Dicing of Gallium Arsenide (GaAs) Wafers with the Laser MicroJet Challenges, Improvements and Safety Issues

Transcription

1 Dicing of Gallium Arsenide (GaAs) Wafers with the Laser MicroJet Challenges, Improvements and Safety Issues Natalia M. Dushkina Laboratory of Laser Applications, The Gem City Engineering Co. Dayton, Ohio 45404, USA Bernold Richerzhagen SYNOVA SA Lausanne, CH 1015, Switzerland GaAs wafers are fragile and brittle and, therefore, the well-developed dicing saw technique, which is widely used in the silicon industry, faces serious problems when used for dicing GaAs wafers. GaAs wafers are very sensitive to changes in the dicing tools and to drifts in the dicing machinery, which makes the dicing difficult and causes some throughput issues. At the moment, the most commonly used dicing saw process is dicing with a 30 µm thick resinoid blade. Although, these blades provide high-quality kerf, they are also very fragile and, thus, have a very short life. Dicing of GaAs wafers with the Synova Laser MicroJet, which implements a YAG- Nd laser beam confined in a water jet, gives exciting and promising results for a general solution of the problem. By means of the Laser MicroJet, wafers as thin as 25 µm can be diced in streets of µm width, providing kerf quality comparable to the dicing saw cut and in some cases even better than the dicing saw. The Synova MicroJet increases the wafer throughput and under certain conditions yields 100% throughput. As far as we are aware, the presented results are the first for laser dicing of GaAs wafers, and, therefore, provoke a detailed discussion about the safety of the new technique. In this paper, we address the advantages and optimization, as well as safety issues of the laser water-jet dicing process for GaAs wafers. Key words: laser cutting, water jet, Q-switched Nd:YAG laser, GaAs wafers I. Introduction Gallium Arsenide accounts for almost three quarters of the total production of compound semiconductors for the last few years, according to a study of Kline&Company, Inc., a leading business consulting firm serving the electronics, chemical and material industries worldwide. 1 Compound semiconductors based on non-silicon wafers have rapidly invaded the semiconductor market in the last decade, which was dominated for more than twenty years by silicon due to its outstanding industrial mastery and low price. GaAs and GaAs-on-Si have significant market potential, both as a substitution technology branch for manufacturing traditional GaAs devices and as a new technology for monolithic integration of GaAs devices and silicon integrated circuits. While logic and memory devices still rely on the well-established silicon technology, the compound semiconductors made a strong impact on the fast-growing market of various communications and photonics devices. As the device and system industries are rapidly

2 becoming a commercially active branch, the substrate production and wafer processing remains mired in the research and development stage. Compound semiconductors are more expensive: they are more fragile and brittle than silicon, their melting points are lower, which compromise fabrication, and the basic boules, from which the wafers are cut, are much smaller in diameter. Therefore, only $3 billion, of the $119 billion of all produced integrated circuits in 2001, accounted for the compound semiconductors, while the much bigger part still belongs to the silicon production. However, the market for compound semiconductors expands rapidly due to the incessantly increasing demands for higher speed of the wireless and broadband communication industry. The growing impact of GaAs in the fields of fast telecommunications and photonics requires sophisticated and less expensive methods for wafer preparation and processing. A particular challenge is the precise and fast dicing of the fairly brittle GaAs wafers. Common saw methods using resinoid blades are close to their limits and it is doubtful if these methods will meet the future demands of flexibility, high cutting speed, production rate and yield. The employment of the Synova Laser MicroJet, on the other hand, increases appreciably the flexibility of GaAs wafer processing. Moreover, it allows arbitrary shape cutting, which is not possible with the conventional saw techniques. However, manufacturing and processing of compound semiconductors, and especially of GaAs, reveals serious industrial hygiene concerns due to hazardous chemical compounds and/or byproducts found in certain processing equipment and environment. In this paper we describe the advantages of the laser water-jet dicing process of GaAs wafers, and discuss optimized cutting parameters, as well as safety issues of dicing with the laser MicroJet. II. The water jet guided laser technology The water jet guided laser technology was invented and developed by one of the authors (B.R.), who called it Laser MicroJet. 2,3 The new technology has a huge market potential for processing of semiconductor wafers and other materials with subtle thermal effect, high speed and high kerf quality. In 1998, the concept was implemented in sophisticated laser cutting and dicing machines produced by Synova SA in Switzerland. The water-jet guided laser technology provides low-temperature laser dicing since the laser beam is coupled in a fine, stable water-jet and conducted to the sample by means of total internal reflection like in a glass fiber (Fig. 1). Thus, the water jet can be referred to as a fluid optical waveguide of variable length. This feature allows a working distance of 2-3 inches and eliminates the problems connected with focusing on the sample when using conventional lasers. The diameter of the water jet is determined by the nozzle size, which might be 30, 50, 75, 100 and 150 µm. The high laminarity of the water jet provides kerf width of the same size as the water jet diameter. The relatively low pressure (10-30 MPa) of the tiny water jet results in a negligible force on the sample; thus, there is no mechanical stress during cutting. This is of great importance when dicing thin and brittle GaAs wafers, which might be as thin as 25 µm. The water jet itself does not cut the sample, but plays a trifold function in the cutting process: 1) it guides the laser beam to the sample; 2) immediately cools the area of interaction of the laser light with the matter, and 3) simultaneously cleans the residues from the kerf.

