Laser Joining of Glass with Silicon

Transcription

1 Laser Joining of Glass with Silicon Reiner Witte, Hans Herfurth, Stefan Heinemann Fraunhofer USA, Center for Laser Technology, Plymouth, MI ABSTRACT New joining techniques are required for the variety of materials used in the manufacture of microsystems. Lasers are emerging as a useful tool for joining miniaturized devices. The beam can be focused to less than.001 allowing localized joining of very small geometries. There is minimal heat input into the part so distortion and change in material properties is minimal. The high quality of the laser welds and the precise process control enable hermetic sealing. Glass to silicon bonds are required in a vast array of opto-electronic components, including laser sources, sensors, switches and multiplexers. Typically, adhesives as well as direct and anodic bonding techniques are used to join the different materials. Adhesive residues, low bond strength, heating of entire parts during joining and poor longterm stability are disadvantages of these conventional techniques. Laser bonding is a promising alternative due to the increased bond strength and high repeatability. Compact and efficient diode lasers equipped with fiber beam delivery in the power range of <50W are applied to the sandwiched glass-silicon structure. During bonding, the laser beam penetrates the upper part and is absorbed at the surface of the bottom part. A clean environment and good part fit-up is required to ensure proper bonding, high bond strength, and hermetic sealing. The process eliminates adhesives; therefore reducing costs due to shorter cycle times, lower maintenance and less inventory. The process requires no handling of toxic liquids and creates no fumes. Furthermore, the bonded parts are free of any residue or contamination, reducing scrap rates. This study investigates the process parameter window and determines the impact on the parts and the stability of the process. The results will lead to the development of several applications in the areas of telecommunications, biomedical devices and MEMS. This includes the encapsulation of MEMS, the covering of sensors and the packaging of biomedical products. The selectivity of the process will be demonstrated on flat coupons. Keywords: Selective bonding, Glass, Silicon, MEMS 1. Introduction Typically methods applied in wafer bonding are anodic bonding and room temperature bonding. Anodic bonding is based on the fact that alkali ions move at high temperatures from one wafer to another facilitated by an externally applied electrostatic field. This effect combined with the occurrence of Van der Waals forces results in a very strong connection between the wafers. However, there are some restrictions to this method. Anodic bonding takes place at elevated temperatures up to 500 C and requires an extended processing time of several minutes. In addition, the process cannot be applied locally; typically the entire contact surface between the parts is being bonded. Extremely high demands on cleanliness and flatness must be fulfilled to bond wafers at room temperature by stacking them and applying an initial clamping force. Laser supported bonding is based on the principle of adhesion of solids. In this case, the heat input by the laser beam initiates the bond. In principle, this method can be selectively applied to very small structures with a laser beam. The minimum size of the bond area is primarily determined by the spot size of the laser beam. This paper includes a general overview on the possibilities of laser-supported bonding and bonding results using different laser sources. An outlook on potential industrial applications will be given. Photon Processing in Microelectronics and Photonics, Koji Sugioka, Malcolm C. Gower, Richard F. Haglund, Jr., Alberto Piqué, Frank Träger, Jan J. Dubowski, Willem Hoving, Editors, Proceedings of SPIE Vol (2002) 2002 SPIE X/02/$

