UV Laser Processing for Semiconductor Devices

Transcription

1 UV Laser Processing for Semiconductor Devices Highly flexible laser-assisted fabrication for gallium nitride based devices Olaf Krüger and Richard Grundmüller The application of reliable laser sources is well-established in several fields of industry including automotive, electronics, and medical manufacturing on macro, micro, and even nanometer scales [1]. In modern semiconductor technology, silicon is the dominating material. Further materials like gallium nitride (GaN) and silicon carbide (SiC) allow for higher operating frequencies and power levels, i. e., high-speed electronics and high-power applications. However, their chemical and physical properties do not only allow extending the limits of silicon-based device performance, but also call for alternative methods of processing. At Ferdinand-Braun-Institut, Leibniz- Institut für Höchstfrequenztechnik (FBH), high-performance devices using GaN-based epitaxial layers are fabricated on SiC and sapphire substrates. Both substrates are very hard and chemically stable, and are thus difficult to pattern by means of classical mechanical or chemical techniques. Hence, laser-assisted technologies to fabricate vias (vertical interconnect access) through SiC as well as laserassisted die separation of GaN-based lasers and light emitting diodes (LEDs) on sapphire substrate have been developed at FBH and are used in device fabrication. Versatile semiconductor device micromachining To meet the special needs of semiconductor device fabrication, a turn-key industrial grade laser workstation (ILS 500 Air, InnoLas GmbH) was customized and placed in an industry-compatible clean room with a two to four inch a Fig. 1 Scanning electron micrographs of KrF laser-produced surfaces in photoresist AZ4562 (Clariant) on silicon using orthogonal mask-dragging techniques: (a) blazed grating (top) and pyramid-like structures (bottom), (b) pyramid-like structures. (Source: FBH) process line for III-V semiconductor devices. Due to its technical maturity and high reliability, a frequency-tripled, diode-pumped solid-state (DPSS) laser was chosen for the laser workstation. The UV laser emitting at 355 nm provides an average power of 4.5 W and a pulse length of < 30 ns at pulse repetition frequencies of up to 100 khz, allowing a high processing speed. Previous tests had proven that such laser type ensures reliable operation as well as high-quality and precise micromachining of SiC for the given application. This laser was integrated into a class-1 lasersafe micromachining workstation. The laser beam travels through a beam attenuator, an aperture module, a beam expansion telescope, a λ/4 plate, and a galvanometer scanner and focusing head. The galvanometer scanner is used to move the laser beam in the XY plane across the workpiece s surface. The beam is focused to a spot size of ~15 µm, using a telecentric F-theta objective with a focal length of 56 mm. A second type of laser extends the basic set-up of the laser workstation, an excimer laser delivering nanosecond pulses with a maximum pulse repetition frequency of 200 Hz. Depending on the excimer gas in use, argon fluoride (ArF) b or krypton fluoride (KrF), the wavelength is 193 nm or 248 nm, respectively. These two additional excimer wavelengths in the UV range significantly extend the variability and potential applicability of the laser workstation. In addition, flexible UV illumination optics (Optec s. a., Belgium) allow for high demagnification illumination (15 ) through a pinhole mask for focal point application as well as for homogenized low demagnification illumination (5 ) for mask projection techniques. The mask stage holds 5" masks (typically chromium on fused silica) and the associated optical system is best suited for direct processing of polymers such as photoresists. The micromachining tool allows us to synchronously move wafer and mask stages and thereby to fabricate blazed gratings and pyramid-like structures on polymer surfaces as depicted in Fig. 1 for photoresist AZ4562 (Clariant). Such 3D structures can be transferred into semiconductor surfaces by either wet or dry chemical etching, e. g. for micro-optical applications such as antireflection layers and beam shaping elements for laser diodes. The laser workstation is equipped with an air-bearing XY stage enabling a positioning accuracy of better than 1 µm. Four cameras and a pattern rec- 26 Laser Technik Journal 5/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Micro Material Processing ognition system allow for automated beam alignment. Two of the cameras are placed above the stage level and allow for front-to-front alignment, i. e. the alignment marks are located on the surface to