Laser Direct Writing of circuit elements and sensors

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1 Laser Direct Writing of circuit elements and sensors A. Piqué a*, D.B. Chrisey a, R.C.Y. Auyeung b, S. Lakeou c, R. Chung d, R.A. McGill a, P.K. Wu e, M. Duignan f, J. Fitz-Gerald a, and H. D. Wu b a Naval Research Laboratory, Washington, DC b SFA, Inc. Largo, MD c Univ. of the District of Columbia, Washington, DC d Geo-Centers, Inc., Ft. Washington, MD e Southern Oregon Univ., Ashland, OR f Potomac Photonics, Inc, Lanham, MD ABSTRACT A novel approach for maskless deposition of numerous materials has been developed at the Naval Research Laboratory. This technique evolved from the combination of Laser Induced Forward Transfer (LIFT) and Matrix Assisted Pulsed Laser Evaporation (MAPLE), and utilizes a computer controlled laser micromachining system. The resulting process is called MAPLE Direct Write or MAPLE-DW. MAPLE-DW can be used for the rapid fabrication of circuits and their components without the use of masks. Using MAPLE-DW, a wide variety of materials have been transferred onto different types of substrates such as glass, alumina, plastics, and various types of circuit boards. Materials such as metals, dielectrics, ferrites, polymers and composites have been successfully deposited without any loss in functionality. Using a computer controlled stage, the above mentioned materials were deposited at room temperature onto various substrates independent of their surface morphology, with sub-10µm resolution. In addition, multilayer structures of different materials were demonstrated by this technique. Multilayer structures form the basis of prototype thin film electronic devices such as resistors, capacitors, cross-over lines, inductors, etc. An overview of the results obtained using MAPLE-DW as well as examples of several devices made using this technique is presented. Keywords: Direct Write, Rapid Prototyping, Laser Induced Forward Transfer, Matrix Assisted Pulsed Laser Evaporation, MAPLE Direct Write 1. INTRODUCTION The ever increasing role played by electronic systems in our everyday life show no signs of slowing down. However, the demand for new products has placed an enormous emphasis towards miniaturization and increased functionality. These trends call for the ability to produce electronic assemblies with reduced weight, volume and cost as well as the rapid fabrication of prototypes for testing new designs and architectures. Considerable progress has been achieved with the use of surface mounted electronic components. This progress, however, still relies on the old strategy of first designing, then patterning, and finally mounting each of the system components on a circuit board. Any changes to the design usually require a start over from the patterning phase. In order to completely sidestep the above described process, a totally new approach is needed. One such approach would be to employ rapid prototyping techniques. The application of rapid prototyping processes to the fabrication of electronic assemblies would make it possible to directly write over any surface all the elements and components called for in the circuit design. Moreover, the time that will be saved would have a great impact in reducing the long delay between concept, design and production. * Correspondence: pique@nrl.navy.mil; Telephone: (202) ; Fax: (202) SPIE s LASE 99, th January 1999, San Jose, CA. Proceedings preprint ( )

