Over the years, silicon wafers have been utilized as a substrate material in circuit production, but there has been a constant need to singulate these products into the device die. This is usually performed using mechanical methods such as a diamond blade dicing saw, but such methods are not feasible as industries turn toward thinner wafers and demand narrow dicing streets.
Since thin wafers are more brittle, it is important to reduce the feed rate so that cracking and chipping can be prevented; narrower streets cannot be processed with the extent of lateral wafer chipping and saw blade width limitation. Moreover, the advent of low-k dielectric materials presented further problems for saw dicing. As a result, companies are searching for suitable alternatives such as laser dicing to reduce expensive consumables.
In the past few years, laser dicing of silicon wafers has become the latest trend. Laser processing of silicon has been tested with a variety of laser systems having different wavelengths and power levels. Usually, longer wavelengths and longer pulse widths will increase the cutting speed on account of increased distribution of energy into the silicon.
On the other hand, these same qualities also result in a process that is sensitive to thermal affects such as cracking, melting, residual stress buildup, and amorphization. In case of silicon wafer die singulation, such thermal affects can reduce die strength and damage sensitive electronics on the die.
New Laser Techniques
A wide range of new laser processes have been recently developed for wafer dicing to reduce kerf width, increase speed, and maintain die strength. Hamamatsu’s stealth dicing technique showed huge potential in dicing application. In this technique, a partially transparent laser beam is focused beneath the surface of the wafer to introduce a controlled fracture of the material in the proposed orientation.
Strength testing of the diced samples proved effective for the laser-induced stress process. In theory, techniques based on laser-induced thermal shock are suitable for uncoated silicon wafers, but in practice the process is restricted by dielectric or metal film stacks on top of the wafer. This would make it difficult for the light to enter deep into the silicon.
In other methods, water is integrated into the laser process to promote better results. Water-jet guided laser process is a popular technique that enables reduced contamination, better speed, and the ability to cut though different materials. However, none of these methods have been widely implemented on the manufacturing floor.
In case of laser ablation wafer dicing processes, optimal fluence concept can be better explained using the diagram in Figure 1, which is based on the theory of exponential attenuation of energy distribution into the material.
Figure 1. Theoretical plot of depth vs. fluence illustrating the inefficiency expected with excessive fluence levels.
In Figure 1, a fluence threshold of 1J/cm2 can be seen. This is the point where minimal material removal initiates. When the fluence is increased to 5J/cm2, meaningful material removal is realized, with an ablation depth of approximately 1.6µm. If the fluence is increased to 15J/cm2, a corresponding depth of ~2.7µm can be observed, which is merely 1.7 times the depth at 5J/cm2.
Rather than increasing the fluence to a high value, if the 15J/cm2 beam has been divided into three individual 5J/cm2 pulses, one could have ablated to the depth (3 x 1.6µm = 4.8µm) and thus acquired 70% better efficiency.
Elliptical Beam Processing
One US patent described the advantages of shaping a beam into an ellipse and slicing along the axis of the beam spot. In one example, inventers demonstrated a 20X speed increase for slicing through a ~600-µm thick polymer or metal film stack when shifting from a round- to an elliptical-beam process.
A longer pulse width will enable heat or energy to enter deep into the material during the duration of irradiation, and will remove more material per-pulse when compared to shorter pulse widths. The amount of heating and thus the level of material removal increases with the square-root of the pulse duration.
In general, shorter pulse widths relate to lower material ablation thresholds, which can have major implications. Study has shown that shorter pulses can result in quality benefits and reduce heat affected zone (HAZ) produced in silicon.
The experiment is performed to characterize the relationship between beam ellipticity and silicon cutting speed for a short pulse width diode pumped solid state (DPSS) q-switched 355nm laser system. For all the tests, double-side polished single-crystal silicon wafers of 2” diameter and ~100-µm thickness were used.
The aim of study is to define the processing benefits for a silicon cutting process by expanding the major-axis of an elliptical Gaussian beam spot and maintaining the minor-axis beam diameter at a fixed value.
Pulseo® 355-20 high-power DPSS q- switched laser system from Spectra-Physics was utilized for all the results presented here. Optimized for 100kHz performance, the laser produces greater than 20W for 200uJ pulse energy output with short, sub-23ns pulse durations.
The optical set-up includes a scanning galvanometer system, Scanlab Hurryscan II 10, coupled with a 100mm focal length telecentric f-theta lens. The scan head is fitted into a precision linear motor XYZ positioning system for complete flexibility with respect to beam focusing and parts positioning capabilities.
The Pulseo® laser beam was then stretched by a factor of 2.5 so as to fill the 10mm scan head aperture. To produce a line focus, a couple of cylindrical lenses of +/- 100mm focal length were introduced into the beam path before the galvo scanner aperture.
Preliminary tests were carried out in which the number of overlapping scans and beam scanning speed were changed to establish the best combination for clean, efficient slicing of the silicon. The accuracy is adequate for overlapping the scans on top of one another on the material. With higher-fluence processing, it is beneficial to use such a high-speed and multi-pass process. This can result in improved quality cuts as there is less cracking, heating or cracking when compared to slower speeds and higher processing speeds.