be laser processed. In case the wafer should be processed on the opposite side (i. e., on the wafer s backside), the sample will be flipped on an openframe vacuum chuck, i. e. marks on the wafer s front will be face down and can be recognized by the other pair of cameras located below the wafer stage. Process flow ensures compatibility a Fig. 2 (a) Optical micrograph of a sapphire target hit by a single laser shot. The diameter of the gold aiming point is 20 µm. The distance between two circles is 10 µm. The sample was laser processed on the back using front-to-backside alignment. The deviation on the target structure shown is X-0.4 µm and Y+0.4 µm (center-to-center). (b) Optical micrograph of a transistor detail with a 7.5 µm wide laser scribe isolating a gold drain connection between two adjacent transistor devices. (Source: FBH) To be compatible with standard semiconductor device fabrication that typically defines fabrication steps and modules as process layers, the laser micro processing step was introduced and specified as a process layer in the device layout. To start with, CAD layout data of the devices are imported into a CAD software package preparing the process code for the laser micromachining tool. With this program called Graffy (Durst CAD Consulting GmbH) various data formats including GDSII and DXF can be imported. The software package allows quick modifications and adjustments in the layout as well as the attachment of technology parameters for laser treatment. All information about positions and shapes of alignment marks and about the laser pattern to be created (e. g. vias and scribing lines) are taken from the CAD layout. Data are then fed into the Graffy postprocessor that automatically generates the program code for laser machining. The CAD system offers high flexibility for pattern generation, which is particularly interesting for rapid prototyping. To precisely position the laser spot on the surface with respect to existing structures on the wafer, a vision system including automated pattern recognition is used to measure the actual wafer position on the stage. In a first step, shift and rotation of the wafer is determined by means of two (pre-) alignment marks on the wafer. In a second step, the fine alignment is performed using proper marks or patterns in or near the field to be processed. Controlling the movement of the stage and of the laser spot by the galvanometer scanner allows for step-and-repeat as well as write-on-thefly processing modes. Such modes are also realized in state-of-the-art lithography tools for semiconductor device fabrication including electron beam lithography systems and optical projection lithography tools (wafer steppers and wafer scanners). The alignment precision of the center of the laser spot with respect to existing structures is better than ± 1 µm, i. e. less than 10 % of the DPSS laser beam diameter. This accuracy can also be achieved when the alignment marks are located on the reverse side of the wafer, i.e. on the side opposite to the one processed by the laser (front-to-backside alignment). The positioning accuracy can be verified using laser target structures patterned on silicon or sapphire by means of standard contact lithography. Different target structures (mesh size and target dimensions) were realized by 2 µm wide gold lines. Nonius-like structures were also processed to allow direct measurement of the overlay precision of scribe lines or indentations created by the laser using a plain optical microscope. A typical result after laser processing is shown in Fig. 2a for backside processing. A single laser shot was deposited onto the target pattern on the wafer s backside. Target structures allow to easily and quickly estimate the positioning accuracy by optical microscopy. The example shown in Fig. 2a demonstrates a beam positioning accuracy of better than ± 1 µm for front-to-backside alignment. In the case shown here, the DPSS laser was positioned by moving the XY stage. The positioning accuracy is nearly the same when using the highspeed galvanometer beam positioner over a mm² target field. Fig. 2b shows another example for the high b beam positioning accuracy on transistor scale. Here, a 7.5 µm wide laser scribe was placed in between two gold air-bridges to isolate a drain connection between adjacent transistors. Laser-assisted via processing High-power AlGaN/GaN high electron mobility transistors (HEMTs) for RF applications are processed on semi-insulating SiC substrates that are typically ~400 µm thick. To boost the transistor performance and to simplify their assembly and chip mounting, a vertical interconnect access with low inductivity between the source contact on the front and the ground electrode on the backside is required. Due to its hardness and chemical inertness patterning of SiC is laborious and time-consuming. Currently, only advanced inductively coupled plasma (ICP) etching can provide significant etching rates of SiC of ~1 µm/min and etching of holes through 400 µm thick SiC is impossible. Laser radiation, however, offers a Fig. 3 Cross-section optical micrograph of a through-wafer via drilled by a DPSS UV laser. The laser hits from the top (backside of the wafer), the laser exit is on the bottom (wafer s front with source pad of transistor).the via s sidewall is completely covered by a metal layer, approx. 5 µm thick. (Source: FBH) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 5/