3 Fig. 1. The mechanism of wafer-dicing process. The water jet guides the laser beam as an optical fiber. Synova MicroJet machines use YAG:Nd lasers in pulsed and Q-switch working regimes with wavelength at 1064 nm. Thus, the water jet guided laser is suited for processing of any material that absorbs at this wavelength. Almost any metal, semiconductor and some ceramics are suitable for the MicroJet cutting, the only limitation being the thickness. Unfortunately, materials that are transparent for the YAG:Nd laser wavelength, like glass and oxide layers, cannot be processed with high quality. To broaden the range of materials that might be cut with the MicroJet, the team of Synova is developing a shorter wavelength laser system using the frequency-doubled YAG laser with a wavelength of 532 nm. III. Properties of GaAs important for the processing with MicroJet The spectral transmission of GaAs is shown in Fig. 2. GaAs absorbs strongly the 1064 nm wavelength of the YAG:Nd laser light, and is therefore an appropriate material for the MicroJet applications. At temperatures higher than 250 o C it starts to show the phenomenon of thermal runaway: that is, the hotter it gets, the more the absorption increases. Its thermal conductivity of Fig. 2. Transmission spectrum of GaAs. 4

4 5x10 1 W/(m K) is more than 2½ times that of ZnSe and its density of 5.31 kg/m 3 is twice that of silicon. 4 The melting temperature of GaAs is 1238 o C. If heated above 480 o C, it decomposes to evolve arsenic vapor, which pressure reaches 1 atmosphere at the melting point. IV. Challenges of dicing GaAs wafers In a drive towards higher production volumes and lower costs, all major players of the GaAs industry have moved or are moving to 6-inch manufacturing capability. Yield improvement is one of the key performance indicators depending strongly on the quality of the wafer post-fab processing since at that stage the wafer has the highest value. Dicing of GaAs wafers is not a trivial process, because GaAs wafers are fragile and brittle. The most commonly dicing techniques at the moment are the saw and scribe/break processes. The saw process involves dicing wafers with 30 micron-thick resinoid blades. Although the resinoid blades provide high-quality kerf, they also raise a lot of handling problems: because of the thickness of the blade, they are very fragile and can easily break; they have very short life (<1000 cuts) and should be changed after almost every one and a half wafers. Due to the blade wear the quality is not consistent and processing speed is low, because blades often break in the middle of a wafer and had to be changed. Therefore, the throughput is very slow and yield is low. The resin-bonded diamond blades also present availability and cost problems due to their complicated manufacturing process. Some of the handling and availability problems were diminished with the newly developed Disco electroplated blades, with strength being their biggest advantage over the resinoid blades. The scribe/break process allows for the reduction of the street size over 50% increasing the number of die per wafer. But this technique is limited to only large die (larger than 1 mm x 1mm), the scribes are unreliable, the operation is slow, and requires precise alignment and a large amount of scrap wafers for quals. In 2001 Tyco Electronics reported a series of scribing improvements that have increased the throughput and reduced the dicing time to 35 minute/wafer. 5 More improvements still need to be made, especially concerning alignment and software problems. V. Cutting of GaAs wafers with Laser MicroJet The laser MicroJet is free of the problems of conventional dicing, such as tool wear, handling and availability, sudden and unpredictable break of the blades or mechanical stress on the sample. A good alignment of the nozzle provides constant quality cutting during the whole working day, as the life of the nozzle depends mainly on the operator s experience to obtain a good alignment, and can last for hundreds of working hours. Figures 3-6 show the capabilities of the LDS-200 machine for cutting GaAs and GaAs/Ge wafers and Fig. 7 reveals some problems and their solutions. Because of safety reasons, all GaAs and other considered toxic materials sample tests are performed at GCE, Dayton, using the LDS machine with a Q-switch laser or the Lasaq long pulse laser machine. The Q-switch laser can cut wafers with thickness 25 µm 300 µm, while the long pulse laser is used for wafers with thickness up to 1-2 mm. Table 1 shows typical parameters for Q-switch YAG:Nd laser MicroJet cutting of GaAs wafers.