2 2. Material The requirements on both parts to be joined, glass and silicon, are very high. As known from anodic bonding in silicon manufacturing, the surfaces must have a high flatness, parallelism and low roughness /1/. The material must also be free of any impurities. The physical properties of the materials such as optical, mechanical and thermal appearance are defined in the following Characteristics Geometrical tolerances and surface quality In addition to the surface cleanliness there are two main specifications that are important for the mechanical properties in wafer bonding: = Flatness = Roughness The surface flatness is a macroscopic measure of the deviation of the wafer s front surface from a specified reference plane, assuming that the backside of the wafer is ideally flat /2/. The total thickness variation (TTV), also known as waviness, is commonly used to specify the surface flatness and describes the difference between the highest and the lowest elevation of the top surface of the wafer. During the bonding process, each wafer is elastically deformed to achieve conformity of the two surfaces /3/. Any flatness defects of the glass and silicon wafers can result in periodic strain patterns (contrast fluctuations) corresponding to typical spatial frequencies of a bonded pair that can be detected by X-ray topography. Larger areas of flatness defects may result in a reduced bond quality or a lack of bonding. The silicon wafer material that was used is a standard material with total thickness variation of less than 1µm. For the glass wafer, a similar value is given referred to as the peak to valley. The glass wafers from Schott have a peak to valley measurement of 0.09mm. After grinding or lapping, wafers are mounted on a flat plate by vacuum or wax. External pressure is applied to push the wafer against the polishing pad, which is moved across the wafer surface. Polishing slurry of a colloidal dispersion of silica (SiO 2 ) powder in an aqueous solution of potassium hydroxide is applied onto the pad to perform the polishing operation. The polishing pad materials are usually poromeric artificial fabrics such as polyester felt or polyurethane. The diameter of the silica particles used is in the range of 40 to 1000 Å, typically 100 Å /4/. The ph-value of the slurry is around and the concentration of silica is typically 3-4% /5/. Today, the polishing technology in wafer processing is able to produce silicon wafers with a roughness in the range of 1 to 2 Å. Chemo-mechanical polishing produces silicon wafer surfaces that exhibit sufficient smoothness and are free of haze and damage. For the silicon wafers, a roughness of 100nm is specified by the customer. The glass wafers did not require special polishing because the surface roughness after manufacturing was better than 3nm. 488 Proc. SPIE Vol. 4637

3 Optical properties The laser-supported bonding is based on the principle of transmission welding with laser radiation, which is well known from laser welding of plastics /6, 7/. This principle requires that one of the parts to be joined is transparent for the laser radiation and the other material is able to absorb the laser energy. The transmission spectrum of glass (Borofloat 33) is shown in Figure 1. For wavelengths between 300nm and 2000nm the transmission is approximately 90%, fulfilling the requirements for transmission welding. Figure 1 Transmission spectrum of Borofloat glass with different thicknesses/8/. Silicon has high absorption properties for wavelengths up to 900nm [Figure 2]; at longer wavelengths the absorption decreases rapidly and the transmission and the reflection increase accordingly. Based on these characteristics, laser sources with a wavelength of less than 900nm should preferably be applied to conduct laser-supported bonding. High power diode lasers adequately cover this wavelength range. After heating of the silicon, the absorption at higher wavelengths increases rapidly to nearly 90%. For this reason, the Nd:YAG laser with radiation at 1.06µm is also appropriate, especially considering the better beam quality and the smaller spot size. Figure 2 Transmission, Absorption and Reflection of 100µm thick silicon /9/ Proc. SPIE Vol

4 Thermal properties The laser beam heats both materials during the bonding process. A close match of the thermal expansion coefficient of both materials is necessary to avoid damage during processing or afterward during the cooling phase. Otherwise, the mechanical stress induced during the heat cycle will result in cracks. Some commercial glass materials exhibit a thermal expansion coefficient that is identical or very close to silicon. Most commonly used in anodic bonding are glasses from Schott GmbH (Borofloat ) and Corning (Pyrex 7070, 7740). Table 1 includes a brief overview on the material specifications. Table 1 Comparison of different glasses and silicon. /10,11/ Manufacturer Product Name Working Softening Annealing Strain Thermal Expansion (*10-6 * -1) Schott Borofloat Corning Pyrex Corning Pyrex n/a Melting Boiling Diverse Silicon The glass material must have a temperature above the softening point (approximately 820) to allow the move of alkali ions into the glass. During processing, the temperature at the interface must be kept below 1400 to avoid melting of the silicon. Melting of silicon would destroy the single crystal and result locally in a polycrystalline structure that exhibits different properties than the original structure. Figure 3 Thermal expansion over temperature of Pyrex glass and silicon. /12/ Delta L/L (PPM) Silicon 200 Pyrex 7070 Pyrex Temperature deg./c 490 Proc. SPIE Vol. 4637