3 Fig. 4 Scanning electron micrograph of a mounted AlGaN/GaN on SiC transistor device with laser vias through the source contact pads ( µm L-band power bar; Source: FBH) Fig. 5 Scanning electron micrographs of cross-sections of a blind via hole after KrF laser drilling terminating about 10 µm above the SiC/GaN interface (left) and after subsequent ICP etching of the SiC (middle). The plasma etching stops at the GaN epitaxial layer. Optical micrograph of a cross-section of a blind via stopping at the source pad of a transistor device (at the bottom, right).the via s sidewall is completely covered by a metal layer, approx. 5 µm thick. (Source: FBH) Institute unique one-step alternative to plasma and wet-chemical etching of highly resistant materials, which makes lithographical formation of etching masks obsolete. At FBH, a laser-assisted technology for fabrication of two types of vias in SiC, i. e. through-wafer vias and blind vias, has been developed. Laser drilling enables throughwafer vias in 400 µm thick SiC Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH) Berlin, Germany The FBH explores cutting-edge technologies for innovative applications in microwaves and optoelectronics. The institute is a competence center for III-V compound semiconductors with strong international reputation providing the full range of capabilities, from design to fabrication and device characterization. For customers in industry and research, FBH develops high-value products and services, including high-frequency devices and circuits for communication and sensor technology as well as high-power diode lasers for materials processing, medical technology, and high-precision metrology. The DPSS laser beam providing a Gaussian-like intensity profile is used to directly drill one hole at a time. A typical pulse energy of 65 µj/pulse at a laser pulse repetition rate of 20 khz is used to create through-wafer micro holes in 400 µm thick SiC. The laser beam is moved on the sample to obtain conical holes with a mean diameter of µm. Without optimizing the drilling process for high throughput it takes about 3 s to drill a 50 µm hole through 400 µm thick SiC. The holes are drilled from the wafer s backside through the transistor s source contact on the front. The drilling position on the backside is precisely controlled using marks on the front and the front-to-backside alignment feature of the tool. After laser processing, the wafer is cleaned in buffered solution of hydrofluoric acid to remove debris [2]. The sidewalls are covered by redeposited material (nanocrystalline silicon, resolidified SiC) that forms a smooth layer with a thickness between 0.5 µm and 4 µm at the laser exit and entrance, respectively. Beneath the deposits, no evidence for laser-induced micro cracks, extended defects, and crystal damage was found [3, 4]. After laser processing, the sidewalls of the holes are metallized by depositing a thin conducting seed layer, followed by electroplating 5 µm of gold. Fig. 3 shows the cross-section of a plated-through via exhibiting a completely closed gold layer. As confirmed by resistance measurements, a vertical electrical interconnect through the 400 µm thick SiC is established. Depending on the via diameter, the through via resistance is mω. Fig. 4 shows a mounted transistor device consisting of five cells with eight gate fingers each (gate width 500 µm). The source pads of this transistor are electrically connected to the back; the holes drilled through by the laser are clearly visible. Electrical measurements of the power performance of such AlGaN/GaN HEMT powerbars at 2 GHz confirmed full functionality [5]. Transfer, output and load-pull characteristics of the devices before and after laser drilling revealed that the transistor s DC and RF performances remain unchanged for gate-via distances 1 µm [6]. Laser-assisted processing of blind vias in SiC An alternative laser-assisted technology has been developed to fabricate blind via holes for AlGaN/GaN HEMTs on SiC [2]. In this case, the wafers are thinned to a