5 Parameter Table 1. Typical Laser Parameters Pulse energy Energy reproducibility Average power Pulse length Repetition rate YAG:Nd laser 2.5 mj <2% 62 W 450 ns 25 khz Fig. 3. Omni-directional cutting with Laser MicroJet. The wafer is GaAs/Ge with thickness (178 µm), kerf width 75 µm, speed 15 mm/s, die size 1mm x 2 mm, s-lines and circles with radius of curvature 0.6 mm and 0.3 mm, respectively. The small dots on the wafer surface are not residues from the cutting process, but features of the top layer. Nozzle with diameter of 75 µm and water jet pressure of 200 bar were used. a) b) c) Fig. 4. GaAs wafer with complex layered structure with thickness 147 µm diced with two passes with speed of 10 mm/s and 60 mm/s, respectively: a) and b) 50 µm nozzle and water jet pressure of 300 bar and c) 50 µm nozzle (vertical line) and 75 µm nozzle (horizontal line).

6 a) b) c) Fig. 5. Comparison of the front side quality with the saw. The wafers are GaAs/Ge with thickness of 178 µm and different top surfaces: a) saw cut with speed 1.8 mm/s, magnification 50 times; b) and c) laser MicroJet kerf with speed of 15 mm/s, magnification 400 times. The wafers were not cleaned after cutting. No chipping or edge cracks are seen on b) and c). The speed factor vs. saw is 8.3. Customers require speed factor more than 4 to consider replacement of the existing saw equipment with the Laser MicroJet. No backside metalization With backside metalization Fig. 6. Backside quality of GaAs/Ge wafer cut with the laser MicroJet : wafer thickness (178 µm), 75 µm nozzle, water jet pressure 200 bar, speed 15 mm/s, magnification 400 times. Careful optimization of the dicing parameters is necessary in the case of backside metalization in order to avoid chipping and usually requires lower speed. a) b) c) Fig. 7. Some problems: a) If the starting point of the cut is on the wafer, the wafer might crack following the crystalline directions. This problem can be avoided by starting the cut outside of the wafer; b) and c) The cross section quality might be improved by reducing the pulse energy, increasing the speed and the number of passes.

7 VI. GaAs and Safety VI.1. GaAs test trial run Pure compound GaAs contains 51.8 wt% arsenic, and is, therefore, considered a hazardous material according to the standards of the Occupational Health & Safety Administration (OSHA). GaAs is described in the Material Safety Data Sheet (MSDS) as toxic by inhalation and a possible human carcinogen. These facts raise a lot of concerns from an environmental, health and safety standpoint in the GaAs industry. As a supplier of the new technology and machines to the American market, we have to provide also information about the potential hazards when working with the laser MicroJet, as well as of the measures that should be observed in order to satisfy the high safety requests of OSHA and the Environmental Protection Agency (EPA). Such information was not available due to the innovation of the technique and the lack of experience in processing hazardous materials with the laser MicroJet. The scanty information that we were able to get from some GaAs manufacturers could not be applied directly due to the different mechanism between the saw and the MicroJet cutting process. Therefore, we performed a six-hour trial run of non-stop dicing of GaAs wafers. The goal of the test was to clarify qualitatively and quantitatively the potential hazards in real working time. Preparing the test, we considered eventual formation of ai-born arsenic and arsine gas, which is acute poison, heavy contamination with arsenic of the wastewater and cutting chamber, as well as some contamination of the working area around the machine, the level of which we could not predict, and, therefore, we took the highest precaution measures for the safety of the operator - bunny-suit, rubber gloves, a respirator with HEPA filter P100 and a PentAir adjustable flow airline hood supplying fresh air from a breather box - air filtration box with carbon monoxide monitor. During the test, the wastewater was entirely collected in a barrel that was afterwards disposed as a hazardous material; the exhaust port of the machine was equipped with high efficiency particulate air filter (HEPA); the ventilation system of the laser room was shut down and all supply and exhaust openings in the room were sealed; access to the room was restricted. All materials used during the test, as well as those for cleaning afterwards, were gathered in a specially provided drum and disposed as solid hazardous. A representative of Ohio Bureau of Workers Compensation, Division of Safety & Hygiene, surveyed the preparation and the trial run itself. VI.2. Sampling and Analysis Summary Here we will discuss only briefly the GaAs trial run and the results of it, since the details will be published soon in a separate paper. 5 During the test we monitored: 1) the presence of arsine gas by three digital arsine gas detectors of electrochemical type (model SEC 1500, manufactured by Sensor Electronics Corporation, Minneapolis, MN, with sensitivity from 0 to 1000 PPB), set to three levels of arsine concentration - 10 PPB (low); 30 PPB (middle) and 40 PPB (high) with alarm warning for immediate danger (2001 TLV=50 PPB; TLV stands for total lethal value).

