High-speed one-dimensional spatial light modulator for ... · High-speed one-dimensional spatial...

High-speed one-dimensional spatial light modulator for Laser Direct Imaging and other patterning applications

Jan-Uwe Schmidt1a, Ulrike A. Dauderstaedta, Peter Duerra, Martin Friedrichsa, Thomas Hughesa,

Thomas Ludewiga, Dirk Rudloffa, Tino Schwatena, Daniela Trenklera, Michael Wagnera, Ingo Wullingera, Andreas Bergstromb, Peter Bjornangenb, Fredrik Jonssonb, Tord Karlinb,

Peter Ronnholmb, Torbjorn Sandstromb aFraunhofer Institute for Photonic Microsystems (IPMS), Maria-Reiche-Str. 2, D-01109 Dresden,

Germany; bMicronic Mydata AB, Nytorpsvagen 9, SE-18303 Taby, Sweden

ABSTRACT

Fraunhofer IPMS has developed a one-dimensional high-speed spatial light modulator in cooperation with Micronic Mydata AB. This SLM is the core element of the Swedish company’s new LDI 5sp series of Laser-Direct-Imaging systems optimized for processing of advanced substrates for semiconductor packaging. This paper reports on design, technology, characterization and application results of the new SLM. With a resolution of 8192 pixels that can be modulated in the MHz range and the capability to generate intensity gray-levels instantly without time multiplexing, the SLM is applicable also in many other fields, wherever modulation of ultraviolet light needs to be combined with high throughput and high precision.

Keywords: Laser Direct Imaging (LDI), Printed Circuit Boards (PCB), Spatial Light Modulator (SLM), Micro Mirror Array (MMA), gray-scale lithography, semiconductor packaging, semi-additive processing, substrate

1 INTRODUCTION

1.1 Laser direct imaging for advanced substrates in semiconductor packaging

Modern electronics packaging increasingly utilizes high-end forms of printed circuit boards called ‘substrates’ and ‘interposers’. The first provide a mechanical support and an electrical interface between integrated circuits and the outside world, the latter act as an intermediate layer used for interconnection routing and as a ground/power plane. Advanced packages require substrates with a high density of interconnects with minimum interconnect line widths and spaces of about 10 µm, in few years even less. Such high density substrates are processed in form of large panels (e.g. 510 mm x 515 mm). To form the interconnect layers, the ‘semi-additive metallization’ process [1] is used (Figure 1): On a copper seed layer, a layer of dry film resist (DFR) is laminated (1). Next, the whole panel is exposed by ultraviolet (UV) light (2), in order to enable patterning of the DFR (3). The spaces in the patterned DFR act as template for the deposition of copper by electroplating (4). The removal of DFR (5) is followed by a flash etch to remove the Cu seed layer (6). As feature size decreases, several wiring layers and their vertical connections (vias) have to be aligned within smaller tolerances to avoid functional errors. Compared to mask-based steppers an exposure by Laser Direct Imaging (LDI) offers higher flexibility. LDI techniques utilize a programmable micromechanical element, a so-called ‘spatial light modulator’ (SLM) to print dose-patterns into the resist and have the potential to combine high resolution, high precision of alignment and high throughput. Small variations in the pitch of existing structures induced by strain in the substrates can be measured for each panel and compensated by appropriate algorithms, such that new layers perfectly match the preceding ones. Micronic Mydata AB has developed a novel LDI5sp laser direct imaging system optimized for this field of application. As a cooperation partner, Fraunhofer IPMS contributed to this system a novel fast one-dimensional diffractive spatial light modulator (SLM), which shall be discussed in the present article.

1 Corresponding author: [email protected]; phone ++49-351-8823-119; fax ++49-351-8823-266; www.ipms.fraunhofer.de

MOEMS and Miniaturized Systems XIII, edited by Wibool Piyawattanametha,Yong-Hwa Park, Proc. of SPIE Vol. 8977, 89770O · © 2014 SPIE

CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2036533

Proc. of SPIE Vol. 8977 89770O-1

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Figure 10: DIC microscopy image of pixel area after sacrificial layer etch. Minimal and homogeneous tilt of idle mirrors.

Figure 11: Close-up onto the wire bonds on one side of the SLM.

Figure 12: Close-up image of the assembled SLM unit showing the wire bonds connecting SLM and FIA boards.

