Novel Noncontact Thickness Metrology for Partially Transparent and Nontransparent Wafers for Backend Semiconductor Manuf
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Novel Noncontact Thickness Metrology for Partially Transparent and Nontransparent Wafers for Backend Semiconductor Manufacturing Wojciech J. Walecki, Vitali Souchkov, Kevin Lai, Phuc Van, Manuel Santos, Alexander Pravdivtsev, S. H. Lau, and Ann Koo Frontier Semiconductor, 1631 North 1st Street, San Jose, CA 95112 ABSTRACT Single probe infrared low coherence optical interferometry has been proven to be an effective tool for characterization of thin and ultra-thin semiconductor Si and compound materials wafers. Its application was however limited to wafers transparent at probing wavelength, and having relatively smooth surfaces. Purpose of this paper is to present an extension of low coherence interferometry to characterization of non-transparent wafers, and wafers with rough surfaces.
INTRODUCTION The metrology of thin (0.1 - 0.2 mm) and ultra-thin (below 0.1 mm) wafers has been identified very early as one of the technology gaps of the metrology for backend silicon device manufacturing. Similar challanges are also encountered in novel compound materials, used in microwave and photonics applications. The most commonly used tools for measurement of geometrical parameters of thin wafers are based on capacitance and air pressure techniques. These two competing technologies have been proven to be reliable and quite accurate solutions for measurement of thickness, bow, warp and related parameters in relatively thick, and well conducting materials. Capacitance method however is not suitable for measuring thickness of semi-insulating and insulating materials, and very thin wafers (thinner than 0.1 mm). Furthermore it has relatively low spatial resolution limited by physical size of probes, and may not be suitable for direct measurement of wafers mounted on insulating materials such as glass, sapphire or plastic tape. Air pressure based sensors are able to measure insulating materials however they cannot be directly applied to wafers mounted on carriers. Both these competing technologies require access from both top and bottom sides of the wafer. For both these methods standoff distance between probes and sensor head is in the range of 0.1 – 4 mm. Very small standoff distance is making it very difficult to integrate these metrologies in in-situ or ultra-clean environments. Optical profilers based on free space optics such as white light interferometers [3], [4], [5] have been succesfully used for measurement of step hights and thickness of transparent materials. They usually are having very small working distance (WD) limited by WD of employed microscope objective. These tools employing visible radiation are not suitable for measurements of nontransparent materials. Some attempts were also performed by our group in past to use triangulation based metrology ([6]) – however in practice this method suffered severe limitations in particular when applied to patterned wafers, turbid media (such as dicing tapes) or multilayer structures. Single probe infrared low coherence interferometry does not suffer from above discuss
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