effective laser decapsulation employing the digital ico laser and haz-methodology
TRANSCRIPT
Effective Laser Decapsulation Employing the Digital ICO Laser and HAZ-Methodology
Gregory Burt AndersonInfineon Technologies Americas, 19401 Victor Parkway, Livonia, Michigan, 48152, USA
Lance Edgar FordhamControl Laser Corporation, 7101 TPC Drive, Suite 100, Orlando, FL 32822, USA
Abstract:Understanding laser decapsulation of semiconductor mold compounds can help failure analysis
labs effectively decap semiconductors containing various bond wire chemistry, including copper chemistry, particularly in components with high filler content. This paper will present a tutorial on how to effectively use the latest laser technology that Control Laser offers, the Digital IC Optimized (ICO) laser, in a decapsulation process.
Introduction:For the past 12-years, or so, Control Laser has been trying to build a laser decapsulation system
that is capable of getting close to the semiconductor die in an effort to reduce and/or remove the reliance on hazardous chemicals. Through this evolution, we have refined the laser, and beam deliveryand conditioning optics as we better understand the interaction of the laser-to-epoxy reaction, and the changes in the industry, such as changes in the mold compound and various bond wire chemistry.
Background:The two most important concepts to understand in creating a laser recipe are how to control
and balance pulse energy and pulse density, of a given pulse profile, for each type of mold compound, and each type of part.
In 2007, we had a breakthrough in our research as we worked with one of our partner-customers in helping them adapt to using our latest technology of the time. We identified a phenomena where if we had a laser recipe that gave us a high pulse density, but low pulse energy, then we could also have similar ablation rates with an inverse laser recipe i.e. where the pulse density was low, but the pulse energy was high. At this time, we had no way to control the pulse profile—the geometric distribution of photons within the pulse over time—so the pulse profile was constant on the older ND:YAG laser technology. Further testing was performed after post-laser-acid decapsulation to reveal that the laser recipes were, in fact, not identical with respect to damage to the die. We would find that a change in the filler of the mold compound, a significant increase in silica and/or
alumina, allowed for the high-pulse energy light to be coupled through, and refocused to, the die causing damage. Realizing this, and also realizing that the lower-pulse energy laser recipe was also more favorable to final acid decapsulation, we began to promote the use of as little pulse energy as possible, without causing recasting, when removing the mold compound with the laser, as a precursor to acid processing.
We eventually changed to a different laser type, which had lower per-pulse energy, and operated at higher-pulse repetition rates (pulse density). This change was effective in allowing us to get closer to the die without damaging it—to about 100-microns—however, when copper-based bondwires were introduced, the recipes that we had long stood by, didn't work anymore since the bond wires were heated up to the point of deformation, or even complete ablation. Finally, in 2012, workingwith a partner laser company, we developed a brand new laser that gave us nearly full control over all aspects of the laser pulse, including the pulse profile, our Digitial IC Optimized (ICO) laser.
The digital laser allows us to not only customize the pulse energy and repetition rates, but it also allows us to customize the pulse profile. In the FALIT™ laser decapsulation system, we use six different pulse profiles, from 0 to 5, with each pulse profile offering more dynamic range in terms of how much control we have over the Heat Affected Zone (HAZ) thus allowing us to get even closer to the die without damaging it.
Illustration 1: The optical pathway the laser can take when ablating mold compounds high in filler content if pulse energy and pulse density are not optimized.
What is HAZ?
HAZ is the total area affected by a laser beam, in our case, during the process of laser decapsulation. The HAZ area extends in the X-Y-Z-planes of a cross-sectioned chip, as shown in this graphic:
Which aspects of the laser recipe affect HAZ?
– Pulse Energy – The number of Joules per pulse of laser light
– Pulse Density – The number of pulses per millimeter, square
– Pulse Profile – The approximate geometrical orientation of the photons within each pulse, overtime
In the FALIT™ software, we have the following laser parameters that affect all of the aspects of the laser process in some way:
– Laser Power – A setting from 0 to 100, as a percentage of overall laser capacity;
– Laser Pulse Repetition Frequency (PRF)– The number of pulses per second emitted by the laser. The range is from 1kHz to 1MHz;
– Laser Pulse Profile – The range is from 0 to 5;
– Speed – The speed of the laser beam over the surface of the material. The range is from 1 to 7620mm/s;
– Fill-Spacing – The distance between raster lines;
– Fill-Style – Whether a horizontal, vertical, or cross (both horizontal and vertical in one process) raster scan is employed.
Which of these parameters affect Pulse Energy and Pulse Density?