thickness of 100 µm and covered by a protecting layer of indium tin oxide (ITO) on their backside. For laser drilling, the ArF excimer laser is used providing a shaped beam that is aligned and focused at the backside to obtain the desired position and via cross section on the sample. By percussion drilling, i. e., without moving target or beam, the hole is formed until the bottom of the hole is close to the interface SiC/GaN (Fig. 5 left). Using a laser fluence of about 20 J/cm² per pulse and a pulse repetition frequency of 50 Hz, laser shots are required to achieve a hole depth of µm, which results in a drilling time of about 2 3 s per hole. In the subsequent process step, the thin remaining SiC is dry-etched using an inductively coupled plasma (ICP) without an additional etching mask, i. e. no lithographic fabrication of a mask is needed. The selectivity of the ICP etching of SiC versus GaN is > 100 : 1, accordingly the etching process will prac- 28 Laser Technik Journal 5/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Micro Material Processing High power UV Laser Sapphire Scribed trenches (lnal)gan layer tically come to a halt at the GaN layer (Fig. 5 middle). A second plasma etching step removes the GaN epitaxial layer by reactive ion etching (RIE), allowing for a clean stop at the metal layer of the front contact. After wet-chemical removal of the protecting ITO layer, a 5 µm thick gold layer is deposited on the wafer s backside and the side walls of the holes (Fig. 5 right), thereby forming an electrical connection between the contact pads on the front and the metal layer on the backside of the device. The electrical resistance of a single via depends on its diameter (cross-section), on the thickness of the gold layer at the side wall as well as on the thickness of the sample (length of the via). Values in the range of mω were measured. In contrast to the through-wafer via process described above, the blind via process does not pierce the source contact metal at the wafer s front, which can be advantageous for further processing (e. g., resist coating) and soldering. Separated wafer pieces Carrier tape Scribed trenches Blade Fig. 6 Schematic of the laser scribing and breaking technology for die separation of GaN-based LEDs and laser devices (not to scale; Source: FBH) Laser scribing for smooth die separation A critical step in processing high-efficiency UV as well as blue LEDs and lasers on sapphire substrates is die separation. Mechanical dicing can produce chipping, cracking, and delamination along the scribe cuts. Group III-nitride materials crystallize in a (hexagonal) wurtzite structure, making it particularly challenging to fabricate rectangular dies by scribing and breaking. Although sapphire exhibits a hexagonal crystal too, the unit cells of a c-plane sapphire substrate and the GaN-based heterostructures are rotated by 30 with respect to each other. This prevents a common cleavage plane of substrate and epitaxial layers. Laser-assisted techniques, such as laser cutting and scribing, are attractive methods to address this issue. Consequently, laser scribing of sapphire wafers has been subject of numerous studies investigating laser systems featuring various wavelengths from the deep-uv to the UV range, with various pulse durations, beam shaping, and power levels. In our case, the nanosecond-pulsed DPSS laser beam is used to scribe the material, followed by breaking on a commercial tool (Fig. 6). Laser processing parameters have to be optimized to avoid damage to the epitaxial layers and to maximize yield and throughput. Due to the large band gap of sapphire of 9 10 ev, the sapphire substrate is optically transparent in the wavelength range from ~200 nm to ~5500 nm. Therefore, absorption at 355 nm (~4 ev) as provided by the frequency-tripled DPSS laser is rather low. However, at sufficiently high energy density in the laser focus, radiation can be absorbed by multi-photon absorption processes and ablation of sapphire occurs. Accordingly, nanosecond-pulsed laser processing of sapphire at 355 nm is rather inefficient and the quality of the process is limited. However, at FBH a laser scribing technology has been successfully developed that allows subsequent cleaving of InAlGaNbased laser [7] and LED chips. Using the automated alignment of the micromachining tool, the laser beam is precisely guided on the backside of the sapphire substrate. The scribe has to be carefully aligned to the crystal plane of the sapphire (a-plane) to avoid zigzag breaking of the epitaxial layers and to obtain a minimum density of terraces and particles on the cleavage plane. The scanning speed of the laser beam, the number of passes, and the position of the beam focus in the material Company