Higher throughput and yields from laser scanning projection in WLP
09/01/2005
To meet the patterning needs of wafer-level packaging (WLP), the IC industry has relied on familiar contact printers and stepper systems. There is, however, a new technology that promises to meet all of the lithography requirements for WLP. This technology employs 1× projection scanning, but uses a high-power 351nm xenon fluoride (XeF) laser and a single stage capable of seamless high-speed scans to print 300mm wafers with high throughput in wafer-bumping applications.
The smallest geometries needed for WLP are the conductor lines in the redistribution layers, which are currently printed at 25µm widths with 12-15µm spaces in between, but industry roadmaps call for 10µm lines and 5µm spaces within the next two years. The wafers for WLP will soon be 300mm, requiring special accommodations for the large area. Printing such a wafer for WLP requires imaging not only repeating die patterns, but also wafer-size patterns: for example, edge-bead removal zones patterned around the periphery and dicing zones patterned between individual chips. WLP employs thick resists for wafer-bumping applications - often more than 100µm thick - requiring large dose and exceptional depth of field. Other key tool factors include alignment and overlay precision as well as throughput, yield, and cost. The various technologies employed - contact aligners, steppers, and scanners - have different virtues for WLP.
Taking a different approach, the HexScan laser-projection imaging (LPI) system adapts a proven large-area lithography method for WLP applications [1]. Current tools resolve lines, spaces, and holes below 10μm with 1μm alignment and a very high exposure speed <20 sec for 12-in. (300mm) wafers. These performance features are achieved with a unique hexagonal seamless scanning projection technique that ensures uniform exposure, and with a single-planar x-y stage for mask and wafer [2-4]. Seamless high-speed scanning of 300mm wafers with 1× laser-projection exposure results in throughput up to 150 wafers/hr in wafer bumping applications.
The basic idea for LPI is illustrated in Fig. 1. The wafer and mask are rigidly mounted on a single-planar scanning stage that moves them synchronously in both x and y directions. The illumination system uses an excimer-laser light source and special beam processing. The laser emits several tens of watts of UV at 351nm, a wavelength ideally suited for imaging conventional photoresists designed for i-line. Specifically, the HexScan system uses a XeF excimer-laser light source that emits in excess of 45W, more than an order-of-magnitude greater than the useful UV radiation from a multikilowatt arc lamp. The laser enables high throughput, even with thick resists requiring very high exposure doses. Further, by emitting UV radiation only, the laser eliminates problems arising from the use of mercury arc lamps such as resist heating and illumination nonuniformity. The laser operation is fully integrated with the LPI system-control software and is run from the main system computer. The beam-processing system illuminates the mask from below with a uniform-intensity, hexagon-shaped region that is typically 50mm in size.
The mask pattern within the illuminated hexagonal region is imaged onto the wafer by a unit-magnification projection lens using a folded image path. The projection lens, the illumination system, and all other optical components are stationary. The sole moving component is the single-planar stage, which is scanned in a serpentine fashion in the x-y plane to achieve complementary overlap between adjacent scans of the hexagonal field. That overlap between adjacent scans results in a cumulative exposure dose from all scans that is totally seamless and uniform across the entire wafer.
By enabling seamless scanning with high-resolution projection imaging, LPI systems can deliver high throughput and yield. Projection imaging eliminates problems commonly associated with contact printers, namely defects and poor alignment precision. When combined with high-speed scanning, a high-power laser also results in higher throughput than conventional steppers.
 |
The table summarizes the key lithography challenges of WLP, and indicates the performance of contact printers, steppers, and laser-projection scanners for each of these challenges. A check indicates that the system is well suited to meet current and future challenges; an X, that the system technology is unlikely to meet the challenges; and X/√, that the technology can likely meet the challenge for certain operating regimes, but not for others.
For current WLP applications, the model 1100 SWE uses a projection lens with a 50mm field diameter and a very large depth-of-focus (DOF) of >500µm, which provides a minimum printable feature size of 10μm (0.4 mil). The system has been used to pattern 10µm features in 70µm-thick JSR resist. Other systems are available with higher- and lower-resolution specifications. Automatic alignment can be performed - either off-axis or through-the-lens - using mask and substrate cameras and various targets (holes, fiducial marks, etc.). Pattern-recognition software activates FiPAS (a fine positioning and alignment system), which corrects x, y, and θ to ±2.5μm (3σ).
 Figure 2. Results showing high aspect-ratio features. 24µm features are patterned in 35µm-thick SU-8 photoresist. |
Similar in design to the 1100 SWE, the new 2100 SPE system gives the same 10μm resolution in 13μm-thick AZ-Clariant 9260 as other Anvik systems designed for large panels [1]. Figure 2 shows 24µm (~1 mil) features imaged in 35µm-thick SU-8 photoresist used for advanced MEMS applications. With an exposure time of 12 sec for 12-in. wafers with common photoresists and an overhead time of 10 sec (for load, unload, and align), the 1100 SWE delivers a net imaging throughput of 165 300mm wafers/hr for any photoresist that develops at ≤150mJ/cm2. Throughput is then limited by the maximum stage scanning speed of 500mm/sec. Slower resists lower throughput, with a 400mJ/cm2 resist increasing wafer exposure time to 22 sec and lowering the net throughput to 114 wafers/hr.
While familiar contact aligners, steppers, and Hg lamp-based 1× projection scanners remain viable technologies for wafer-level packaging, the laser-projection imaging system described here promises to be an extendable, cost-effective solution, especially for high-volume WLP applications.
Acknowledgments
HexScan and FiPAS are trademarks of Anvik Corp.
References
- K. Jain, M. Zemel, M. Klosner, “Large-Area Excimer Laser Lithography and Photoablation Systems,” Microlithography World, Vol. 11, No. 3, pp. 8-10, August 2002.
- US Patent 5,285,236, issued Feb. 8, 1994; K. Jain, “Large-Area, High-Throughput, High-Resolution Projection Imaging Systems.”
- US Patent 5,652,645 issued July 29, 1997, “High-Throughput, High-Resolution, Projection Patterning System for Large, Flexible, Roll-Fed Electronic-Module Substrates.”
- K. Jain, et al., “Large-Area, High-Resolution Lithography and Photoablation Systems for Microelectronics and Optoelectronics Fabrication,” Proc. IEEE, Vol. 90, p. 1681, 2002.
Marc A. Klosner is a research staff member at Anvik Corp., 6 Skyline Dr., Hawthorne, NY 10532; e-mail [email protected].
Marc Zemel is director of engineering and product manager at Anvik.
Shyam Raghunandan is a project manager at Anvik.
Kanti Jain is the founder and president of Anvik.