– Pulse Energy: Laser Power, PRF (with exceptions)
– Pulse Density: PRF, Speed, Fill-spacing
The exceptions for the PRF, thus it's effect on pulse energy, are that each pulse profile has an optimum PRF, called PRFO . At PRFO both the pulse energy and the average output power are at their highest.
– Below PRFO - the pulse energy is capped and is constant, regardless of the repetition frequency. The average output power is, however, reduced as the repetition frequency descends, at a linear rate;
– Above PRFO - the average output power is constant regardless of the pulse repetition frequency. However, the pulse energy is reduced as the repetition frequency ascends.
Each of the six pulse profiles have a different PRFO. As the pulse profile number increases, so does the PRFO by a significantly larger amount than the previous pulse profile. This expansion of PRFO
for each of the subsequent pulse profiles gives us more dynamic range to fine tune the laser power applied to the part since pulse energy remains constant under PRFO. As the specific PRFO for each pulse profile is proprietary information, they are intentionally omitted from this paper.
How does each pulse profile affect the laser ablation process?
– Pulse Profile 0 – The high initial pulse energy, and the long tail of even-dwindling energy can punch through materials while keeping the material primed for the next pulse so that ablation can be more effective in some materials. As PRFO for this profile is the lowest, the period between effective pulses is longer than the higher-numbered pulse profiles;
– Pulse Profile 1 – The shorter pulse duration, and more average energy distribution allows for the pulse to have more punching power without the added heat of profile 0. As the PRFO for this pulse profile is significantly higher than the previous pulse profile, the period between effective pulses is shorter;
– Pulse Profiles 2 through 5 – As the profile number increases, the pulse duration becomes shorter and the pulse energy is lower. At pulse profile 5, the period between the effective pulses is the shortest.
Waste pulse energy is the energy that is absorbed by the material without causing ablation. Asabsorption without ablation causes heat within the mold compound, waste energy has a significant affect on HAZ so our goal is to reduce waste energy as much as possible. This concept rules out the use of pulse profile 0 since this pulse contains a lot of energy that does not cause ablation, thus HAZ for this pulse profile is large. Since pulse profile 1 consists mostly of energy that causes ablation, it is an ideal candidate for mold compound ablation.
What does speed do?
The scan speed alters how many laser pulses occur in a straight line per millimeter for a given PRF, and how closely each pulse stream is from one point to the next. Speeding up the scan speed reduces the number of pulses per linear millimeter and slowing it down increases the number of pulses.
What does fill-spacing do?
If scan speed alters the pulse density in a straight line, fill-spacing alters the pulse density in the direction perpendicular to the scan direction. It can also be thought of as fill-spacing determines how many raster scan lines are in one millimeter. At some point, the fill-spacing parameter will cause the laser beams of neighboring raster lines to overlap each other by some amount.
How does one determine when to change the speed and fill-spacing?
Since both fill-spacing and speed are dependent upon the kerf, or cut width, made from the High-Fluence Zone of the laser beam, and since the kerf of the laser beam is dependent upon the material that it is ablating, determining when to change these values is derived through inspection. There is a theory that the more dense the mold compound, the tighter the fill-spacing and slower the speed need to be, and vise versa. If either the speed or the fill-spacing values are too high, a row or grid pattern can be seen in the mold compound.
Illustration 3: An exaggerated cross fill-style pattern showing a comparison between two different speeds, with a constant fill-spacing, and how ablation rate within the scan line is different for each.
Illustration 2: A horizontal fill-style shows how fill-spacing affects surface texture and ablation rates.
We set the Fill-spacing so that we no longer have ridges and well-defined scan lines in the material through a process of overlapping the subsequent scan line with the previous scan line by a percentage of around 47% to 79% of the kerf. If we then adjust Speed so that the same overlap from the Fill-Spacing occurs in the linear direction of the scan line, we will have about 100% coverage of thedecapsulation area. For high pulse energy recipes, it is sufficient to have just 100% coverage, and this is what we used in our older ND:YAG laser systems. The problem with having higher pulse energies, and lower pulse density, is that the dynamic range of how much the energy can be controlled is limited. However, using the new Digital ICO laser, which gives us high dynamic range, combined with repeatable pulse-to-pulse stability, we tend to choose a higher pulse density, and lower pulse energy recipe to achieve better results. For example, we might have a recipe that gives us a speed at which we can achieve 400% pulse coverage over a decapsulation area, at a pulse energy that is four-times lower than the older laser recipe, while maintaining the same average laser power so that the materialstill ablates properly. Now, however, we split up the pulse energy into four different, closely spaced, pulses. Then by adjusting the speed, we can adjust how many pulses per dot of decapsulation area there will be, thus we can fine-tune the amount of ablation energy delivered to the part.