2 Rapid Prototyping (RP) techniques have offered some of these advantages for the manufacture of mechanical components for some time, where it is now possible to generate an actual 3-dimensional working part from a CAD drawing using CAM tools. The development of RP techniques for the fabrication of electronic devices is only recent. They offer the capability to deposit or pattern the different types of materials that make an electronic device without the use of masks or patterns, and as such they are known as direct write processes. Direct write technologies do not compete with photolithography for size and scale, but rather complement it for specific applications requiring rapid turnaround and/or pattern iteration, conformal patterning, or for modeling difficult circuits. Current technologies are either subtractive, i.e. they remove material from the part or additive, i.e. material is added instead. Examples of direct write technologies for fabricating or modifying metallic interconnects and/or other passive elements include laser trimming, ink jet printing, laser chemical vapor deposition (LCVD), Micropen and laser engineered nano-shaping (LENS). However, none of these techniques is yet capable of operating in air and at room temperature while maintaining sub-10 µm resolution as well as being compatible with the broad classes of materials required for electronic assemblies and not requiring ex situ processing. In addition, none of these techniques is capable of operating in both additive and subtractive fashion. By combining some of the major positive advantages of laser induced forward transfer (LIFT) and matrix assisted pulsed laser evaporation (MAPLE), a novel laser driven direct write technique has been developed. This technique has been called MAPLE Direct Write (MAPLE-DW). This paper will outline the approach to the MAPLE-DW process, and some of its advantages, such as the ability to perform in situ laser micromachining, surface pretreatment and annealing. In addition, details on the fabrication of gold interconnect lines and nichrome thin film resistors using LIFT will be provided. Finally, examples of ferroelectric single layer capacitors as well as a ferrite core inductor fabricated using the newly developed MAPLE-DW technique will be shown LIFT 2. BACKGROUND Over the past decade, several direct write techniques based on laser-induced processes have been developed for depositing electronic materials for a variety of applications. Among these techniques, laser induced forward transfer or LIFT has shown the ability for direct writing of metals for interconnects and mask repair and also dielectric materials such as simple metal oxides. The LIFT process utilizes the focused beam of a pulsed laser to remove a thin pre-deposited film from a laser transparent substrate, called the ribbon. The removed material is then redeposited onto a second substrate placed in close proximity ( 25 µm) to the ribbon. The area coated per laser pulse depends on the size of the laser spot striking the ribbon as well as the gap between both substrates. LIFT was first demonstrated using metals such as Cu and Ag over substrates such as silicon and fused silica utilizing excimer or Nd:YAG lasers 1,2. There are several experimental requirements for LIFT to produce useful patterns including: 1) the laser fluence should exceed the threshold fluence for removing the thin film from the transparent support, 2) the target thin film should not be too thick i.e., less than a few 100 nm, 3) the target film should be in close contact to the substrate, and 4) the absorption of the target film should be high. Operating outside this regime results in problems with morphology, spatial resolution, and adherence of the transferred patterns. Repetitive transfer of material allows control of the film thickness deposited on the substrate. Laser induced modification of the transferred material can occur through the transparent substrate after deposition. Overall, LIFT is a simple and powerful technique that can be used on a wide variety of target films MAPLE A new vacuum deposition technique, known as Matrix Assisted Pulsed Laser Deposition, or MAPLE 3 has been developed at NRL for depositing thin and uniform layers of chemoselective polymers 4,5,6 as well as other organic materials, such as carbohydrates 7. This technique is a variation of the conventional Pulsed Laser Evaporation process in that it provides a more gentle mechanism for transferring complex organic molecules from the solid to their vapor phase. In MAPLE, a matrix consisting of a frozen solution of the organic compound dissolved in a relatively volatile solvent is used as the laser target. When the laser strikes the surface of the target, it causes rapid vaporization of the solvent molecules. Part of the thermal energy acquired by the solvent is transferred to the organic molecules. When these molecules become exposed to the gas -target interface, they are transported into the gas phase with sufficient kinetic energy to be desorbed from the target surface without being denatured in the process. A film will be formed onto a substrate placed opposite to the target, while the solvent is pumped away. 2

3 2.3. MAPLE Direct Write The MAPLE-DW 8 process combines several of the advantages of LIFT with MAPLE in order to produce a novel laser driven direct write technique capable of transferring various materials such as metals, ceramics and polymers. The MDW process has been used successfully to deposit numerous types of materials in air and at room temperature. The resolution with which these materials have been deposited is of the order of 10µm onto a variety of substrates such as silicon, fused silica, polyimide and several types of circuit board materials. Because the MAPLE-DW process uses a highly focused laser beam, it can easily be utilized for micromachining, drilling and trimming applications, by simply removing the ribbon from the laser path. Thus MAPLE-DW is both an additive as well as subtractive direct write process. The key to the MAPLE-DW process is the development of a suitable matrix containing the material to be transferred. This matrix is then used to form a very uniform coating on the surface of a transparent substrate, i.e. the ribbon. The matrix is chosen so it strongly absorbs the laser wavelength being used, generally, laser fluences below the ablation threshold of the matrix are used. The purpose of the matrix is to hold the material in place until heating from the laser pulse causes the matrix to decompose resulting on the material being ejected from the ribbon and transferred to the nearby substrate. Because the laser fluences employed are lower than the ablation thresholds of the materials being transferred, no decomposition ever takes place, so the functionality of the transferred material is never affected. Similarly to LIFT, MAPLE-DW requires that the ribbon be held in close proximity to the substrate. In MAPLE-DW, the area coated per laser pulse also depends on the size of the laser spot striking the ribbon as well as the gap between the ribbon and the substrate. Figure 1 shows a simple schematic diagram of a MAPLE-DW system. Patterned Laser Forward Transferred Material Material to be Deposited Transparent "Ribbon" Pulsed Laser Energy Objective Micromachined Channel Micromachined Through-Vias Substrate Figure 1. Schematic diagram showing the basic elements of a MAPLE-DW system. 3

