]. For copper, profilometry gives a 1.5 µm amplitude of roughness. It follows that direct ablation in metals is not a viable solution.
At last, the direct laser ablation depends mainly on the material and on different considerations such as processing time, roughness and proximity effects. Actually, the direct micro-machining is very attractive in the case of polymers [7]. We tested the case of polycarbonate (see figure 2), but this material is neither adapted for the CO2 laser beam nor for transmitive D.O.E.
4.3 Indirect ablation process.
First, a thin film of photoresist with a 1-2 µm thickness is deposited on a substrate (metal, silicon). This layer acts as a mask for a further step. The hologram pattern is then reproduced in the resist. The transfer is made with the ablation writer described above. The main difference with direct etching is the ablation rate of photoresist: it is higher as in metals. So, this process is quite faster than direct etching.
The insolation and the development of photoresist is also possible with the excimer laser. Thus, the process is the same as direct writing. The required laser fluence is much lower than in photoablation process. By directly machining the desired phase structures into photoresist, we can eliminate the developing step in generating D.O.E. We present on figure 3 an ablated pixel with a Shipley photoresist on glass. It shows also a 128 x 128 pixels holographic pattern transferred in Shipley photoresist.
After the patterning, the most adapted technology is employed to create the modulation relief. The indirect method is compatible with plasma or reactive ionic etching processes in the case of silicon or metal substrates. For binary design, electrolytic deposition of copper is a high speed and a low cost solution. Figure 4 summarizes the different possibilities with this indirect process and shows the relief structure obtained with other described processes.
We have successfully reproduced diffractive patterns on resist. We have realized relief modulation with chemical electroless deposition or ionic etching (Figure 5). The required etching depth is 2,79 µm; the pixel width is 50 µm. The relief on the second element with copper deposition is too rough. Resulting noise decreases the efficiency in reconstruction. The image is also affected. The represented diffractive optics encodes an image. The incident laser light of a CO2 laser is spontaneously transformed in a desired shape. We have applied this principle in marking applications of electronic components.
Precise control of the excimer laser fluence makes possible the manufacture of an arbitrary surface profile within limits imposed by laser wavelength and beam delivery optics. Besides, multi-level structures are possible in this indirect process. First, we transferred in the resist layer the multi-level structures with the L.A.L. Different pulses with the same fluence give relief modulations. The required depth in the deposit surface layer is also achieved with an acoustical monitoring in real time [5], because the ablation rate tends to decrease with the number of laser pulses. The prototyping may be possible in one step as in direct writing systems. Because of the difference between the etching rate of photoresist layers and substrates, the ionic etching is well adapted.
5. Conclusion.
Our aim was to demonstrate the possibility to use a low cost direct or indirect process for the fabrication of D.O.E. for CO2 laser beam. In this article, we discussed the different technologies and their advantages. We studied two processes based on ablation writers; their main advantage is that they do not require high cost masks for the transfer of diffractive patterns. The first solution uses direct ablation micro-machining with excimer laser, but it is limited to polymers. The processing time and the roughness had to be considered in each case to evaluate limitations. In the second process, the relief modulation is obtained by indirect transfer using an intermediate photoresist. This layer acts as a mask for a further etching or deposition step. The potential application of D.O.E. for CO2 laser in the electronics industry are, for example, the fast marking of components, brazing and thermal treatments.
Acknowledgments:
The work was supported by ANVAR. The excimer laser is located at IREPA Laser, Pôle API, 67400 ILLKIRCH, France.
References:
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