Parts having mold compound with less filler generally require a higher fill-spacing overlap as the High Fluence Zone is smaller; parts with more filler, generally will require less overlap. Once one pass of the laser has run, the material will show whether or not the fill-spacing is close enough as the grid pattern will be evident if the fill-spacing overlap should be increased. While it is possible to perform laser decapsulation to some degree if the fill-spacing overlap is not ideal, one will not be able to get as close to the underlying structures without damaging them if the fill-spacing overlap is not optimized because the pulse density will not be sufficient to ablate the material in between the pulse grid of the laser ablation area. Current research is underway to help calculate the optimum fill-spacingfor a part given certain material characteristics.
Illustration 4: A recipe comparison between the old ND:YAG laser technology and the new Digital ICO laser
Different Fill Styles:
There are three main fill styles that are used in the FALIT software, they are:
– Horizontal – Creates a horizontal raster scan over the part;
– Vertical – Creates a vertical raster scan over the part;
– Cross – Creates both a vertical and horizontal raster scan over the part.
The most common fill style is the Cross fill style as this provides the most even ablation since native characteristics of the laser beam can cause the vertical or horizontal axis of the beam to be non-uniform. However, since the Cross fill style is essentially two passes of raster scans over the part, one should understand that each run of the laser will cause double the pulse density to be imparted for each pass of the laser. Also, if the pulse energy is not sufficient enough to ablate the material, recast can form by having such high pulse density as the material heats up, or, in some cases, there will be uneven ablation since lower pulse energies tend to have more difficulty fracturing the larger filler material.
Some customers prefer to use either the Vertical or Horizontal fill styles for a number of reasons, such as moving the laser in the direction of the vacuum, or because they want to be more cautious with the decapsulation of the part.
Focus!
Laser focus is one of the most important aspects of the laser ablation process. Understanding what laser focus is, and how it contributes, or disrupts, laser ablation is another key to proper laser ablation of mold compound. In fact, the latest research shows that if we maintain the most precise focus, combined with the understanding of the other aspects of laser ablation described in this tutorial, we can decap mold compound all the way to the underlying structures without damage, in some cases.
Illustration 5: From left-to-right: Cross Fill Style; Vertical Fill Style; Horizontal Fill Style
What is Laser Focus?
The laser light within the Digital ICO laser propagates at a single wavelength of approximately 1,064nm, which means that it can be focused to a relatively sharp point, in contrast to longer wavelengths, or light sources with multiple wavelengths.
Within the area of Depth of Focus, the laser is said to be in focus. However, in practice, because of spherical aberrations and other concepts beyond the scope of this tutorial, the laser focus is not so easily defined; at any given point within the Depth of Focus, the laser beam size can be a different, thus the beam will have a different energy density at different points along the Depth of Focus, which will affect the High Fluence Zone, previously mentioned. This means that if we want uniform ablation as we go farther into the part, we need to ensure that the decapsulation area stays at the uniform focus point – the same focus point as at the start of the ablation process.
Illustration 6: A simplified diagram showing the concept of laser focus.
How does one set the laser focus?
At our factory, we setup the FALIT systems so that the laser is focused to the top of the provided self-centering stage and that stage becomes the reference focus position. So, the most accurate way to set the laser focus is to measure the thickness of the device to be ablated, taking into consideration any pins that might lift the device off of the stage of the laser system, and offset the motorized z-axis of the system by the thickness of the part. In some systems, this focus adjustment is automatic.
To maintain the same focal point as the laser etches away at the mold compound, the z-axis should be offset by the amount of ablated material so that the new laser focus is actually below what used to be the surface of the device. In our FALIT system, where we employ a confocal camera, and where the camera focus and laser focus are nearly the same, we can simply offset the z-axis until the image from the camera comes into focus. Other methods, to be more precise, would be to remove thepart from the system and measure the displacement between the top of the device and the decapsulation area. As a general rule, the amount of ablation will be the same as one traverses fartherinto the device. A displacement sensor is an optional feature to the FALIT that can come in helpful when determining the ablation depth and proper focus point in an automated fashion.
Illustration 7: Setting the optimum laser focus by offsetting the z-axis by the thickness of the device to be decapsulated
Examples:
The following are some examples where the information in this tutorial has been beneficial in effective laser decapsulation.
Illustration 8: The shiny part of the aluminum bond wires shows where the laser caused some minor ablation to the outer layer of the wire. For the next iteration of this part type, we reduced the laser energy in an effort to try to mitigate this issue.