4 3. RESULTS AND DISCUSSION Fused silica quartz discs 5.0 cm dia. of various thickness were used as ribbon supports. These discs were coated by e-beam evaporation with 150 nm thick layers of gold for the LIFT of conducting lines and with 150 nm thick layers of nichrome for the LIFT of the resistors. For the MAPLE-DW experiments, a matrix containing the materials to be transferred was used to coat the discs. Various substrates were used for the transfer experiments including silicon, glass, polyimide and various types of circuit boards such as FR-4 and Rogers RO4003. In all the transfer experiments a 25 micron spacer was used to separate the coated side of the ribbons from the substrates. Both the substrate and ribbons were held in place using a vacuum chuck over an X-Y translation stage. The output from an excimer laser operating with a KrF mixture (248 nm, 10 ns pulse) was directed through a circular aperture and then through a 10x UV grade objective. By changing the aperture size, beam spots from 8 to 50 microns were generated. The laser fluence was estimated by averaging the total energy of the incident beam over the irradiated area. A simple test pattern containing examples of passive circuit elements such as metal lines for interconnects, single layer capacitors, coplanar resistors and inductors with a rectangular cross section core was developed for testing purposes. The test pattern was prepared in the form of a CAD file which was then translated into machine code using a software package developed by Potomac Photonics, Inc 9. The machine code routines were then used to control the substrate position as well as the laser firing during the fabrication of each structure described in this work. These routines contained information about the laser spot size in use, the relative shift required between laser pulses, as well as the required overlap between layers. Figure 2 shows the layout that was used. 8 mm Inductor Capacitors Resistors Conductive Lines 5 mm Figure 2. CAD design pattern comprising of typical circuit elements used for evaluating the MAPLE-DW process Conductive lines Using the gold coated ribbons, Au conducting lines were deposited using a 25 micron laser spot size following the patterns indicated in Figure 2. It was found that in order to improve the morphology of the transferred gold it was necessary to operate at laser fluences only slightly above the ablation threshold of the gold films. At higher fluences, any part of the laser pulse which is not absorbed by the Au layer on the ribbon can interact with the gold already transferred over the substrate and ablate it. Furthermore, multiple passes were required in order to build the gold lines to the desired thickness of 4

5 10 microns. The overlap between those passes also had a marked effect on the morphology of the final line. Figure 3 shows SEM images comparing two gold lines made by LIFT. In the first one, neither the laser fluence nor the overlap between passes had been optimized. The second line clearly shows the improvement achieved after optimization. The best results were obtained for laser fluences between 550 and 600 mj/cm 2 and 12.5 µm overlap between passes. Once the entire pattern of interconnect lines from Figure 2 was completed, any debris which might have accumulated on the sides of the lines during the transfer was removed by rastering the laser along the edges of the lines using the same 25 µm spot with the ribbon removed. Microscope images of the resulting conductive gold lines are shown on Figure 4. The surface roughness of the lines reflect largely the underlying roughness of the substrate on which they were transferred. The average conductivity of these lines was measured to be 7.5 x 10-7?m at room temperature which is about 30 times higher than that of bulk Au (2.4 x 10-8?m). Figure 3. SEM images showing the effect of varying the fluence and the overlap between passes in the fabrication of Au conducting lines. The fluence was too high and the overlap did not provide good uniformity for the line shown in the left image. The right image shows a line made after both these parameters had been optimized. The scale bar indicates 10 µm. Figure 4. Au lines deposited by LIFT on RO4003 circuit board using the tests pattern of Fig. 2. The Au linewidth is approximately 30 µm. A final laser trimming pass was performed along both sides of the line. The scale bar indicates 125 µm. 5

