J. Pei

IN SITU ACOUSTIC TEMPERATURE TOMOGRAPHY OF SEMICONDUCTOR WAFERS

Spatial temperature distribution in semiconductor wafers during rapid thermal processing is obtained by means of acoustic tomography. Ultrasonic Lamb waves are excited in the wafer by acoustic transducers bonded to spring‐loaded quartz support pins located at the wafer periphery. The Lamb wave time of flight in the wafer is used to measure the average wafer temperature with ±0.5 °C accuracy for a S/N ratio exceeding 55 dB. Spatial temperature information is gathered by electronic switching of the transmitter and receiver transducers. Tomographic reconstruction techniques are then used to calculate the temperature distribution from the measurements with different pixel maps. Using eight transducers, the thermal image of a 10 cm, (100) silicon wafer is obtained with 2×2 cm2 pixel resolution. Thermal image rates of 2 images/s are achieved by the system enabling real‐time control of wafer temperature distribution during rapid thermal processing.

Lamb Wave Tomography and Its Application in Pipe Erosion/Corrosion Monitoring

Abstract Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. To excite Lamb waves, mode conversion of bulk waves or photoacoustic excitation often are used. Both of these approaches suffer from the need for liquid couplant or ablation of materials to reach a good signal-to-noise ratio. In this paper, we propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor velocity changes. A pair of pin transducers is used to excite and detect the A 0 mode Lamb wave in the pipe wall, and the wave velocity is obtained by time-of-flight measurement. Any change in the pipe wall thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of the A 0 mode. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of measurements of plate thickness and erosion/corrosion in a ...

Rapid thermal multiprocessing for a programmable factory for adaptable manufacturing of ICs

This paper presents an overview of research at Stanford University on the development of concepts of a programmable factory, based on a new generation of flexible multifunctional equipment implemented in a smaller flexible factory. This approach is demonstrated through the development of a novel single wafer Rapid Thermal Multiprocessing (RTM) reactor with extensive integration of sensors, computers and related technology for specification, communication, execution, monitoring, control, and diagnosis to demonstrate the programmable nature of the RTM. The RTM combines rapid thermal processing and several other process environments in a single chamber, with applications for multilayer in-situ growth and deposition of dielectrics, semiconductors and metals. Because it is highly instrumented, the RTM is very flexible for in-situ multiprocessing, allowing rapid cycling of ambient gases, temperature, pressure, etc. It allows several processing steps to be executed sequentially in-situ, while providing sufficient flexibility to allow optimization of each processing step. This flexibility is partially the result of a new lamp system with three concentric rings each of which is independently and dynamically controlled to provide for better control over the spatial and temporal optical flux profile resulting in excellent temperature uniformity over a wide range of process conditions namely temperatures, pressures and gas flow rates. The lamp system has been optimally designed through the use of a newly developed thermal simulator. For equipment and process control, a variety of sensors for real-time measurements and a model based control system have been developed. >

In situ thin film thickness measurement with acoustic Lamb waves

In situ thin film thickness measurement is an important problem in semiconductor processing, which is currently limited by the lack of adequate sensors. Most of today’s available techniques are restricted to certain type of films and many have difficulties in performing the measurement in situ. The fact that the velocity of an ultrasonic Lamb wave traveling in a silicon wafer is changed by the thin film coating on the wafer surface can be used as a monitoring method for basically any type of film—opaque, transparent, metal, or insulator. The acoustic sensors are easily implemented into plasma or CVD environments. We have demonstrated the technique in an aluminum sputtering system in which we measure Al film thickness with a resolution of ±100 A. Even better resolution can be achieved for SiO2, copper, and tungsten films. This system has a variety of potential applications, not only in film thickness measurement, but also in characterization of film properties and multilayer deposition process control.

Lamb wave tomography and its application in pipe erosion/corrosion monitoring

Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. To excite Lamb waves, mode conversion of bulk waves or photo acoustic excitation are often used. Both of these approaches suffer from the need for liquid couplant or ablation of materials to reach good signal to noise ratio. In this paper, we propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor velocity changes. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of measurements of plate thickness, and erosion/corrosion in a section of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate.