4 SLM CHARACTERIZATION

4.1 Fundamental resonance of mirror tilt movement

For a current spring design the resonance frequency has been measured at ambient pressure using a MSV-300 Microscope Scanning Vibrometer (Polytec) by modulating the data-voltage with a rectangular voltage function. The resonance frequency has been determined to be higher than 1.3 MHz.

4.2 Planarity of chip within pixel area

An imperfect planarity of the SLM within the pixel area will lead to errors of the generated SLM image. The imaging system and software of an LDI system may compensate or correct for certain errors of SLM shape: e.g. a cylindrical bow of the MMA can be compensated by the imaging optics. Planarity specifications therefore have to be considered with respect to the specific imaging system. The planarity within the large pixel area is measured using a Wyko® NT9800 optical profiler. Single measurements covering an area of 1.2 mm x 1.6 mm sampled with 4.7 µm resolution are stitched together to cover the whole pixel area. Figure 13 shows a typical height map of the pixel area after subtraction of bow.



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Table 1: Summary of SLM parameters.

LDI SLM Parameter Value

Chip dimensions [mm] 15 x 88

Dimensions of pixel area [mm] 4 x 82

Fill grade of active area [%] > 90

Number of pixels 8192

Mirrors per pixel 268

Mirror shape hexagonal

Mirror material Al-alloy

Operating wavelength [nm] 355

Mirror reflectance at 355nm [%] > 85

Max. mirror deflection [nm] > 110

Pixel resonance frequency [MHz] > 1.3

Achieved contrast Up to 1000

Mirror RMS planarity [nm] < 7

4.5 Deflection test of assembled SLM-unit

The functionality of the completely assembled and wire bonded SLM-units is verified by a deflection test using a Wyko® NT9800 optical profiler. Herein the SLM unit is addressed through the FIA boards to include FIAs and wire bonds in the test. All pixels are addressed with the same voltages to check for non-working pixels. The lateral resolution for the interferometric measurement is chosen such that the deflection amplitude of each individual mirror can be determined. The pixel area is covered by stitching a set of subsequent measurements. Figure 16, a plot of mean pixel deflection vs. pixel number, illustrates a typical result. All pixels are functioning. Due to design-related systematic differences in the routing of data electrodes, the observed deflection values vary slightly with pixel number in a systematic way. Irregular features, like the one at pixel 3072, are attributed to small variations in hinge width caused by small tolerances of lithography. Before the SLM is finally used in a LDI system, the mentioned systematic effects are compensated by a calibration procedure. After calibration for all individual pixels the voltages required to reach certain deflection values can be retrieved from look-up tables. A two-dimensional representation of the same data, i.e. a map of deflection for each individual mirror of the SLM (inset in Figure 16) can be used to check for possible non-working mirrors and to retrieve their positions.

5 SLM PERFORMANCE IN LDI EXPOSURE SYSTEM

5.1 Description of exposure system

Figure 17 is a schematic of the LDI5sp system developed by Micronic Mydata AB. The coherent light source of the exposure system is a high-power, 355 nm diode-pumped solid state laser. The SLM is illuminated by the rectangular laser beam. Light reflected into the 0th diffraction order passes the aperture in the Fourier plane of the optical system, while the higher diffraction orders of the SLM are blocked. Behind the lens system, the beam is reflected via a rotating prism into one of four imaging arms rotating in synch with the prism before it is imaged onto the substrate. During exposure the intensity modulated line focus is laterally scanned in an arc across the substrate panel, always keeping the same orientation similar to a windscreen wiper. After it has passed the panel edge, the laser beam hits the next facet of the prism and is directed into the next imaging arm to expose the next arc. Since at the same time the panel is steadily moved, subsequently exposed arcs are seamlessly stitched together until the whole panel has been exposed. To maximize throughput, the twin table system aligns the next panel already while the preceding panel is being exposed. After exposure, the aligned panel is briefly flipped upwards to enable the passage and unloading of the already exposed panel. The tool exposes dry film resist on substrate panels with dimensions up to 510 x 612 mm² at a resolution better than 10 µm L/S. The constant-speed operation of both rotor arms and panel stage contribute to a highly precise pattern placement. Feature edges can be smoothly shifted laterally utilizing the gray-leveling capability of the presented SLM. An adjustment of exposure dose is possible by tuning the speed of rotation for the imaging arms. Based on the high SLM modulation rate and high optical efficiency of the SLM, a high write speed and efficient use of laser power are achieved.