6 3.2. Coplanar Resistors The 5 coplanar resistors shown in Figure 2 were made by using nichrome ribbons. A 25 µm laser spot was used at a fluence of 1.5 J/cm 2 to generate the rectangular resistor pads. The overlap between successive passes was optimized in order to improve the uniformity of the nichrome structures. The thickness of the nichrome was about 10 µm. The measured resistances ranged from 65 to 190? and they scaled with respect to cross section and length as expected. The resistivity of the transferred nichrome was considerably larger than that of bulk and is likely due to the high degree of porosity present in the transferred nichrome as well as oxidation of the alloy during transfer. Figure 5 show a micrograph of the resistors. 400 µm Figure 5. Various size nichrome coplanar resistors fabricated by LIFT Capacitors and Inductors For the fabrication of the capacitors and inductor a hybrid approach was used. First, a 3 µm thick gold layer was e-beam deposited over bare Rogers RO4003, a hydrocarbon ceramic composite circuit board used for RF applications. The bottom electrodes were then patterned with the laser. Using a 25 µm laser spot and a fluence of 3 J/cm 2 the gold was ablated in order to generate the bottom electrode patterns shown in Figure 2. Then a ferroelectric layer consisting of BaTiO 3 (BTO) in the case of the capacitors or a ferrite layer consisting of Y 3 Fe 5 O 12 (YIG) in the case of the inductor was deposited by MAPLE-DW. Finally, the top Au electrodes were deposited by LIFT using the same conditions employed for making the conduction lines. Figure 6 illustrates the above steps schematically. For the capacitors, a 25 µm laser spot at a fluence of 400 mj/cm 2 was used to fabricate 20 to 30 µm thick BTO layers. For the inductor, similar parameters were used to fabricate a 20 µm thick YIG core. In both cases the morphology and thickness of the BTO and YIG layers was quite uniform, and the surface roughness variations observed with a profilometer were due primarily to the imperfections of the underlying substrate. Figure 7 shows the profilometer scan from one of the capacitors made. 6

7 1 2 3 Figure 6. Schematic showing the fabrication steps for the parallel plate capacitors. (1) Patterning of the bottom Au electrode, (2) MAPLE-DW of the dielectric layer, (3) LIFT of top Au electrode. Figure 7. Profilometer scan showing the uniformity across the capacitors fabricated by MAPLE-DW. The parallel plate capacitors were evaluated from frequencies ranging from 1 MHz up to 1.8 GHz using a HP4291A impedance analyzer. The capacitance ratio between the large and small capacitors was close to their area ratio (4:1) as expected with some variations attributed to non-uniformities on the BTO transfers. All the capacitors made showed capacitances between 2 and 40 pf and disipation factors between 0.11 to These capacitors were then annealed in a furnace at 200 C for two hours. After the annealing step, the capacitances dropped by about 40% while the dissipation 7

8 factors decreased by an order of magnitude. From these results, the effective dielectric constant of the capacitors was estimated to be around 25 after the annealing step. Figure 8 shows a micrograph of one of the BTO capacitors pairs made. The inductance of the four turn YIG core inductor was 9 nh at 1 MHz. The inductor exhibited very high losses and the effective permeability was estimated to be about 70. This result can be attributed to the fact that the inductor made had a very small YIG core with a large number of air gaps. Figure 9 shows a micrograph of the inductor. Figure 8. BaTiO 3 capacitors with Au electrodes made by MAPLE-DW. The larger capacitor was 1.6 mm x 1.6 mm other is 25% smaller. Figure 9. Four turn inductor with YIG core fabricated by MAPLE-DW. 8