5 3. Processing 3.1. Sample preparation Commercially available wafer material and glass samples were used for the investigation of lasersupported bonding. The materials meet the requirements for flatness and surface quality, however special treatments are necessary to remove the oxide layer from the silicon and to achieve the needed cleanliness of the sample. The surfaces must be free of particulate, organic, and metallic contamination because the cleanliness has a direct effect on both the structural and optical properties of the bonding interface as well as on the resulting electrical properties of the bonded materials Surface cleaning The cleaning techniques applied must remove all contamination from the surfaces without degrading surface smoothness. Similar to very large scale integration (VLSI) device fabrication, a silicon surface with a high degree of smoothness and flatness is also a key concern in laser-supported bonding. A hydrogen-peroxide-based (RCA) wet cleaning solution (see Table 2) that is typically used for wafer bonding was also found to be appropriate for laser-supported bonding. Table 2 Cleaning of silicon with hydrogenperoxide-based solution. 50% unstabilized H 2 O 2, 50% NH 4 OH, 37% HCL /1/ Cleaning step Compositions by Volume Operating Temperature (deg. C) Operating Time (min) Designed to Remove Ethanol C 2 H 5 OH 25 5 (Supersonic) Dust, fat RCA 1 NH 4 OH:H 2 O 2 :H 2 O 0.25:1: (Supersonic) Particles, organics, some metals Rinse Di H RCA 1 RCA 2 HCl:H 2 O 2 :H 2 O 1:2: Alkali and heavy metals Rinse Di H 2 O 25 5 RCA Laser sources The surface of glass is less reactive with its natural ambient. Therefore, it is not necessary to apply the same cleaning schedule as is applied for silicon. After removing the organics and particles with RCA1 and rinsing with DI water, the glass samples are ready for use. Two different laser types with different wavelengths and output power were investigated: a 30 W diode laser and a 1000 W Nd:YAG. Diode lasers have significantly improved in recent years and represent a promising alternative to conventional laser sources. The system used provides laser radiation at a wavelength of 808 nm and delivers the beam through a 600 um step index fiber to the workpiece. A minimum spot diameter of 750µm is achieved at a focal length of 50mm. The Nd:YAG laser beam has a wavelength of 1064nm and delivers a maximum power of 1000 W through a 300µm fiber. With this setup, a focus diameter of 150µm can be achieved using a 100 mm focal length. Proc. SPIE Vol

6 Compared to the diode laser, significantly higher intensities in the focal spot are achievable with the Nd:YAG laser. However, the diode laser radiation is better absorbed by the silicon surface (see Figure 2). The objective for the comparison was to determine if a bonding process still can be established using the highly cost effective diode laser Clamping To achieve intimate contact between the silicon and glass samples, a pneumatic fixture (Figure 4) was used, that can apply pressure between 1MPa and 30MPa. A transparent cover plate made of fused silica applies the clamping pressure to the sample surface and allows access of the laser beam. A three-axis motion system was used to scan the surface of the samples with the laser beam. This set-up offers high flexibility to vary speed, focal position and feed direction. Figure 4 Schematic drawing of processing glass to silicon wafer bonding F Laser Beam F Cover-Glass Galss-Sample Silicon-Sample Aluminum Base Plate F In the investigation using the 30 W diode laser, a layer of polytetrafluorethylene (PTFE) was used to control the heat flow from the silicon into the aluminum base plate. The PTFE layer serves as an insulator that allows sufficient heating of the material even with the low power diode laser. Without the PTFE layer it was not possible to achieve any bonding. During the tests with the Nd:YAG laser, isolation was not necessary due to the higher intensity in the focal spot of this laser. 4. Results The investigation included tests where the samples were stationary relative to the laser beam (static tests) and tests where the samples were moved under the stationary beam (dynamic tests). During the static tests, the laser beam was used to bond a single spot. The heat input was controlled by the laser power and the beam-on-time. Figure 5 shows the matrix of settings for the beam-on-time and the power that was investigated to determine appropriate conditions for bonding. It is distinguished between parameter settings that result in good bonds and settings that lead to defects. Typical bonding defects include lack of bond strength and crack formation during and after processing. The tests using the diode laser show a small parameter window where bonding is possible. 492 Proc. SPIE Vol. 4637

