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Jonny Corrao

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Case Study: GaN Wafer Dicing

Posted by Jonny Corrao on Wed, May 30, 2018 @ 02:34 PM

CHALLENGE:
To develop a singulation process for GaN wafers that consistently provides high quality and yields.

CASE STUDY:
GaN-scribe-breakIntegra evaluated the quality of die singulation of mechanical dicing versus scribe and break. A 120µm thick 4" GaN wafer was used for the evaluation.

SOLUTION:

  • Mechanical dicing exhibited superior topside and backside quality compared to scribe and break.
  • The mechanical dicing cut was very clean on the topside of the die with none of the chipping issues that one might normally expect from a hard III-V material.
  • Backside quality from mechanical dicing was acceptable per Mil-Std specifications.
  • Scribe and break demonstrated inferior topside quality compared to mechanical dicing due to the inability to maintain consistent and sufficient force during scribing of the GaN material.
  • Backside quality was also substandard compared to mechanical dicing due to the excessive force required to break and separate the hard III-V material. 

RESULT:
As a result of this evaluation, Integra was able to develop and qualify a high-quality production process for singulating GaN wafers using mechanical dicing.

Interested in finding out more or have your own challenge for Integra to solve?

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Dice Before Grind (DBG) for Medical Devices

Posted by Jonny Corrao on Thu, May 12, 2016 @ 12:11 PM

CORWIL was recently approached by a medical customer who had developed a tiny wearable device that included a small bare die with innovative packaging. The product required a very small and strong die due to the customer’s very tight and unconventional package requirements. The die were on a 300mm wafer and had tight streets and low-k dialectrics.

Knowing that the backside and edge quality were key, the CORWIL team used Dice Before Grind (DBG) to reduce die breakage and chipping typically caused by the conventional method. DBG reverses the usual process of fully dicing the wafer after grinding. In DBG, the wafer is first trenched, or partial-cut, to a depth greater than the final target thickness. The wafer is then thinned to the final target resulting in die separation. After grind, the wafer goes to the in-line DBG Mounter, which mounts the wafer and gently peels off the protective grinding tape, completing the process.

Since the die are singulated at the final target thickness, wafer-level breakage is greatly reduced. Additionally, as a result of the die separation occurring during the grinding process, the backside chipping associated with thin-wafer dicing is kept to a minimum. DBG can also provide improved die strength depending on the application. For these reasons, DBG is an excellent process for processing wafers with high-quality backside requirements.

Top: DBG Bottom: No DBG
 
 
Corwil-medical-device-technology

Want to Learn More About Die Singulation?

Posted by Jonny Corrao on Wed, Nov 06, 2013 @ 01:00 AM

wafer-dicingDie singulation is the process of isolating individual IC’s from a wafer. There are a variety of methods for die singulation with the most common being conventional dicing, laser dicing, scribe and break, and dice before grind (DBG). 

Conventional dicing is the current industry standard for die singulation. Conventional dicing typically utilizes diamond enriched resin-bonded blades on high precision saws to cut through materials like silicon, alumina nitride, sapphire, gallium nitride, and mold compound.

Conventional dicers are equipped with a porous ceramic chuck to hold the work piece during
dicing. A blade mounted on a high speed spindle cuts the material while high pressure water nozzles flood the work piece and blade to provide cooling. Standard dicing feed rates range from 0.5 to 3.0 inches per second depending on the material, material thickness, and quality requirements.

In addition to providing cooling...

The chilled deionized water used during dicing provides lubrication to remove particles generated during saw. Re-ionized water can be used instead of deionized water to lower resistivity and minimize ESD effects on ESD sensitive products. Surfactant can also be added to the process water for ESD purposes, additional lubrication, and to minimize corrosion in copper embedded bond pads.