8 2) the air-born arsenic was monitored by five detectors using pre-weighed 37-millimeter mixed cellulose ester filters in-line with calibrated SKC Airchek 224-PCXR-4 high-flow air pumps (method reference #NIOSH 7900). The samples were collected within the operator s breathing zone and in areas of worse case exposure: inside the exhaust chamber, on the top of the machine and around it. 3) the contamination of the room by As and GaAs particles, as seven wipe samples were taken from the cutting chamber, table next to the operator, and room floor and walls immediately after the trial run was completed. Additional samples were taken after cleaning of the equipment and room. The samples were analyzed at NATLSCO Laboratory. 4) the contamination of the wastewater, as a sample was taken every 15 minutes directly from the tank of the machine. The samples were later analyzed by TestAmerica, Inc., using methods EPA and EPA ) personal safety - according to the MSDS, acute poisoning from GaAs is unlikely (NIOSH #LW ), but high atmospheric concentrations may lead to systematic toxic effects of arsenic poisoning. Therefore, the operator had a medical check for arsenic the day before and after the GaAs trial run. The results from the GaAs trial run, as well the Permissible Exposure Level (PEL) and recommended precaution measures, are summarized in Table 2. As we expected, the contamination of highest degree was of the wastewater, where the concentration of arsenic was about 1000 times higher than the EPA s current maximum allowable amount. The highest concentration of air-born arsenic and particles deposition, which was 13 and 30 times higher than the OSHA cancer hazard, respectively, was measured inside the cutting chamber, while outside the machine the contamination level was lower than the OSHA arsenic standard. The fact that Arsine gas was not detected was not surprising for us. Since the laser beam is coupled in a water jet and the laser pulses are very short (around 450 ns), the time for interaction of the laser light with matter is very short, and immediately followed by the cooling effect of the water. Therefore, one could suppose that the average temperature in the cutting spot is not high. This hypothesis was proved by an experiment performed a week before the GaAs test. For its purpose an infrared camera and a frame-grabbing system was used to monitor the cutting process. 5,7 The recorded temperature at any working conditions did not exceed 160 degrees Celsius. The simultaneous action of laser beam and water jet keep the average temperature in the cutting point far below the level of decomposition of the material and generation of arsine gas in dangerous concentrations. The concentration of inorganic arsenic in the human body, determined after the test, half of the limit considered as carcinogenic level. In summary: 1. No arsine gas was detected inside and outside the cutting chamber; 2. The main concerns should be the wastewater - severe measures for filtering using arsenic filter and proper recycling should be taken under consideration. 3. An exhaust system with high efficiency dust/mist filtration, arsenic filter or closed recycling of the waste water, and wet post-cleaning of equipment and facility, are highly recommended, as well as 4. Full personal protection rubber or plastic gloves, HEPA respiratory filter, glasses or protective shield. The personnel working full 8-hour shifts on a daily basis should

