Wafer Processing with Short-pulsed UV DPSS Lasers
Expanding New Applications Development
By Marco Mendes AND Jongkook Park, J. P. Sercel Associates
Ultraviolet (UV) diode-pumped solid-state (DPSS) lasers are robust, production-capable systems used in a wide range of applications in microfabrication, surface treatment, and materials processing. The potential of these lasers for semiconductor and other industries leads to the development of applications that often cannot be performed by other mechanical, chemical, or laser fabrication methods. DPSS lasers offer beam quality and a high repetition rate, providing a small beam size for micro features. Typically, the DPSS lasers used are vanadate (Nd:YVO4)-based, and produce an IR beam at approximately 1 μm. Efficient frequency conversion allows use of these lasers at 355 and 266 nm, with several watts of available power.
DPSS lasers are used widely in micromachining. Silicon, sapphire, CVD diamond, III-V semiconductors (gallium arsenide, indium phosphide, gallium phosphide), and III-nitrides, such as gallium nitride and aluminum nitride, are routinely machined by DPSS lasers in structuring, via drilling, dicing, and cutting applications (Figure 1). These lasers are also used for micromachining other ceramics, polymers, and metals.
Figure 1. 4” sapphire wafer scribing with a UV DPSS laser system.
Recently, frequency-multiplied DPSS lasers demonstrate fairly high pulse energies, even at UV wavelengths and high kHz repetition rates. A focused-beam spot with short pulse duration creates extremely high irradiance, resulting in instantaneous vaporization of materials during wafer scribing. In conventional laser scribing, a simple far-field imaging technique is used, where the laser beam is sharply focused onto a small spot, and then delivered to a target wafer. However, the focused beam spot in the conventional, far-field imaging does not have sufficient flexibility to adjust for optimum intensity, which is determined by the light absorption properties of a particular target.
Optimum laser intensity is important to achieve the desired processing result, because either excessive or insufficient laser intensity will introduce imperfections into the laser scribing process. The achievable minimum spot size of the focused beam, which is directly related to resolution of the scribing process, is limited in conventional laser scribing. Consequently, a method that avoids the drawbacks of existing techniques is needed. Adequate beam shaping/delivery optics have recently been developed to adjust the laser intensity while minimizing the beam waste. Properly optimized laser intensity with a highly resolved beam spot increases semiconductor wafer scribing speed, while minimizing heating and collateral material damage.
Recent improvements in short-pulse-width, short-wavelength, DPSS lasers have resulted in robust, production-capable systems that offer a wide range of flexibility for modifying pulse shape, repetition rate, and beam quality. Harmonic generation allows the user to adequately choose the most appropriate wavelength for a wide range of materials-processing applications.
DPSS Laser Applications
The development of high-repetition-rate UV DPSS lasers makes them particularly suitable for cutting, marking, scribing, and via-drilling applications. For a number of applications, UV DPSS lasers are used at the focal point of the imaging lens on a direct writing technique. The use of CAD-conversion software minimizes setup time for new parts, and allows efficient, high-quality machining. Sapphire, quartz, CVD diamond, and glass are materials cleanly cut up with high-quality, narrow kerfs up to 500-µm thick, using special processing techniques.
Via drilling with the DPSS laser is performed using different techniques, the choice of which depends on via diameter and required accuracy. Percussion drilling is used to create small-diameter vias (3 to 10 μm), while vias larger than the beam diameter can be machined using trepanning/helical drilling. In this technique, the spot beam is moved one or several times across a defined path until breakthrough is achieved. One method of DPSS application involves machining of small holes in fused-silica films, with diameters varying from 10 μm to several millimeters within the same wafer. For machining blind vias with smooth, flat-bottomed surfaces, or vias with minimal taper, excimer lasers are typically used, rather than the quasi-Gaussian beam profile typical of DPSS lasers, because a constant density distribution on target is required.
Figure 2. 2.5-μm kerf in GaN on sapphire wafer. Narrower kerfs result in higher yields in die count per wafer.
Laser scribing is currently an area receiving special focus. These lasers are routinely used to scribe blue LED and sapphire wafers at speeds of 50 to 75 mm/sec, leading to a throughput of more than 10 wph (for standard 2" diameter wafer with die size of 350 × 350 μm) while creating a kerf as narrow as 2.5 μm (Figure 2). With its high throughput and minimal impact on LED performance, the process is tolerant of wafer warp and bow, and delivers much faster scribing speed than traditional mechanical methods. For III-V semiconductors, such as GaAs and InP, a cut depth of 40 μm is typically achieved at speeds of up to 150 mm/sec. Very high scribing speeds with minimal affectation are also achieved for some metals, silicon, and other ceramics.
LED Wafer Scribing
Using short-wavelength (157 to 248 nm) excimer and UV DPSS lasers increases die yields in LED wafer scribing without appreciable loss of brightness, and also prove superior to traditional diamond-scribing methods in terms of throughput and long-term product reliability. These improvements are due to the nature of UV lasers, and their method of application to the scribe-and-break process (front- vs. back-side processing).