Thin film effects in ultrasonic wafer thermometry

We use an ultrasonic technique where the temperature dependence of lowest order anti-symmetric Lamb wave velocity in the silicon wafer is utilized for in-situ temperature measurement in the 20-1000°C range. In almost all wafer processing steps, one or more layers of thin films are present on the wafers. The effects of these films on temperature sensitivity is investigated. A theoretical model for Lamb wave propagation in general multilayered plates is developed using the surface impedance approach. This model is utilized to calculate the effects of anisotropy and thin films on temperature coefficients in semiconductor wafers. Calculations predict 2.38E-5 (1/°C) sensitivity for a 10 cm (100) silicon wafer with 238b anisotropy. The same figures for gallium arsenide are 2.2B-5 (1/°C) and 8.7%. Thin film effects are considered for various materials commonly used in semiconductor processing. The density and shear elastic constants of film materials are found to be effective parameters in determining sensitivity figures. The frequency dependent sensitivity calculations show that it is possible to minimize effects of aluminum and silicon dioxide on silicon wafers by choosing the frequency-thickness products around 1.6 MHz-mm and 3.3 MHz-mm in temperature measurements, respectively. Using a simple propagation model, the time of flight sensitivity is calculated and compared with experimental data obtained from a Rapid Thermal Processor

In situ thin film thickness measurement using ultrasonics waves

The fact that the velocity of an ultrasonic Lamb wave traveling in a silicon wafer is changed by the thin film coating on the wafer surface can be used as a monitoring method for basically any type of film-opaque, transparent, metal or insulator. We excite and detect Lamb waves using PZT transducers on one end of quartz buffer pins that are in contact with the back side of the wafer. The time of flight of the ultrasonic wave is calculated and measured to be linearly related to the film thickness. The acoustic sensors are easily implemented into plasma or CVD environments. We have demonstrated the technique in an aluminum sputtering system. We measure Al film thickness with a resolution of ±100 Å. Even better resolution can be achieved for SiO2, copper and tungsten films. This system has a variety of potential applications not only in film thickness measurement, but also in characterization of film properties and multi-layer deposition process control

In-Situ Temperature Monitoring in Rtp by Acoustical Techniques

A new technique utilizing the high sensitivity of acoustic wave velocity to temperature is used to measure the wafer temperature in RTP. Acoustic energy is coupled to a Lamb wave mode in the wafer using the quartz support pins already present in most rapid thermal processors. The tips of the pins are sharpened to have point contact with the wafer and acoustic transducers are bonded to the other end to excite and detect acoustic waves. By measuring the pin-to-pin time of flight of Lamb waves, it is possible to monitor the wafer temperature in-situ in the 20 - 1000°C range with ±5°C accuracy. Increasing SNR to 50dB by spring loading the pins and using better electronics, it is possible to improve this figure to ±1°C. Also a modified system with multiple spring loaded pins is constructed and wafer temperature mapping is performed using tomographic reconstruction techniques. The resulting images are in good agreement with thermocouple readings and can be used for temperature control and rapid thermal processor design.

Plate Tomography with Dry Contact Lamb Wave Transducers

Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. To excite Lamb waves, mode conversion of bulk waves or photo acoustic excitation are often used. Both of these approaches suffer from the need for liquid couplant or ablation of materials to reach good signal to noise ratio. In this paper, we propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor velocity changes. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of measurements of plate thickness, and erosion/corrosion in a section of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate.

Plate thickness and transducer distance dual inversion with dry contact ultrasonic Lamb wave transducers

Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. Recent development in dry contact Lamb wave transducers enables one to efficiently excite Lamb waves without the aid of liquid couplant. However, in order to perform thickness measurement in a plate-like structures, the transducer to transducer distance has to be fixed and known to a good accuracy, which places strict mechanical requirements during actual implementation. In this paper, we propose a novel technique that utilizes spectrum analysis on the receiver signal and invert for both the plate thickness and transducer to transducer distance. A pair of pin transducers are used to excite and detect the A/sub 0/ mode Lamb wave in a plate and the receiver waveform is captured for a windowed FFT. The signal phase changes with both the plate thickness and the transmitter to receiver distance, but with different sensitivity coefficients. Therefore, with proper calibration, we are able to use an optimization algorithm to perform simultaneous thickness and distance inversion. With the relaxed requirement for known transducer to transducer distance, this technique extends the advantages of the dry contact transducers with added implementation flexibility.