9 perform a medical check and urine test every six months and keep tracking records of the results. Table 2. Data for the Arsenic Concentration During the GaAs Test. Arsine gas, ppm Air concentration of Arsenic, µg/m 3 Water concentration of Arsenic, µg /L Presence of Arsenic in the human body, µg/l PEL Detected during the test Recommendations TLV=0.05 Not detected 10 (OSHA cancer hazard) (in the cutting chamber) 4 (outside the machine) BEI =35 50 (EPA) 2 (in the waste water without filtering) <20 Wipe (surface) sample results, µg/cm 2 1 (see 1-3 below) 5.2 (before the test) 9.4 (after the test) 30 (in the cutting chamber) (table next to Synova machine) VII. Conclusion and Acknowledgments Exhaust system with a particulate filter Closed recycling or Arsenic filter Gloves and a HEPA filter Post-cleaning necessary The results and conclusions from the GaAs test reveal that dicing of GaAs with the Laser MicroJet might be a safe working process, if the precautionary measures recommended above are meticulously observed. The new technology allows omni-directional and free-shape cutting with radius of curvature as small as twice the nozzle diameter, i.e., 100 µm 200 µm. The laser MicroJet provides dicing speed for GaAs wafers 7-10 times higher than the saw and a constant burr- and chipping-free cutting quality for wafers without transparent oxide or photoresist layers on the front or back wafer surfaces. The cutting parameters depend strongly on the wafer structure, the front wafer surface material and the availability of backside metalization, should be optimize for any series of wafers. More studies and improvements still need to be made in order to meet the broad variety of requirements of our customers. 1 BEI (Biologic Exposure Index) defined by ACGIH (The American Conference of Governmental Industrial Hygienists. 2 EPA s current maximum allowable amount of Arsenic per liter of water (EPA = Environmental Protection Agency). 3 Not well defined. The generic method for establishing an acceptable surface residual level used by GaAs manufacturers follows the scheme: Take the airborne standard (10 µg/m 3 for As); Use the average amount of air inhaled during an 8-hour shift (10M3); Assume total retention of the inhaled material during the shift; Calculate the effective dose (100 µg for As); Apply this calculated value to the typical surface area that has been wiped (100 cm 2 ); The standard is then 100µg/100m 2 or 1 µg/m 2.

10 The authors thank the Management of The Gem City Engineering Co. for their support of the Synova project and Prof. Rado Kovacevic from SMU for organizing the temperature monitoring experiment! Special thanks are addressed to the customers, who provided the samples and additional information! References 1. Kline&Company, Inc., Little Falls, N.J., PRNewswire via Comptex, May 7, Richerzhagen, B., B. Richerzhagen, "Development of a System for Transmission of Laser Energy," Ph. D. Thesis work, EPFL, Switzerland, B. Richerzhagen, G. Delacrétaz, R.P. Salathé, "Complete Model to Simulate the Thermal Defocusing of a Laser Beam Focused in Water," Optical Engineering, vol. 35, No. 7, 1996, Ready, J.F., (ed.), LIA Handbook of Laser Materials Processing, (1 st ed.), Laser Institute of America, Magnolia Publishing, Inc., 2001, p Dushkina, N.M., Safety Concerns in Dicing of GaAs Wafers with Synova Laser MicroJet, to be published. 6. Clansky, K.B., Ed. Suspect Chemical Sourcebook: A Guide to Industrial Chemicals Covered Under Major Federal Regulatory and Advisory Programs. Roytech Publications, Inc. Burlingame, CA Update, p. xlvii; section 3, pp.86, The experiment was performed by M. Valant in the frame of a collaboration work with Prof. Radovan Kovacevic, Director of SMU Research Center for Advanced Manufacturing, Southern Methodist University, Richardson, Texas. Meet the Authors Natalia Dushkina is Ph.D. in Physics, author of more than 35 scientific papers and presentations at international conferences in the areas of optical properties of semiconductors, optical methods and laser applications. After five years research in Japan, Dr. Dushkina moved to Bowling Green State University, Ohio. She is the Manager of the Laboratory of Laser Applications at The Gem City Engineering Co., Dayton, OH, since August Bernold Richerzhagen (born 1964 in Cologne, Germany) received his MSc in mechanics from the Technical University of Aachen, Germany, and his PhD in micro-technology from the Swiss Institute of Technology, Lausanne, Switzerland. He is the inventor of the water jet guided laser technology. Since this invention in 1994, he has published a great number of articles on combining laser and water jet for which he has received several awards. He is the CEO of SYNOVA SA, Lausanne, an incorporated company manufacturing high precision laser machines, which he has founded in 1997.