9 3.4 Gas Sensors A new type of gas sensor based on conductimetric techniques 10,11 can be fabricated using composites made from a dispersion of a conducting material such as graphite and a non-conducting polymer. The resulting matrix is conductive and its resistance will change when exposed to different vapors. A ribbon made with a 4 µm thick layer of Polyepichlorohydrin (a chemoselective polymer) mixed with graphite was used in order to test the ability of the MAPLE-DW process to transfer polymer materials as well as composites 12. A series of conductive patches across gold electrodes were produced that showed sensitivities of the order of parts per million (ppm) when exposed to several gases. This work is still in its preliminary stages, and more detailed analysis will follow. However, it clearly demonstrates that MAPLE-DW can also be used for direct writing of functional polymer materials as well as composites. Figure 10 shows an optical micrograph of one of the prototype chemoresistors fabricated by MAPLE-DW. 500 µm Figure 10. Photograph of a PECH/graphite chemoresistor gas sensor fabricated by MAPLE-DW. 4. SUMMARY A novel direct write process for rapid prototyping of electronic circuit elements was developed by combining two laser based processes, LIFT and MAPLE. The new technique is called MAPLE Direct Write or MAPLE-DW. Materials such as metals, dielectrics, ferrites, polymers and composites have been successfully deposited without any loss in functionality by this technique. MAPLE-DW was used to fabricate gold interconnect lines, nichrome resistors, barium titanate parallel plate capacitors and a YIG core inductor. All these components were fabricated in air and at room temperature with sub-10 µm resolution. Using the MAPLE-DW setup developed for this work both subtractive processes such as laser machining, trimming and cleaning as well as additive processes such as LIFT and MAPLE-DW were demonstrated. 9

10 5. ACKNOWLEDGEMENTS We would like to thank Tim Schaeffer of the Mayo Foundation for the high frequency measurements of the capacitors and inductor made for this work. Financial support for this work was provided by DARPA. 6. REFERENCES 1. J. Bohandy, B.F. Kim, and F.J. Adrian, J. Appl. Phys. 60, 1538 (1986). 2. J. Bohandy, B.F. Kim, F.J. Adrian and A.N. Jette, J. Appl. Phys. 63, 1558 (1988). 3. R. A. McGill, D. B. Chrisey, Method of Producing Thin Film Coating by Matrix Assisted Pulsed Laser Deposition, US Navy Case No. 79, R.A. McGill, R. Chung, D.B Chrisey, P.C. Dorsey, P. Matthews, A. Piqué, T.E. Mlsna, and J.L Stepnowski, IEEE Trans. On Ultrasonics, Ferroelectrics and Frequency Control, 45, 1370 (1998). 5. R. A. McGill, D. B. Chrisey, A. Piqué, T. E. Mlsna, SPIE Proceedings Vol 3274, pp , A. Piqué, R.A. McGill, D.B. Chrisey. J. Callahan, T.E. Mlsna, in Advances in Laser Ablation of Materials, MRS Proceedings, vol 526, p. 421, A. Piqué, D.B. Chrisey, B.J. Spargo, M.A. Bucaro, R.W. Vachet, J.H. Callahan, R.A. McGill and T.E. Mlsna, in Advances in Laser Ablation of Materials, MRS Proceedings, vol 526, p. 421, D. B. Chrisey, R. A. McGill, A. Piqué, Matrix Assisted Pulsed Laser Evaporation Direct Write, US Navy Case No. 79, Potomac Photonics Inc Nicole Drive, Lanham, MD J.W. Gardner, M. Craven, C. Dow and E.L. Hines, Meas. Sci. Technol., 9, 120 (1998). 11. J.V. Hatfield, P. Neaves, P.J. Hicks, K. Persaud and P. Travers, Sens. Actuators B: Chem., 18, 221 (1994). 12. R. A. McGill, D. B. Chrisey, A. Piqué, Fabrication of Patternable Electrically Conductive Thin Films for Chemoresistor Chemical Sensor Applications with Laser Evaporation Techniques, US Navy Case No. 79,