7 Figure 5 Radiation time vs. laser power for static diode laser tests good bonds broken or cracked bond Laser Power [W] Due to the heat input of the laser beam into the silicon wafer, thermal and mechanical stress results in the bonded parts. In the dynamic experiments, it was found that the duration of the heat input is relevant for producing cracks in the glass. During the process, the average temperature is steadily increasing in the silicon. At the beginning of the bond line, the parts are already connected while the rest of the silicon is expanding because of the heat input. The glass is less subjected to this effect because the absorbed energy in the glass is negligible so that only the connection to the silicon heats up the glass. At the end of the bond line, both materials will cool off with the result that the silicon bulk material is much warmer than the glass during bonding. Since silicon shrinks more than glass, mechanical stress is induced and cracks occur if the stress exceeds the strength of the bond. Furthermore, the energy input must be well controlled to achieve localized bonding and avoid melting of silicon. Melting of the silicon will result in a poly crystalline structure with changed electrical and mechanical properties. Bonds where melting of silicon occurred also exhibit a rough interface. Examples of static and dynamic bonding are shown in Figure 6. The left picture is a single pulse with a beamon-time of 2 sec and a laser power of 27W. The lines in the middle and the circle on the right side are also done with 27W. The speed for the lines is 0.07m/min and for the circle 0.08m/min. The length of the lines is 3mm and the diameter of the circle is 3mm. Figure 6 Samples after processing with diode laser. Due to the different beam characteristics and beam quality of the Nd:YAG laser, there are differences in the clamping and the required power. The higher intensity in the focal spot enables bonding without using a PTFE Proc. SPIE Vol

8 layer between silicon and the aluminum base plate. The speed can be increased up to 200mm/min at a focal spot diameter of 300µm. The sample in Figure 7 shows a circle with a 3mm diameter that is bonded at a speed of 100mm/min and 47W laser power. Figure 7 Bonded circle with Nd:YAG laser at 100mm/min and 47W. 5. Summary and perspective Localized laser bonding of glass to silicon has been investigated using a diode laser and an Nd:YAG laser. The main differences between these laser types are the beam quality and the output power. The investigation included the testing of bonds on spots, straight lines and circles. Bonding was successfully established with both laser types, however the tests show that the usable parameter window to achieve good bonds is narrow. Either cracking of the glass wafer or a lack of bonding is observed if the heat input deviates from the appropriate settings. It was also found that cleanliness and surface conditions of samples and appropriate clamping are crucial to bond the materials. The results exhibit a high potential for laser bonding to address an increasing need for localized joining of materials in the manufacture of miniaturized components. Future work will include the further development and optimization of the bonding process for glass-to-silicon and the characterization of the bond strength, durability and residual stresses. 6. References: /1/ Q.-Y. Tong, U. Goesele, Semiconductor Wafer Bonding, John Wiley & Son., 1999; /2/ T. Takahagi, A.Ishitani and S. Wakao, Chemical structure and reactivity of a silicon single crystal surface fluorinated by xenon fluoride, J. Appl. Phys., 76, 3140, 1994 /3/ K. Hofmann, G.W. Rubloff and R.A. McCorkle, Defect formation in thermal SiO 2 by high-temperature annealing, Appl. Phys. Lett., 49, 1525, 1986 /4/ C.C. Payne, Silica sol composition for polishing silicon wafers, U.S. Patent No. 4,462,188 (1984) /5/ R.M. Mandle, Process for preparing a polishing compound and product, U.S. Patent No. 3,298,807, 1967] /6/ T. Nilsson, C. Lampa, Diode laser welding of plastics, Proceedings of the 7 th Nordic Conference in Laser Processing of Materials, Acta Universitatis Lappeenranteaensis 84, 1999, Proc. SPIE Vol. 4637

9 /7/ H. Puetz, D. Haensch, H.-G. Treusch, S. Pflueger, Laser welding offers an array of assembly advantages, Modern Plastics, September 1997, ] /8/ Schott AG, Borofloat 33 product information, 2001 /9/ Fraunhofer Institut fuer Laser Technologie (ILT), Aachen, 2000 /10/ Schott AG, product information 2001 /11/ Corning Inc., product information 2001 /12/ Corning, Glass Silicon Constraint Substrates, 1999 Proc. SPIE Vol