Illustration 9: Lowering the laser energy helped to mitigate the etching on the aluminum bond wires, but now we either have recasting caused by the material heating up to the glass transistion temperature and hardening, or because the laser energy is too low to fracture the material.
Parts such as these TO-263 parts, which have a high geometric aspect ratio, and a more dense mold compound, tend to be more difficult to ablate with the laser. The TO-263 decapsulations above were performed with the older ND:YAG laser technology, which doesn't give us the dynamic range that we have with the latest laser technology, the Digital ICO laser. With the ND:YAG technology, the minimum thickness of mold compound that we need to leave over the die is between 100-300-microns. However, with a properly dialed-in recipe, the Digial ICO laser gets us closer.
The following decapsulations were performed with the Digital ICO laser, and show just how close we can get to the die, for some part-types, by using the methods outlined in this tutorial.
Illustration 10: A balanced recipe of pulse energy and pulse density, controlling HAZ, has produced an effective decapsultion in this image. The aluminum bond wires do not appear to be damaged, and the mold compound is relatively flat to the other devices decapsulated during these tests.
Illustration 11: This is a SOIC-20 with two die and copperbond wires. While the die are out of focus, it is apparent,judging by the polyimide passivation layer, the die have been decapsulated without significant damage.
Inspecting these parts further, we see what appears to be some breakthrough, but the shiny parts we see could also be silica that are sitting directly on top of the passivation layer, giving us a glimpse of what's underneath, a lensing effect.
Illustration 12: One of the two die inside the SOIC-20 package showing the passivation layer of polyimide nearly intact. Electrical testing will prove whether or not breakthrough occurred.
Illustration 13: Zooming in from the image above, on some of the areas that are potential breakthrough, we see silica filler sitting ontop of the passivation layer without apparently any epoxy underneath. Electrical testing will show if this is actual breakthrough or not.
Illustration 14: The same recipe from the SOIC-20 was used on this SOT-23. After inspection, we see that the passivation layer has been penetrated.
Illustration 15: A closer look from the above image gives a better view of the penetrated passivation layer.
The recipe consisted of three passes of the cross-type fill-style, with the following laser parameters:
Laser Power: 35%
PRF: 30kHz
Speed: 500mm/s
Fill-Spacing: 0.075mm
Pulse Profile: #1
The ablation rate was measured with a displacement sensor after the first pass, and the z-axis was modified by this amount so that the laser was always in optimum focus. Because there was breakthrough, but not so much breakthrough so as to cause significant damage, then it was understood that adjusting the laser power will be the best decision for the next part. The Laser Power was adjusted to 30% from 35% for the last pass only, the first two laser passes had the same recipe as before. This would ensure that the majority of the mold compound would be ablated before the last pass. The results follow:
Illustration 16: SOT-23, copper bond wires, after laser ablation. The passivation layer appears to be intact. Electrical testing to follow will show if the part is still functional.
Electrical testing was performed to see if the part was still functional. The test was performed against a gold-standard part.
Illustration 17: SOT-23, copper bond wires. A closer look shows that the passivation layer appears to be intact. There seems to be some scratching at the lower right corner, but the part appears to have withstood the laser decapsulation.
Illustration 18: VBRDSS is normal
Illustration 19: VGSth is normal
The results show that VBRDSS, VGSth, and IGSS are all normal. This a P channel device, so VBRDSS, and VGSth, have been inverted for readability.
Conclusion:
We've presented an effective laser decapsulation process as well as a tutorial for understanding the underlying concepts of what our research shows to be the contributing factors to breakthrough by the laser during decapsulation. Controlling HAZ is the key to laser decapsulation of semiconductor mold compound down to the underlying features without damaging them. While we cannot yet say conclusively that the laser is a full-decapsulation method, we've certainly come a long way since the laser was used in this capacity. There is continuing development within Control Laser and with our partner-customers, such as with Infineon, that help us to push this relatively green technology into the future.
Illustration 20: IGSS is normal
Acknowledgments:
Eric Lezon – Infineon Technologies
Andrew Saxonis – Analog Devices
Thomas Gannon – Analog Devices
Anthony Tollis – Analog Devices
Sharon Furcone – Freescale Semiconductor
References:
– “Fundamentals of Laser-Material Interaction and Application to Multi-scale Surface Modification.” Chapter 4. Matthew S. Brown and Craig B. Arnold.
– “Laser Material Processing”, Fourth Edition. [ISBN 978-1-84996-061-8, e-ISBN 978-1-84996-062-5]. Chapter 2. William M. Steen and Jyotirmoy Mazumder
– “Implementing HAZ Methodology for effective Decapsulation”, Anderson, G. B., Confidential document.