## 1 Introduction  
Since [PERSON]'s law [1] stating that the number of transistors per silicon chip doubles every two years, circuits on the chip have increasingly become more integrated and multi-layered, necessitating flatter wafers for the chip substrate, and consequently allowing for higher-resolution photolithography [2]. Chemical mechanical polishing (CMP) is a wafer-planarization technique, in which the wafer is pressurized against a rotating polishing pad in conjunction with chemical slurry; the wafer surface is thus smoothed and flattened through mechanical polishing and chemical reaction [3]. Over the past few decades, CMP has emerged as a critical processing step in advanced semiconductor manufacturing due to a number of advantages in addition to global planarization of the wafer surface [4].  
The extent of wafer planarization, however, is affected by the removal rate of wafer material. This removal rate is in turn highly dependent on the roughness of the polishing pad surface, which has numerous micro-asperities [5]. Therefore, it is important to maintain proper surface roughness on the polishing pad during CMP to achieve high quality of wafer flatness. For this purpose, it is imperative to develop metrology that can efficiently assess the roughness of the pad surface in situ. Previously, there have been several measurements of the surface roughness of the CMP pads by obtaining their texture or contour maps using stylus profilometry [6], atomic force microscopy (AFM) [7], scanning electron microscopy (SEM) [6], coherence scanning interferometry (CSI) (often also called white light interferometry (WLJ)) [8], and laser scanning confocal microscopy (LSCM) [9]. Despite promising results using these classical metrology technologies [6; 7; 8; 9], the measurements of pad in situ have been scarce; most of the technologies are inaccessible to the pad when CMP is conducted on wet or submerged pads in slurry mixture. A recent effort has shown the possibility of the pad surface measurement under wet conditions using full-field optical coherence tomography (FF-OCT), which is a variant of a high-resolution OCT [10]. Due to the double-arm interferometer utilized in FF-OCT, however, interference is overly sensitive to small external perturbations as a result of separate optical-beam paths. This condition is inappropriate for evaluating the pad in a process exposed to substantial ambient noise such as factory vibration. More recently, advanced confocal technology based three-dimensional (3-D) surface profilers have been developed by several surface metrology companies [11, 12, 13]. By combining the metrologies with an appropriate immersion objective, the profilers are able to measure the wet pad on the polisher, which is necessary for both lateral beam scanning, using either a rotating multiple pinhole disk [11] or a digital micromirror device (DMD) [12, 13]) and vertical scanning of the objective with a piezo drive to generate a 3-D reconstruction of the pad surface.  
In this paper, we present a new interferometric method for on-site pad measurement. This novel metrology technique utilizes a common-path phase-shifting interferometry, contributing to existing techniques by enabling precise and reliable evaluation of the pad surface in wet conditions and noisy environments. The surface-mapping performance of the proposed system was tested and validated using a well-known nano-structure. A commercial CMP pad was then examined using the proposed system.