[email protected] silicon and generally, two types of anisotropic

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Last updated: December 31, 2019

[email protected] is a partnership established by the DefenceScience & Technology Agency (DSTA) and Nanyang Technological University(NTU). A Memorandum of Understanding was signed between NTU and DSTA on 25thMarch 2003 on the establishment of [email protected] for the coordinating and developmentof long term technology programmes. [email protected] was officially declared open by MrTeo Chee Hean, Minister for Defence, Singapore on 7th September 2007.

1.1 Internship ObjectivesThe scope of this internship was to investigate andoptimize sample preparation techniques for advanced application-specificintegrated circuits (ASICs) with global uniformity using tools such asprecision milling/polishing system and optical imaging.More specifically, the aim was to thin the siliconlayer from the backside of the wafer chip to below 10?m.

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The need for such a thin and compact product is essential to meetingform factor requirements as every micron matters in ultra-compact products. 2.   Literature Review2.1  MechanicalGrindingMechanical grinding is one of the most commonmethod used for wafer thinning and it usually involves 2 steps starting withcoarse grinding followed by fine grinding. Typically, the bulk of the grindingis completed during coarse grinding and this is the main process that willreduce the thickness of the wafer chip significantly. However, coarse grindingof the chip will result in micro-cracks which damages the silicon. Finegrinding will then complete the whole process by removing the damage fromcoarse grinding and reducing the surface roughness.

2.2 Chemical EtchingChemical etching of silicon is another method usedto reduce the thickness of silicon and generally, two types of anisotropicetchants are used. They are potassium hydroxide (KOH) and tetramethylammoniumhydroxide (TMAH) which is an alkaline organic solution. KOH shows poorselectivity between silicon and silicon dioxide. It is also not compatible withthe complementary metal-oxide-semiconductor (CMOS) process. However, TMAH isCMOS compatible and shows a better selectivity between silicon and silicondioxide as compare to KOH. 3.

   Methods3.1  AutomatedSelected Area Polisher (ASAP)ASAP is designed to tin and polish small selectedareas in electronic packages and wafer-level dies. It is mainly used for thebackside or front side preparation of semiconductor chips or packages. Figure 1: ASAPThe first step is mounting the chip onto the sampleholder plate. The chip is then held securely on all sides with screws.

Next,the sample holder plate is heated up to 80oC. By heating it up, thepackage chip will “relax” thus flattening the die and reducing stress duringsilicon thinning. Depending on the die or cavity size, the appropriate diametertool size would be as seen in Table 1 below. Tool Diameter Suggested Die/Cavity Size Range (X or Y Dimension) 1mm 3mm to 6mm 2mm 6mm to 9mm 3mm 9mm to 15mm 5mm 15mm upwards  Next, tilting adjustment and centring was carriedout. Using a 5-point tilt auto generate, it is only used during the initialsetup as it overrides any previously established recipe set points. The startand end points are first set before the machine automatically find the 5 pointsand calculate the tilt.

The tilt is then manually adjusted using the X Y tiltknobs once the process is finished. Once the tilt is less than 0.01 turns ineither clockwise or anti-clockwise direction, the next step can begin.  The tool head is then raised and locked, allowingthe tool to be removed from the tool spindle and replaced with a fine diamondtool which is used to thin down the silicon via milling. The locking screw isthen unlocked and lowered gently till it reaches the bottom end. Once the start and end points are set up, pressingXY centre would bring the tool to the centre of the cavity defined. The forceis then set to be on and Z is brought down till the force reads 100g and atthat point, Z is set as 0 in order to track Another factor or setting to take note of was thetravel pattern of the tool.

There were 8 different patterns available butgenerally only ASAP-1, ASAP-1 X and ASAP-1 Swap X-Y were used for standardremoval. Several other settings to take note of were the table speed and toolspeed. The settings are very flexible and may change depending on the sampletype and cavity size. Figure 2: Table travel patternsThe general guide for backside thinning of a sampleusing a sample is as shown in Table 2 below. This is a guide and the procedurediffers according to different samples and requirements. For the case of thewafer chip that I was working on, Step 1 and 2 was not necessary as thebackside of the silicon was already exposed, showing the silicon.

Thus, fromthere, all that had to be done was to thin down the silicon to the desiredlevel before polishing. Table 2: Six step process for backside thinning of a packaged deviceStep 3 – Fine DiamondThe main purpose of using the fine diamond tool isto remove the bulk thickness of the silicon and this is done in the presence ofa lubricant called extender fluid. Generally, silicon is removed in incrementsof 50?m at a time. The processwas stopped when the desired depth of around 100?m was achieved.Step 4 – Blue DiamondPasteAfter using the finediamond tool, the Xylem tool was used for both the blue and yellow diamondpaste but each tool was to be used for each respective diamond paste onlywithout switching.

The blue diamond paste and extender fluid was applied to thesurface of the silicon and spread over the cavity. The process was then run forthe desired time and once it was completed, the sample was cleaned thoroughlyto remove all traces of the blue diamond paste. This was to ensure that noscratches would be left on the next step due to particles of blue diamond pasteleft behind.Step 5 – Yellow DiamondPasteAnother Xylem tool wasused for the yellow diamond paste and similarly to the blue diamond paste, theyellow diamond paste and extender fluid were applied to the to the siliconsurface and spread over the cavity. The sample was also cleaned thoroughly toremove all traces of the yellow diamond paste.Step 6 – Colloidal SilicaXybove tool was used and a few drops of colloidalsilica was applied to the silicon surface. The sample was not heated at thisstage as if the colloidal silica was heated and dry out, it would causeprecipitation of the silica and it would be difficult to remove.

Once theprocess was completed, the sample was cleaned thoroughly.

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