InnoLas Systems GmbH Krailling, Germany Fig. 7 GaN-based LED device on sapphire substrate (L W H = 1000 µm 600 µm 430 µm) mounted with its epitaxial side down on an aluminium nitride submount and TO socket, i. e. the sapphire side is facing upwards. The die was separated by laser scribing and subsequent breaking. (Source: FBH, Schurian) InnoLas Systems GmbH produces laser systems for micro material processing, particularly in the photovoltaic industry, for semiconductors, electronics and precision engineering. InnoLas Systems designs and manufactures machinery solutions as stand-alone systems or for inline integration, perfectly adapted to industrial production conditions. Worldwide service and support ensures smooth setup and reliable operation. InnoLas Systems utilizes innovations in laser technology to produce highly-efficient and reliable processing systems WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 5/

5 are adjusted to avoid excessive mechanical and temperature stress on the wafer. If the scribe depth is too small, the sapphire does not break exactly along the scribe and terraces with high density are formed. If the groove is too deep, UV radiation of the laser penetrates the sapphire and is absorbed by the GaNbased epitaxial layer, which damages or degrades the functional layer. The opening of the grooves is about 20 µm wide. The scribe depth is adjusted to the thickness of the wafer. A scribe depth of about 390 µm is suitable for breaking 440 µm thick sapphire substrates into dies with a size of 850 µm 700 µm using a commercial scriber/breaker tool (Dynatex). Fig. 7 shows a mounted LED chip singularized by laser scribing and subsequent breaking. Smaller dies were obtained from thinner wafers. As previously published, the presented technology has also been applied to produce smooth facets for high-performance laser diodes both on un-thinned sapphire and on bulk GaN substrates [7]. State-of-the-art power levels and threshold current densities have been achieved. Summary The feasibility of laser-assisted via fabrication for AlGaN/GaN HEMTs on SiC as well as of laser scribing for die separation of GaN-based LEDs and lasers on sapphire has been proven on device level. A complete process flow from CAD wafer layout to laser processing has been established. For reliable laser processing a high beam positioning accuracy of ± 1 µm with respect to the device pattern on the wafer was verified by optical microscopy using test target structures. The direct laser drilling and scribing approaches offer highly flexible ways to create patterns, e.g., for rapid prototyping. It has been demonstrated that direct laser micro processing is well compatible with semiconductor device fabrication. Thus, laser micro ablation turned out to be a promising alternative to the combination of conventional lithographic, plasma, and micro mechanical processing of hard and inert materials. DOI: /latj [1] M. C. Gower: Opt. Express 7 (2000) 2, 56 [2] O. Krüger et al.: UV laser drilling of SiC for semiconductor device fabrication, J. Phys.: Conf. Ser. 59 (2007) [3] T. Wernicke et al.: Appl. Surf. Sci. 253 (2007) 8008 [4] O. Krüger et al.: Appl. Phys. A 93 (2008) 1, 85 [5] O. Krüger et al.: IEEE Electron Dev. Lett. 27 (2006) 6, 425 [6] T. Wernicke et al.: Abstract Book WOCS- DICE th Workshop on Compound Semiconductor Devices and Integrated Circuits. J. Stake, Ed., Göteborg, Sweden: Chalmers University of Technology (2006) [7] J. R. van Look et al.: IEEE Photon. Technol. Lett. 22 (2010) 6, 416 Authors Olaf Krüger received his Ph. D. in chemistry from the Humboldt-Universität Berlin (HUB), Germany, in Afterwards he stayed as a scientist at the HUB. From 1995 to 1997 he was working at the California Institute of Technology, USA. In 1997 he joined the Leibniz-Institut für innovative Mikroelektronik (IHP), Frankfurt/Oder, Germany. Since 2001 he has been with the Berlin-based Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH), where he is presently Head of the Process Technology Department. His current research interests include laser micromachining for semiconductor device fabrication and novel techniques for device assembly. Richard Grundmüller received his diploma degree (Dipl.-Ing. FH) in mechanical engineering from Fachhochschule München in In 1995 he started his own business as founder and CEO of InnoLas GmbH. Currently he is CEO/President of InnoLas Systems GmbH. Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4, Berlin, Germany, Tel.: +49 (30) , Fax +49 (30) , olaf.krueger@fbh-berlin.de InnoLas Systems GmbH, Robert-Stirling-Ring 2, Krailling, Germany, Tel.: +49 (89) , Fax +49 (89) ; richard.grundmueller@innolas.com 30 Laser Technik Journal 5/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim