Advantages of Micromachining Polyimide Using a Mode-Locked High Repetition Rate

Newport and its Spectra-Physics Lasers Division have been supplying photonic and laser solutions to the semiconductor industry for more than 10 years. With the advent of the mode-locked, quasi-cw ultraviolet lasers in 2000, a new method of material processing was introduced which utilized low energy picoseconds pulse at a high repetition rate.

The duration of picoseconds pulse helps in removing materials with a high peak power, while high repetition rate enables material processing at a higher speed which otherwise is not possible with a standard power nanosecond pulse Q-switched laser.

These aspects of mode-locked laser processing hold significant importance in a number of laser applications in the microelectronics and semiconductor industries demanding higher throughput rates and accurate processing of features. Moreover, features can be processed with reduced thermal damage with high peak power and picoseconds pulse width.

In experiments performed in the past, it has been shown that a 355nm mode-locked laser is capable of processing materials like polyimide at a processing speed that is much higher than that of a 355nm Q-switched laser even when both lasers displayed the same average power.

In case of materials like copper, it was noted that at the same average power a nanosecond Q-switched laser was comparatively better at removing materials than that of the picoseconds mode-locked laser. This article describes the advantages and disadvantages of 355nm Q-switched and mode-locked lasers for processing of copper, silicon and polyimide.

It also investigates a process parameter space that leverages the features of each laser to obtain the best processing results in terms of processing quality and throughput.

Q-Switched Lasers

Mode-locked UV DPSS lasers and Q-switched UV diode pumped solid-state (DPSS) lasers are two of the most common products used in the microelectronics packaging and semiconductor industries.

A Q-switch produces high loss in the laser cavity which in turn inhibits lasing as the gain medium is being pumped. This pumping action introduces energy into the laser medium, and when this energy increases to the highest value, the Q-switch is set to allow lasing.

Due to the high gain developed in the laser medium, laser output power instantly increases to a high value. Nevertheless, the intensity of this pulse instantly reduces all the stored energy from the gain medium and the output reduces as quickly as it has risen.

Mode-locked Lasers

Mode-locking is a technique used to obtain the highest repetition rates and shortest pulses of around 100MHz. Standard durations of mode-locked pulses vary from a few femtoseconds to a few picoseconds. As a rule, two things are required to obtain mode-locked performance with transform-restricted pulses and these include a consistent mode-locking mechanism and some form of dispersion compensation.

Mode-locking mechanisms can be passive or active. With the introduction of mode-locked and Q-switched lasers, two entirely different processing regimes have materialized. The Q-switched laser processing regime utilizes high energy per pulse (µJ/pulse), low repetition rate (kHz), and a nanosecond pulse width, while the mode-locked laser processing regime uses very low energy per pulse (nJ/pulse), very high repetition rates (MHz), and a picosecond pulsewidth.

Although data from the prior study demonstrates which laser delivers better results at a particular material thickness, it does not show what exactly happens when a decrease or increase is observed from that specified material thickness. Also, in the earlier study, peak power was different, though the average power for both types of lasers was found to be same. These aspects can affect processing results.

Therefore, to compare both set of lasers, the present study focused on distinguishing mode-locked lasers against the processing ability of Q-switched lasers for the same peak power and same average power. In addition, different scribe depths obtained by both lasers at different cutting speeds were examined. Here, the aim was to establish which laser performs better in terms of material removal and quality of the processed features for a specified material.

Experimental Set Up

In this study, two lasers were used: a Q-switched HIPPO™ laser operating at 80kHz with a 12ns pulse width and a mode-locked, quasi-cw Vanguard™ laser operating at 80MHz with a 12 ps pulse width. Both lasers were configured to offer output at 355nm wavelength. Figure 1 shows the experimental set-up.

Figure 1. The experiment set-up

The HIPPO Laser

The HIPPO series (Figure 2) from Spectra-Physics is a diode-pumped solid state range of high power Q-switched lasers that can generate high output powers along with very short pulses. With pulse widths shorter than 11ns in the UV and 15ns in the IR, high peak powers can be obtained, thus making the HIPPO laser source suitable for all micromachining applications.

The Vanguard Laser

The Vanguard™ UV (Figure 3) from Spectra-Physics is a diode-pumped solid state laser that can generate very low noise, quasi-CW ultraviolet output. It is characterized by long-term stability, high TEM00 mode quality, and long lifetime. Using advanced mode-locking technology, this robust laser provides 4W of quasi-CW, UV output at 355nm. Te Vanguard serves as an excellent solid-state alternative to replace CW ion lasers in various applications.

Figure 2. The Spectra Physics HIPPO lasers

Figure 3. The Spectra Physics 2.5W Vanguard laser

Results and Discussion

The Table 1 shows the process parameters selected for the experiment.

Table 1. Various process parameter conditions

Polyimide Silicon Copper
Hippo Vanguard Hippo Vanguard Hippo Vanguard
Wavelength, nm..... 355 355 355 355 355 355
Rep. Rate, kHz..... 80 80000 80 80000 80 80000
Pulse Width, ns..... 12 0.012 12 0.012 12 0.012
Max. Power, W..... 2.5 2.5 2.5 2.5 2.5 2.5
Max. Peak Power, kW..... 2.6 2.6 2.6 2.6 2.6 2.6
Spot Size, urn..... 10 10 8 8 4 4
Fluence, J/cm2..... 40 0.040 62 0.062 249 0.249
Intensity, GW/cm2..... 3.3 3.3 5.2 5.2 20.7 20.7

Polyimide

Figure 4 depicts the variations in cut quality of a scribe performed at same average speed of 400mm/s between the Vanguard and HIPPO lasers. Apart from slight variations in edge quality, both scribes have the same appearance. Figure 5 shows the polyimide machining comparing scribe depth obtained at a specified cutting speed.

Figure 4. Comparing mode-locked Vs Q-switched cut quality in polyimide

Figure 5. Polyimide machining comparing scribe depth achieved at a given cutting speed

As noted in the earlier study, results do suggest that in case of thinner films, mode-locked laser is considerably better in material removal efficiency and it enables scribing of polyimide film at a faster cutting speed. The result also suggests that as the thickness of the material increases, the “digging out” of material becomes more difficult for mode-locked laser pulses with low energy picoseconds. Therefore, the cutting speed for both lasers is almost equal for 80-micron deep scribes.

Silicon

For silicon scribing studies for the same average power of 2.5W at the surface of sample, different cutting depth obtained at a specified cutting speed is illustrated in Figure 6.

Figure 6. Silicon machining comparing cutting depth achieved at a given cutting speed.

Results in Figure 6 demonstrate that in case of thinner films, the mode-locked laser is significantly better in material removal efficiency, and it enables scribing of silicon at a faster cutting speed. However, for the deeper scribing, the “digging out” of material becomes much harder for low energy picoseconds pulses. As a result, the scribing speed for both lasers becomes equal for scribes that are 8-micron deep.

Copper

Figure 7 depicts the variation in scribes produced by means of a HIPPO laser for the same average speed for a different number of scans. For Vanguard laser, the same effect of cleaner removal by multiple scans was noted (Figure 8). Figure 9 illustrates the difference in cut quality between the Vanguard and HIPPO lasers.

Figure 7. Copper machining using a Q-switched HIPPO laser at same average speed using single and multiple scans.

Figure 8. Copper machining using a mode locked Vanguard laser at same average speed using multiple scans at different scanning speed.

Figure 9. Comparing mode-locked vs Q-switched cut quality in copper

Conclusion

The present study validated the trend observed in the prior studies. When compared to the Q-switched laser, the mode-locked laser appears to be more efficient in machining material with a low thermal conductivity like polyimide against material with a higher thermal conductivity like copper.

For a specified material type and average laser power, there exists a cross over thickness above which Q-switched lasers are more efficient in removing material and below which mode-locked lasers are more efficient. However, material removal efficiency of the mode-locked laser becomes much better as processing speed increases irrespective of the type of material type.

In case of copper and silicon, material removal via multiple high speed scans is cleaner when compared to single low speed scan. Generally, for a higher cutting speed, material removal efficiency of mode-locked lasers is higher.

When material is moved slowly, more time is available for thermal penetration which helps in more material removal, while a material is moved at a higher speed, there is less time for thermal penetration and ultimately pulse separation restricts machining capability of Q-switched lasers.

Hence, for materials having higher thermal conductivity Q-switched laser provides a better option, and for materials having low thermal conductivity, mode-locked laser offers a better choice.

About Spectra-Physics

Spectra-Physics, a Newport Company, is the world's premier supplier of innovative solutions for precision laser applications. Our broad portfolio of lasers are proven in 24/7 applications and are backed by our global support team. Our innovation driven culture strives to create breakthrough products that achieve new levels of cost-performance that power our customers' success.

This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.

For more information on this source, please visit Spectra-Physics.

Citations

Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Spectra-Physics. (2018, August 06). Advantages of Micromachining Polyimide Using a Mode-Locked High Repetition Rate. AZoM. Retrieved on September 26, 2020 from https://www.azom.com/article.aspx?ArticleID=11455.

  • MLA

    Spectra-Physics. "Advantages of Micromachining Polyimide Using a Mode-Locked High Repetition Rate". AZoM. 26 September 2020. <https://www.azom.com/article.aspx?ArticleID=11455>.

  • Chicago

    Spectra-Physics. "Advantages of Micromachining Polyimide Using a Mode-Locked High Repetition Rate". AZoM. https://www.azom.com/article.aspx?ArticleID=11455. (accessed September 26, 2020).

  • Harvard

    Spectra-Physics. 2018. Advantages of Micromachining Polyimide Using a Mode-Locked High Repetition Rate. AZoM, viewed 26 September 2020, https://www.azom.com/article.aspx?ArticleID=11455.

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit