Acoustic Temperature and Film Thickness Monitor and Method

This invention relates generally to an acoustic temperature and film thickness monitor and method and more particularly to an acoustic temperature and film thickness monitor and method for measuring the temperature and deposited film thickness of a semiconductor wafer during processing, such as rapid thermal processing, evaporation, ion etching, etc.

In rapid thermal processing of semiconductor wafers a wafer is supported in a processing chamber by a plurality of pointed quartz pins, usually three. The low thermal conductivity of the quartz as well as the small contact area at the support point minimizes heat transfer from the wafer into the support rods thereby assuring uniform wafer temperature. The wafer is rapidly heated by thermal radiation from a single or an array of tungsten-halogen lamps which are arranged to provide uniform temperature throughout the wafer.

It is extremely important that the wafer temperature be continually monitored during wafer processing. It is also desirable to monitor the thickness of films which are deposited as the wafer surface during processing. An ideal temperature and thickness sensor has the following characteristics: fast response time to provide accurate and repeatable real time temperature independent of changing wafer conditions, must not disturb the wafer temperature distribution, be insensitive to wafer processing gases and pressures, and inexpensive.

Different temperature dependent physical phenomena have been used to measure the temperature of wafers during processing. One of the most popular temperature sensor, based on lattice vibrations, is the thermocouple. Although the thermocouple is fairly accurate, its must contact the wafer. There are several problems with the contacting technique. First, thermocouples are metallic and serves as heat sinks on the wafer producing temperature non-uniformities. The thermocouple material contacting the wafer contaminates the wafer. Finally, unless the thermocouple is welded or bonded onto the wafer it is difficult to ensure a reliable contact. Thus, thermocouples, although used extensively for calibration purposes are generally not used during actual processing. The most widely used temperature measurement technique in rapid thermal processing is pyrometry. Pyrometry, however, has the significant limitation that its measurements are strongly dependent on the emissivity of the wafer. The emissivity of the wafer is dependent on several factors including film thickness, surface roughness, surface material, and temperature and is difficult predict. Other method of temperature measurement based on emission (black body radiation) are multi-wavelength pyrometry, infrared spectral correlation, and ellipsometry. Another temperature dependent phenomena is thermal expansion. In one such temperature measurement technique, a grating is placed on the wafer and the moire pattern created by the interference of a laser light grating and an etched grating is studied to measure the local thermal expansion, and thus the temperature. Thermal expansion of the thickness of the wafer and the length of the wafer are also used to measure the temperature. The wide range of temperature measurement schemes reflects the fact that temperature monitoring and control is currently a very important issue in semiconductor processing.

It is an object of this invention to provide an acoustic temperature and film thickness monitor and method for monitoring semiconductor wafer temperature and/or deposited film thickness during processing.

It is another object of the invention to provide an acoustic temperature and/or film thickness monitor and method which employs Lamb waves.

It is another object of the invention to provide an acoustic temperature and/or film thickness monitor in which Lamb waves are excited in the semiconductor wafer through the quartz supports which support the wafer in the processing chamber.

It is another object of the invention to provide an acoustic temperature and/or film thickness monitor which employs Lamb waves excited in the wafer at two different frequencies.

It is another object of the invention to provide an acoustic temperature monitor and method which has a fast response and provides repeatable temperature in real time.

The foregoing and other object of the invention are achieved by exciting acoustic waves in a wafer being processed and measuring the time of flight or velocity of the acoustic waves as they travel through the wafer and converting the time of flight or velocity measurement into temperature and/or film thickness.

A wafer temperature monitor for monitoring the temperature of a wafer as it is being thermally processed including means for generating in said wafer acoustic waves so that the waves propagate through said wafer, means for providing an output signal representative of the time of flight or velocity of said Lamb waves as they propagate through the wafer and means for convening the time of flight or velocity signal to temperature and/or film thickness.

The foregoing and other objects of the invention will be clearly understood from the following description when read in conjunction with the drawings of which:

FIG. 1 schematically show a wafer supported by quartz rods in the processing chamber of rapid thermal processing equipment;

FIG. 2 is a top plan view of the wafer, wafer guides and wafer supports;

FIG. 3 is an enlarged view of the bottom of a quartz support rod showing an acoustic transducer for exciting extensional acoustic waves in the rod which induce acoustic waves in the wafer;

FIG. 4 is an enlarged view of a quartz support rod showing an acoustic transducer which receives extensional acoustic waves induced in the quartz support rod by acoustic waves in the supported wafer;

FIG. 5 shows an anti-symmetric Lamb wave;

FIG. 6 shows the output of the transducers and the time of flight measurement;

FIG. 7 is a schematic diagram of one embodiment of time of flight measurement electronics;

FIG. 8 shows a calibration measurement of acoustic time delay and temperature as measured by a thermocouple at difference lamp powers;

FIG. 9 shows the measured temperature dependence of Lamb wave velocity;

FIG. 10 shows an actual temperature measurement during rapid thermal processing compared to the temperature taken by a thermocouple;

FIG. 11 shows another time of flight measurement technique; and,

FIG. 12 shows multiple transmitting and receiving rods for obtaining a temperature profile of the wafer by using tomographic techniques.

Referring to FIGS. 1 and 2, there is shown a wafer 11 supported in a processing chamber 12 by quartz rods 13 a-c. The wafer is held in position by spaced guides 14 a-c. The chamber 12 includes a window 16 through which thermal radiation 17 projects. The thermal radiation may for example be supplied by tungsten-halogen lamps. The radiation rapidly heats the wafer as it impinges on the surface of the wafer. The quartz support rods 12 include pointed tips 18 which provide minimal contact area to the wafer, thereby minimizing the contacting effect of the quartz pins.

A PZT longitudinal or shear wave acoustic transducer is bonded at the end of spaced quartz support rods. Referring particularly to FIGS. 1 and 3, the transducers 19 are of predetermined thickness which determines the frequency of acoustic waves and have metalization 21 and 22 for application of electrical voltage thereacross to generate the acoustic waves. Preferably, the lower end of the corresponding quartz rod 12 is metalized on the bottom and along the side edge as shown at 23, whereby electrical connection can be easily made to the spaced metalization 21 and 22. Voltage applied across the metalization causes mechanical displacement which launches longitudinal or shear acoustic waves at a frequency dependent on the transducer thickness along the length of a quartz pin. The longitudinal or shear wave in a quartz pin is coupled into the silicon wafer via the sharp point 18 at the end of the quartz rod. The sharp point localizes the point of excitation allowing for a more controlled coupling of acoustic energy into the wafer. The sharp tip also has the effect of amplifying the displacement caused by the longitudinal or shear wave.

If a low frequency transducer is employed, Lamb waves are excited in the wafer. If a high frequency transducer is employed, longitudinal, or shear waves are propagated through the pin. At high frequency the thickness of the wafer is several wavelengths of the excited waves. Thus surface, longitudinal and shear waves are excited separately. A continuation of these waves at low frequency excite Lamb waves. As a numerical example, at 100 mHz, the wavelengths are 50 microns, 55 microns and 85 microns for the surface, shear and longitudinal waves, respectively. Thus, these waves can be excited separately. All of the acoustic waves have velocities which are dependent on temperature. Only surface and Lamb or plate waves are affected by film thickness. The velocity is measured by measuring the time it takes the particular acoustic wave to travel from the exciting pin to a receiving pin.

Among the various acoustic modes generated in the silicon wafer by the quartz pin is the zero order anti-symmetric Lamb wave illustrated in FIG. 5. The Lamb wave propagates along the plane of the silicon wafer, and its velocity dispersion characteristics are a function of wafer bulk temperature. For any frequency of excitation, the velocity of a Lamb wave will vary with temperature, and for any temperature, the velocity of the Lamb wave will vary with the frequency of the transmitted wave. The Lamb wave travels across the wafer from one quartz rod to a spaced quartz rod. The Lamb wave is coupled to the spaced quartz rod and generates a longitudinal wave in that quartz rod. The resulting displacement at the end of the pin is converted into a voltage by the associated transducer 19. The same excitation and detection takes place for the other acoustic waves.

Referring to FIG. 6, a pulse 26 is shown applied to the transmitting transducer, the pulse generates a longitudinal wave of predetermined frequency in the associated quartz rod. As described above the longitudinal wave is coupled to the wafer to generate a Lamb wave in the wafer; however, the wave is also reflected and is therefore detected by the transmitting transducer which provides an echo signal 27. The Lamb wave travels to the spaced rod and generates a longitudinal wave 28 in the other spaced quartz rod. The time of flight of the Lamb wave in the wafer is obtained by subtracting the time it takes the longitudinal wave to travel in the two spaced quartz rods from the total time between the transmit pulse of the first transducer and the output of the second transducer. Assuming the rods are of equal length, this time is equal to the time represented between the transmit pulse and its echo output. Therefore the time of flight is obtained by counting the time between like excursions of the waves 27 and 28, FIG. 6. Zero crossing of the pulse echo signal is used to start a time interval counter and the zero crossing on the time of flight signal is used to stop the counter thereby providing the time interval. In a second method two separate time measurements are made. One for the time between transmit and pulse echo and the other for the time between the pulse and the output signal. The pulse echo time is subtracted from the total time of flight in a computer.

Referring to FIG. 7, the implementation of the latter method is illustrated. The delay generator 31 provides a trigger signal to the pulse receiver 32 which generates pulses to excite the transducer at one of the quartz rods. At the same time it enables the time interval counter 33. The echo signal stops the time interval counter 33 which provides the time interval between time of initiation of the pulse and reception of the echo signal. The delay generator 31 also activates the time interval counter 34 which is turned off when a signal is received from the transducer 19 associated with the rod 12. These outputs are applied to a computer which subtracts the two signals. The computer 36 is also connected to a thermocouple readout 38 which can be used to calibrate the monitor.

