Low cost wafer metrology using a NIR low coherence interferometry

Abstract

In this investigation, a low cost Si wafer metrology system based on low coherence interferometry using NIR light is proposed and verified. The whole system consists of two low coherence interferometric principles: low coherence scanning interferometry (LCSI) for measuring surface profiles and spectrally-resolved interferometry (SRI) to obtain the nominal optical thickness of the double-sided polished Si wafer. The combination of two techniques can reduce the measurement time and give adequate dimensional information of the Si wafer. The wavelength of the optical source is around 1 μm, for which transmission is non-zero for undoped silicon and can be also detected by a typical CCD camera. Because of the typical CCD camera, the whole system can be constructed inexpensively.

Applied Photoelasticity for Residual Stress Measurement inside Crystal Silicon Wafers for Solar Applications

“…Infrared light source is used because the crystalline silicon optical band gap is about 1 μ m and hence, silicon is transparent for wavelength greater than its band gap . However, a NIR wavelength is preferred to avoid perturbations due to speckle phenomena or/and parasitic IR radiations from motors and hot light sources like halogen or tungsten.…”

Metrology of undoped double-sided polished silicon wafer: surface, thickness and refractive index profile measurements

“…As known in Figure 1, the light in the range of 1-1.1 μm can go through the wafer and also be detected at the camera. Even though the transmittance of a Si wafer and the sensitivity of the camera are not so high in this wavelength region, they are enough to be used in wafer metrology [15].…”

“…If the illumination light has a finite spot size, SRI gives the nominal optical length of the illuminated area. Once SRI measures the optical lengths of the target, the measurement results can support to determine the scanning range of LCSI in order to reduce the acquisition time [15]. Moreover, these nominal values can be more accurate than those determined by LCSI.…”

“…In this method, only a single measurement result can provide all measuring parameters of a Si wafer to expect to obtain. This technique is based on low coherence scanning interferometry (LCSI) and spectrally-resolved interferometry (SRI) using a NIR light, which is around 1 μm, for which transmission is non-zero for undoped silicon and also detectable by the typical visible CCD camera [15]. Because of not using an expensive IR CCD camera and an optical source, the proposed technique is cost-effective.…”

Sub-sampling low coherence scanning interferometry and its application: refractive index measurements of a silicon wafer

“…Recently, the NIR light in the range of 1-1.1 μm has been used for LCSI using a SLD and a typical CCD camera as a cost effective solution [9]. In the case of measuring a double-sided polished Si wafer, LCSI can provide two correlograms, which correspond to the reflected beams from the front surface and the rear surface of the wafer, respectively.…”