The blades used in conventional dicing vary in size depending on the material thickness and saw street width. The saw street is the distance between the outer edge of each die on a wafer. As the blade cuts through the material a saw kerf is generated. The kerf includes the extra material removed in addition to the blade width. The saw kerf is typically an additional 10-20µm wider than the actual blade width. On a 40µm wide blade, for example, the actual material removed, or kerf, would be 60µm. As a result, blade selection is highly dependent upon street width. 

Material thickness also plays a critical role in blade selection. With thick materials wide blades are a necessity in order to provide adequate blade strength to cut through more material. Additionally, the dicing blade edge requires an adequate clearance and engagement area to effectively cut. The clearance required for a blade is called the blade exposure. A tall skinny blade is more unstable and prone to blade wobble and breakage when cutting thick materials. Therefore, material thickness and sufficient exposure is another key variable in blade selection. As a rule of thumb, the thicker the material the wider the blade needed, the taller the exposure and, in turn, the wider the required street width.

Finally

With conventional dicing, chipping is the main quality concern. Chipping quality is governed by feed rate, cut mode, blade width, blade concentration, and blade grit. Typically, the higher the feed rate the larger the chipping. Two different cut modes are typically used, step cut and single pass. Single pass uses one blade to cut all the way through the material. Step cut uses two blades to cut at different depths in the wafer. Single pass provides greater throughput, but larger chipping compared to step cut. Blade grit and concentration are selected based on whether topside or backside chipping is critical and whether metal peeling or chipping is of concern. 

What is Wafer Thinning?

Posted by Jonny Corrao on Mon, Sep 30, 2013 @ 09:05 PM

wafer-thinningWafer thinning is the process of removing material from the backside of a wafer to a desired final target thickness. The two most common methods of wafer thinning are conventional grind and chemical-mechanical planarization (CMP).

Conventional grinding is an aggressive mechanical process that utilizes a diamond and resin bonded grind wheel mounted on a high speed spindle to perform the material removal. The grind recipe dictates the spindle RPM, rate of material removal, and the final target thickness of the work piece. Harder materials like sapphire typically require slower feed rates compared to more forgiving materials like silicon.

The wafer is positioned on a porous ceramic rotating vacuum chuck with the backside of the wafer facing upwards (towards the grind wheel). Both the grind wheel and wafer chuck rotate during grind. Deionized water is jetted onto the work piece to provide cooling and wash away material particles generated during the grind. A grinding tape is applied to the front side of the wafer to protect the devices from being damaged during thinning.       

wafer-thinning-process

 

For conventional grinding the thinning is a two-step process. 

  1. The first step is a coarse grind that performs the bulk of the material removal. 
  2. The second step is a fine grind. The fine grind typically removes 30µm of material or less and provides the final finish on the backside of the wafer. Standard finishes for conventional grind include 1200 grit, 2000 grit, and poligrind.

1200 grit is a rough finish where the grind striations are clearly visible. 2000 grit has improved roughness compared to 1200 grit and the grind marks are less apparent. Poligrind is a near-mirror finish with the smoothest roughness. Poligrind also provides the highest wafer and die strength as the high grit wheel removes the most subsurface damage. As a rule, as the grit increases the wafer strength and smoothness improves while the wafer warpage and subsurface damage decreases.

Polish is another finish of conventional grinding. A polished finish is a mirror finish. This provides the least warpage and highest die strength of all finishes. Mechanical polishing requires a separate process and equipment from conventional grinding. Mechanical polishing is a minimal removal process of only 2-3µm of material and is typically only performed on silicon.  

In CMP, abrasive chemical slurry is used with a polishing pad to perform material removal. CMP provides greater planarization compared to mechanical grinding, however, it is considered a “dirtier” and more costly process. The wafers are mounted to a backing film, such as a wax mount, which can be difficult to remove or leave a residue on the front side of the wafer. 

CMP does have the advantage of being more forgiving when it comes to processing
hard or exotic materials like tungsten, but the cost-benefit and cleanliness of mechanical grinding compared to CMP should always be factored when determining the method of wafer thinning.