LASER

PATTERNRASTERRE

SLM Real-time data path that enablesadvanced pattern alignment

ALIGNMENT CAMERAS

Analog Spatial Light Modulation(SLM) with high modulationfrequency

Rotor -based optical system withhigh scanning speed and lowoverhead time

e

ROTOR

Twin -table stage system thatenables concurrent alignmentand exposure

A

AIN

4Mk

Mi-My AB 2013 -06 -18 10:45 F D14,9 x800 100 um

5.2 Exposure results

Examples of patterned substrates are shown to illustrate the successful utilization of the presented SLM in Micronic Mydata’s LDI5sp Laser Direct Imaging systems. Figure 18 shows a 15 µm high dry film resist on a copper seed layer patterned with 6 µm minimum line/space dimensions. Figure 19 shows a 25 µm high dry film resist on a copper seed layer structured with a pattern typical for the routing of interconnects around a pad. The minimum line/space dimensions in this image are 10 µm. Using the presented new SLM, contrast values (here defined as ratio of maximum and minimum intensity at the exposed substrate) of up to 1000 have been measured.

Figure 17: Schematic of Micronic Mydata’s LDI5sp system

Figure 18: 15 um DFR on Cu seed layer, patterned with a micro-VIA interconnect test pattern. Minimum line/space feature size is 6 um.

Figure 19: 25 um DFR on Cu seed layer, patterned with a structure typically used in the pad region of a substrate. Minimum line/space feature size is 10 µm.



6 SUMMARY

Fraunhofer IPMS has developed a fast one-dimensional analog spatial light modulator (SLM) in collaboration with Micronic Mydata AB. The SLM supports a pixel rate of >10 billion grayscale-pixels per second and handles tens of Watt of laser power at a wavelength of 355 nm. The SLM has successfully passed all performance tests and is now utilized in Micronic Mydata’s novel LDI5sp series of Laser Direct Imaging systems optimized for the processing of advanced substrates for semiconductor packaging. A first Micronic-Mydata LDI5sp tool equipped with the IPMS-SLM has reached acceptance status at a final customer.

7 ACKNOWLEDGEMENTS

The authors wish to acknowledge Per Askebjer and Jarek Luberek for their valuable contributions to this development.

REFERENCES

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[3] Lakner, H., Duerr, P., Dauderstaedt, U., Doleschal, W., and Amelung, J., "Design and fabrication of micromirror arrays for UV-lithography," Proc. SPIE 4561, 255 (2001).

[4] Solgaard, O., Sandejas, F. S. A., and Bloom, D. M., “Deformable Grating Optical Modulator,” Optics Letters 17 ( 9), 688-690 (1992).

[5] Schmidt, J. U., Bring, M., Heber, J., Friedrichs, M., Rudloff, D., Roessler, J., Berndt, D., Neumann, H., Kluge, W., Eckert, M., List, M., Mueller, M., Wagner, M., “Technology development of diffractive micromirror arrays for the deep ultraviolet to the near-infrared spectral range,” Proc. SPIE 7716, 77162L (2010)

[6] Haspeslagh, L. et al., "Highly reliable CMOS integrated 11MPixel SiGe-based micro-mirror arrays for high-end industrial applications," Proc. IEDM, 655-658 (2008).

[7] Dauderstaedt, U., Askebjer, P., Bjornangen, P., Duerr, P., Friedrichs, M., List, M., Rudloff, D., Schmidt, J.-U., Mueller, M., and Wagner, M., "Advances in SLM Development for Microlithography," Proc. SPIE 7208, 720804-2 (2009).

[8] Martinsson, H., and Sandstrom, T., "Gray scaling in high performance mask making," Proc. SPIE 5853, 1031-1042 (2005).

[9] Schmidt, J. U., Friedrichs, M., Bakke, T., Voelker, B., Rudloff, D., Lakner, H. “Technology development for micromirror arrays with high optical fill factor and stable analogue deflection integrated on CMOS substrates,” Proc. SPIE 6993, 69930D (2008)

[10] Sandstrom, T., Askebjer, P, “SLM Device and method for combining multiple mirrors for high power delivery,” US-Patent US 8’531’755 B2, filed in 02/2010



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