TERAHERTZ TIME OF FLIGHT DETECTION FOR ABSOLUTE THICKNESS MEASUREMENTS OF SINGLE SIDE POLISHED SILICON WAFERS

We present a solution for a challenge in semiconductor manufacturing, which is the measurement of the absolute thickness of single side polished silicon wafers. Our thesis is that terahertz time of flight detection can be used in non-destructive measurement applications in precision engineering such as the example discussed here. Here we discuss the application of the method to measure the thickness of single-sided silicon wafers. A transmission mode terahertz time of flight system was designed, constructed and used to demonstrate this technique. This is a challenging measurement in a manufacturing environment and a terahertz approach offers many advantages. Furthermore, the absolute thickness measurement was performed. The results show that the terahertz time of flight metrology system is repeatable and has a measurement speed of less than 2 minutes making it suitable for high volume process control. INRODUCTION Terahertz radiation refers to far infrared or sub millimeter radiation and its frequency lies between the microwave and infra red region of the electromagnetic spectrum. In terms of wavelength, terahertz radiation corresponds to 10 12 Hz and hence 0.3 mm in free space. The advent of ultrafast lasers heralded the start of the terahertz era. Technological advances and applied research in terahertz radiation has increased exponentially ever since. The terahertz spectral region lies between the electronic part and the optics part of the electromagnetic spectrum and is referred to as the terahertz “gap”. Photon energies in the terahertz region are below the electronic state transition energies in material that are necessary for optical sources. Moreover, terahertz photons cannot excite electrons above the band gap energy in a photodiode. On the other hand of the gap, terahertz frequencies are too high to be directly generated and detected by microwave technology. These technological challenges in generating and detecting terahertz radiation are overcome using ultrafast optics [1,2]. As a result of the growing interest in terahertz radiation, a multitude of system configurations have emerged for various applications in terahertz research and development. The first demonstration of a terahertz imaging system for non destructive testing was done by Hu and Nuss [3] at AT&T and Bell labs. They used photoconductive antennas as a source and detector of terahertz radiation. A mode locked Ti: Sapphire laser at a wavelength of 800 nm was used to excite the photoconductive antenna. Other research groups independently demonstrated the applications of terahertz radiation using different configurations. A commonly used design was the use a photoconductive antenna to generate terahertz radiation and a non linear electro-optic crystal for terahertz detection [4]. Photoconductive antennas are the most widely used sources for generating terahertz radiation. They are used in most terahertz systems because of the high power and signal to noise ratio. Free space electro-optic sampling measures the amplitude and phase of the terahertz electric field in the time domain. In this method, the change in polarization of an ultrafast probe beam is measured. The measured polarization change can draw an exact picture of the terahertz field. NON DESTRUCTIVE TESTING USING TERAHERTZ RADIATION Applications in precision engineering and precision metrology are relatively unexplored. Terahertz radiation has potential for precision metrology of internal structures or defects in insulating materials such as coatings, ceramics, insulation and many composite materials that are difficult or impossible to characterize otherwise. Examples include paint film thickness, voids, crack, and internal geometries [5]. Terahertz radiation provides unique advantages over other non destructive techniques. The property of terahertz radiation to penetrate insulating material opaque at other wavelengths enables various applications. Terahertz metrology has great potential in inprocess monitoring of some aspects of the semiconductor manufacturing process. The concept of production line inspection using terahertz radiation has been demonstrated by Yasui et al [5]. They successfully designed and implemented a terahertz paintmeter system that was employed to measure the thickness of paint films. They also studied different non destructive testing methods and demonstrated the use of terahertz radiation to measure the thickness of paint films. In their technique, the temporal delay between reflected pulses is measured and converted into thickness information. Other technologies such as ultrasonic thickness meters are currently used for this purpose and offer a great degree of portability and reliability. However, such devices perform contact measurements and can be impractical in inprocess inspection. Given these advantages, we decided to design and construct a terahertz time of flight system that could be used for nondestructive measurement applications. OPERATIONAL PRINCIPLE The fundamental components of the terahertz system are, femtosecond laser, terahertz source, terahertz beam optics and the terahertz detector. FIGURE 1. Sampling of terahertz waveform by femtosecond laser pulse. (a) Terahertz waveform. (b) corresponding femtosecond optical pulse. (c) measured terahertz waveform obtained by sampling (a) and (b). Figure 1 illustrates the sampling of the terahertz signal by a femtosecond optical pulse. A single femtosecond pulse generates one terahertz pulse. Identical copies of the femtosecond pulse, and hence the terahertz pulse, are produced at the repetition rate of the laser. Making the two optical path lengths equal ensures that each terahertz pulse interacts with the corresponding femtosecond pulse. By translating the delay line we look at a different set of identical pulses. Hence we are not looking at a single pulse but the average shape of many terahertz pulses METROLOGY OF SINGLE SIDE POLISHED/ETCHED WAFERS Infrared transmission has been used in the semiconductor industry for measuring silicon crystals. Such systems are generally limited to measuring only double side polished silicon wafers. A rough back surface destroys the spatial coherence of the light enough to eliminate interferometric fringes. Schmitz et al. describe the use of infrared interferometer developed at the National Institute of Standards and Technology (NIST) to measure the thickness variation in silicon wafers. The interferometer makes use of interferometric fringes and as a result can only measure double-side polished silicon wafers [6]. A similar interferometer was implemented at UNC Charlotte. The interferometer is setup in the Twyman Green configuration utilizing infrared radiation. A tunable laser is used for this setup [7]. In both of these configurations, the reflection from the back surface needs to be sufficiently specular, as the measurement is based on the spatially coherent interference condition over the area of the wafer. If the surface is rough the interference fringes will not be observed and the measurement will not be possible. Our terahertz metrology instrument demonstrates a fast measurement of single side polished or etched silicon wafer.