Referring the FIG. 8, a wafer is irradiated at different lamp powers to elevate its temperature to different levels. The time delay obtained by the time interval counter is then calibrated, whereby, during processing, the delay in microseconds provides the reading of temperature. The computer is provided with either a look-up table which correlates delay to temperature, or a polynomial, such as in FIG. 9, which relates velocity to temperature. Each of these can be obtained from a calibration run and used to convert time of flight into temperature.

In one example, the monitor was tested and the results shown in FIG. 10 were obtained. This shows a good correlation between acoustic thermometry and thermocouple measurements.

Referring to FIG. 11, the Lamb wave delay or velocity can be obtained by the use of beams 41, 42 from laser 43, 44 such as helium-neon lasers. The beams 41, 42, focused at two locations 46, 47 at the surface of the wafer. The angular reflection of helium-neon beam is modulated by the Lamb waves in the wafer. The position of the reflected beam is measured by bi-cell detectors 48, 49. The bi-cell detectors provide output signals as the Lamb wave passes locations 46, 47. The signals can be processed electronically to obtain the time delay which can then be used to convert the time of flight to a temperature by a look of table or a polynomial.

A CW sinusoidal wave may be generated by the transmit transducer. By keeping the propagation characteristics and the quartz rods constant and measuring the phase and amplitude difference between the input to the transmit transducer and the output of the receiver transducer the phase changes in the Lamb waves propagating in the wafer can be measured. The phase change can be used to derive the velocity of the Lamb wave and in turn temperature or film thickness measurements. Because the phase detection scheme relies on measuring the differences in phase and amplitude rather than measuring the absolute value, the initial temperature or film thickness needs to be known.

A combination of the two techniques can be used. The pulse echo technique can be used to measure the initial temperature or film thickness prior to processing. During processing when detection time becomes crucial the phase and amplitude technique can be used.

The surface wave in particular can be used to measure temperature and film thickness in the same fashion as we used the Lamb waves. The dependence of the surface wave velocity on temperature and film thickness is different from that of the Lamb waves; however, these functions can be evaluated and used to find temperature and thickness.

The quartz pins can be spring loaded and made to clamp the wafer against stops protruding in from above the edges of the wafer. With spring loading, reliable Hertzian contacts between the quartz tips and the wafer can be made at more than three points. FIG. 12 shows a plurality of contact points 51 on the periphery of the wafer. With a large number of quartz pins located at the perimeter of the wafer, temperature mapping of the wafer will be possible using tomography techniques. One transmit pin 52 will launch Lamb waves which fan out from the point 52 and are received by a plurality of receive pins 53 located near the edges of the wafer. Because the Lamb waves will travel different areas of the wafer which will be at different temperatures, the velocity of the Lamb waves (adjusting for anisotropy) will vary. By transmitting the acoustic wave which fans out from sequential transducer pins, and by receiving the fan beam, a plurality of velocity measurements are obtained. This information can be processed by a computer to provide a tomographic temperature map of the wafer.

In many applications, it is not only important to monitor the temperature of the wafer, but it may also be important to monitor the thickness of films which are being grown on the surface of the wafer. The velocity of the Lamb waves in the wafer depends on temperature, frequency and thickness: V=F[temperature(T), thickness(h) & frequency(f)]. Thus, transducers attached to different pins are selected of different thicknesses to operate at different frequencies. With two measurements of velocity V.sub.1 and V.sub.2, we have two equations: V.sub.1 =F(T, h, f.sub.1) and V.sub.2 =F(T, h, f.sub.2). Since f.sub.1 and f.sub.2 are known, the equations can be solved for temperature (T) and thickness (h). Using the tomographic technique described above with two frequencies, the temperature and surface profile of the wafer can be obtained.

The pin--pin method of coupling and receiving ultrasonic waves can be used for material characterization. For instance, one pin could send and the other receive signals due to plate modes in a wafer and the velocity of the acoustic wave is measured. Longitudinal and shear waves in the bulk can also be used to determine material constants in a sample. Contact between the pins and samples could be removed, the same rotated, and then contact reestablished and a new measurement of the speed of the acoustic wave made. This process is repeated to acquire a measure of the wave velocity as a function of angle. Thus, the anisotropy is measured. This measurement could be used with other modes of propagation to measure surface wave and bulk wave properties of samples. In this fashion, this technique competes well (no water necessary) with acoustic microscope measurements of material properties.

As described above, a cone point contact is used to launch the acoustic waves in the material. A wedge can be used as a line source of acoustic waves instead of a point source. The waves from such a source would be more directional than the fan shape from a point.

Thus, there has been described a novel method of measuring parameters of wafers, such as temperature and thickness during processing and for providing profiles of such parameters.