Current Issue The International Reference for Chip-Scale Electronics, Flip-Chip Technology, Optoelectronic Interconnection and Wafer-Level Packaging July 2002 Email the editor

By Dr. Marc Klosner, Shyam Raghunandan, Chris Nunes, Marc Zemel and Dr. Kanti Jain, Anvik Corp., Hawthorne, N.Y.

To meet the patterning needs of wafer-level packaging (WLP), the IC industry has traditionally relied primarily on lithography methods adapted from other process steps. We are referring, of course, to the two dominant technologies that have sustained the microelectronics industry for decades-proximity printers and stepper systems.

Proximity printers (sometimes referred to as proximity aligners) are most commonly used today for low-resolution, large-area patterning needs, such as printed circuit board manufacture.

Step-and-repeat tools, alternatively, are used for high-resolution, small-area patterning applications, specifically for IC fabrication.

Technologies Evolving in Parallel

Both of these technologies evolved in parallel with the development of the semiconductor industry itself, and each has thus been adapted for optimum performance in its intended application.

Given the successes of proximity and stepper systems, in addition to their well-established infrastructure, it is no wonder that the industry has turned to them to meet the challenges of new applications such as WLP.

There is currently an ongoing evaluation within the IC community as to whether stepper or proximity printing technology is better suited for WLP applications.(1, 2) This evaluation is focused on the tradeoffs between the pros and cons for WLP.

Figure 1. The Anvik HexScan™ LPI system for large-area, high-resolution lithography on 12-inch wafers.

The tradeoffs occur because the lithography requirements for WLP are different from the applications for which steppers and proximity printers have been designed.

These requirements include large-area patterning, micron-level resolution, large depth-of-focus, and, of course, high throughput and yield. Traditionally, proximity printers have struggled to meet the resolution, depth-of-focus and yield requirements, while steppers are not capable of high-throughput large-area patterning.

In this article, we describe a new technology that has been developed to meet these specific requirements.

Called "HexScan," it employs seamless scanning laser projection imaging (LPI) to deliver high-resolution patterning over large areas, including 300 mm wafers, and eliminates many of the shortcomings of proximity printers and steppers.

Lithography Requirements for WLP

For WLP processes, as with any microelectronics fabrication application, the selection of a lithography technology begins with the resolution requirements.

The smallest geometries generally needed for WLP are the conductor lines in the redistribution layers-currently printed at 25 µm widths with 12 µm spaces in between. Industry roadmaps are calling for 10 µm lines and 5 µm spaces within the next two years.

Gold bumping, however, presents more demanding requirements, with resolution needs approaching 1 µm. Steppers can easily achieve these resolutions, but proximity printers will be challenged to meet these requirements with efficiency and reliability.

Additionally, given that within two years 300 mm wafers will become commonplace, high-resolution, large-area lithography is essential for the success of WLP. "Large-area" does not simply refer to the imaging of a repeating pattern over the entire surface of a 300 mm wafer-in which case a stepper would be well suited for the process.

Instead, the term refers to wafer-size patterns that must be imaged over the full wafer. Large-area includes, for example, edge-bead removal zones patterned around the periphery of the wafer and dice zones patterned between individual die.

These requirements are best met by using a large-area mask with an integrated layout that is fully imaged over the entire wafer. Large-area exposure units with the necessary resolution are clearly better suited for this application than steppers.

Figure 2. Anvik's high-resolution, high-throughput laser projection imaging technology, showing scanning with a single planar stage and hexagonal illumination beam.

Thick Resists

Another important requirement for WLP is the capability to pattern thick resists for wafer-bumping applications, which use resists as thick molds to create arrays of solder bumps. Each bump is typically over several hundred microns in diameter.

Resists over 100 µm thick are commonly used, and thus the exposure tool should have a depth of focus that is large enough (>several hundred microns) to maintain high-resolution images throughout the entire thickness of the resist.

This resist requirement could be a difficult challenge for proximity printers, which offer a limited depth of focus, and are sensitive to nonuniformities in the mask-to-wafer gap.

Other key factors that must be weighed in evaluating steppers and proximity printers include throughput and yield. Steppers offer higher yield but operate at lower throughputs compared with proximity printers, due to the overhead time required to step-to and align at each die site.

We also note that steppers offer very high precision alignment capabilities using through-the-lens alignment methods, an advantage that will become more important as WLP evolves towards finer geometries.

Table 1 summarizes the key lithography challenges of WLP, and compares the performance of proximity printers, steppers and large-area HexScan lithography for each.

A Laser Projection Imaging System

The HexScan combines a novel lithography technology with elegant opto-mechanical system engineering. The HexScan system, pictured in Figure 1, delivers:

• High-resolution projection imaging on 300 mm wafer feature sizes (lines, spaces, holes and vias) from tens of microns down to 1 micron

• Very precise alignment, layer-to-layer or mask-to-wafer alignment down to 0.3 micron

• High exposure speed-under 20 sec exposure speed for 300 mm wafers

These performance features are achieved with a unique hexagonal seamless scanning projection technique and a single-planar x-y stage configuration that provides superior optical and scanning efficiencies, and combine high-resolution, large-area imaging with high-precision alignment capability.

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