Using full power output of the laser beam at a pulse repetition frequency of 100kHz, the galvo scanner was utilized to scan the focused beam across a 100-µm thick single crystal <100> silicon wafer. For a specified scan routine, both the scanning speed as well as the number of scans in certain cases was changed.
After completing the scan routine, the spacing of the cylindrical lens was increased to generate a longer major axis and the same minor-axis beam width of ~9µm was maintained. Then, the scan routine was again performed with the modified line-focus beam. The entire processing was carried out in normal air- atmosphere ambient environment.
The samples were examined once adequate parameter space was explored. Inspection included optical surface microscopy utilizing transmitted and reflected to check for cut-through and kerf width of the material. Through this approach, it was easy to ascertain the range of parameters for which silicon of 100-µm thickness can be completely cut. However, the samples have to be cleaved for cross-sectional analysis to obtain depth information for less than 100-µm deep laser scribes.
The samples were cleaved along the laser scribe, exposing the laser- scribed area for measurement with the optical microscope. This way, an entire set of laser cutting depth data was obtained for a range of beam ellipticities and speeds. Further information relating to the sidewall surface roughness was obtained with a scanning white light optical profiler system, Zygo NewView 7200.
The preliminary experimental results were quite significant. While optimizing the beam’s ellipticity, laser scribes with different scan speeds along with number of scans were produced. Although high fluence processing needs high-speed, multi-pass processing for optimum results, it was observed that following certain amount of beam elongation, this requirement no longer exists. In other words, a single beam scan at a specific speed acquires similar or same effective and quality cutting speed as multiple scans at a higher speed.
More rigorous test results included a range of scribe depth measurements for 11 ellipticities and 8 scan speeds. This data is shown in Figure 2.
Figure 2. Scribe depth data plot showing that for a particular scribe speed, there is an optimal ellipticity for achieving a maximum cut depth in silicon.
In case of scan speed of 300mm/s, ellipticity values between 20 and 35 results in cutting via the wafer. For speeds more than 300mm/s, increased beam ellipticity leads to deeper scribes; however, beyond a specific point, this trend reverses and beam stretching results in reduced depth. To describe this phenomenon, a range of optical microscope images of the 600mm/s data set is shown in Figure 3.
Figure 3. Cross-sectional and top-down views through optical microscope of laser scribes at 600 mm/s and ellipticity values from 19 to 77.
In Figure 3, the microscope images clearly show how the scribe depth varies with increasing ellipticity. The highest cutting depth for a specific speed and the average fluence at which this depth is obtained are shown in Figure 4.
Figure 4. Optimized cutting depth, and fluence at which this depth is achieved, versus scan speed.
Surface Finish Quality
The sidewalls of certain scribes produced during the tests were examined and analyzed using optical profilometry. Given that minimal chipping and smoother sidewalls result in better die strength, this is an essential feature of any dicing process. Figure 5 displays a 3D optical profile image and roughness measurement data created for a ~97-µm deep scribe.
Figure 5. 3D optical profile image and sidewall roughness measurements of ~97- µdeep scribe machined at 400mm/s; red dotted square indicates region analyzed for RMS and Ra measurements.
In the initial experiments, it was observed that following certain degree of beam ellipticity (about 16:1 elongation), there is no major benefit of utilizing a high-speed, multipass process for slicing 100-µm thick silicon, while a single scan at 350mm/s can easily cut through the same thickness of silicon. In addition, a single-scan dicing process, at speeds of less than 2m/s, can be managed by latest linear motor stage motion technology.
This technology delivers better precision than the scanning galvanometer technology. Lastly, the removal the f-theta lens in the optical set up ensures that a shorter focal length final focusing optic can be utilized to produce a smaller minor axis of the ellipse. This would increase the fluence and enable a more elongated beam and faster cutting speeds.
The characterization of sidewall roughness with the optical profilometer further shows the quality and efficiency that can be obtained with elliptical beam processing with 355nm, short-ns pulse widths. The 0.39 Ra measurement value presented here compares positively to those reported for 80-µm thick silicon cutting with pico- and femtosecond pulse widths.
For the ~9-µm minor axis beam diameter and pulse energy utilized, it is clear that beam ellipticity optimization helped in determining the fastest silicon cutting speed for different cutting thicknesses of interest.
High-speed dicing of thin silicon wafers has been described through a line-focus, fluence optimization approach. With 18W laser power on-target and 9-µm minor axis beam diameter, full-cut scan speed for 100-µm thick silicon wafers was observed to be from 350 to 400mm/s. For various silicon thicknesses, the optimal ellipticity and fastest cutting speed for acquiring this speed has been determined. At 1m/s scan speed, it was possible to make a >50-µm deep cut in silicon.
With ellipticities more than 16:1 ratio, it was observed that high-speed, multi-pass process was not required to produce high-quality results, but instead a single scan at the optimal fluence was observed to give better quality and efficiency results. Moreover, optical profilometry test denotes that short pulse, high pulse overlap, and large beam ellipticity produce high-quality cuts with reduced sidewall roughness. These results confirm the benefits of laser dicing over traditional saw-dicing methods for thin silicon wafers.
This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.
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