UV laser processing allows a much narrower scribe-kerf width than other technologies. Coupled with front-side processing, this increases the number of usable die that can be produced per wafer. One example involves a 2" blue LED wafer on sapphire, with 250 × 250 µm devices. Using traditional diamond scribing with typical 50-µm streets (300-µm die pitch), there will be ~24,400 die on the wafer; 22,200 usable die at a 90% die yield. Using UV laser scribing, the street width can be reduced to 20 µm (a 270-µm die pitch). At this die pitch, the wafer contains ~27,750 die (a 14% increase due to narrower streets), and will produce 27,475 usable die at a 99% yield - a 24% total increase in usable die.
GaAs Wafer Scribing
Scribing GaAs wafers with UV DPSS lasers is an alternative method of scribing for separation for brittle-compound, semiconductor wafer materials (Figure 3). These lasers rapidly process wafers with kerfs to <3 µm in thin or thick wafers without edge-chipping in all the III-V materials, including “IV” materials such as Si and Ge, and straight, accurate, cleaner cuts - particularly for GaAs wafers. GaAs wafers are expensive, so wafer real estate is valuable. The tighter, narrower, and cleaner cuts achievable with UV laser scribing provide better die-count-per-wafer and higher yields due to fewer damaged die. The laser scribing process operates within 20-µm streets or narrower.
Figure 3. GaAs wafer, diced and expanded. Wafer dicing is well-suited for UV DPSS lasers with their highly focusable, pinpoint-bright beams.
With the scribe-and-break process, PCM structures must be designed with through-dicing lanes. The diamond scribe does not scribe continuously through the structure, so the structure must be designed with dicing lanes, which creates issues for testing. With laser scribing, PCM design is no longer an issue. The structures can be designed in a way that benefits the tests being completed, rather than the requirements of the separation method. Laser scribing is not interrupted when there is no dicing lane.
With the traditional methods of separation, the dicing lanes must be free of thin films and metal because it increases the wear on the blade during the saw process, reducing the life of the blade or causing it to “blow” during the cutting process. With scribe-and-break, thin films and metal in the dicing lanes makes the diamond tool skip and bounce, creating areas that are not scribed and do not break; causing the rest of the wafer to break off the scribe lines. Thin films and metal in the dicing lanes do not limit laser scribing, allowing for higher yield, lowering the number of reworks during in-line production, and eliminating wear to the laser. The ability to dice through thin films increases throughput for the photo processes. Currently, all wafers must be patterned to the edge of the wafer - including partial fields - so the street can be etched clean, creating a clean dicing lane to the edge of the wafer. When the wafers are not patterned to the edge, the yield takes a big hit.
Traditional methods of die separation require lots of time. A 4" diameter wafer with a die-size of 0.300 × 0.360 mm has ~55,000 die. A wafer with this die count takes ~4 hours with the saw (saw speed = 6.5 mm/sec); ~2 hours with scribe and break (scribe speed = 12.8 mm/sec); and ~3 minutes with laser scribing (laser scribe speed = 150 mm/sec).
Laser scribing gives the added benefit of options when determining shipping methods. The scribe-and-break process requires that wafers be scribed on a film frame and transferred to a grip ring for shipping. Scribed wafers must be stretched to prevent the die from “rubbing” together when the tape flexes. The laser scribe gives the option of shipping wafers on a film frame. The die are separated through the active levels by the laser scribe, and will only “rub” together in non-active material.
Current die separation methods lose yield due to chip-outs. Chip-outs are eliminated with laser scribing, and have a large impact on product yield. Laser scribing will increase wafer throughput on final automated wafer inspection. Currently, wafers must be stretched to prevent chip-outs caused by the die “rubbing” together. The die do not stretch uniformly, causing inspection time to increase. Each die must be aligned for the automatic inspection to be performed correctly. Occasionally, the yield is impacted because the die will not align. Laser scribing allows for the wafers to be inspected on a film frame, which greatly reduces inspection time, and allows all products to go through the auto inspection process.
The demand for microfabrication, surface treatment, and materials processing in the semiconductor industry is increasing. Short-pulsed UV DPSS lasers also continue to increase their average power, providing rugged industrial packages. Currently, a major effort is underway to create reliable ultra-short (pico- and femto-second) pulsed laser systems for industrial use. The ongoing development of laser systems, new machining techniques, improved beam delivery optical systems, and enhanced knowledge of laser/material interactions will continue to advance the development of new applications in the future.
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MARCO MENDES, Ph.D., and JONGKOOK PARK, Ph.D., laser applications engineers, may be contacted at J. P. Sercel Associates, 17D Clinton Dr., Hollis, NH 03049; 613/-595-7048; E-mail:firstname.lastname@example.org and email@example.com