METHOD AND APPARATUS FOR DETECTING THERMAL WAVES

Abstract: Measurement of the change in reflectivity of the sample surface is a function of the changing surface temperature. The apparatus includes a laser radiation probe (52) that is directed on a portion of a sample (22) which is periodically heated by another laser beam (34). A photodetector (56) is aligned to sense the intensity changes in the reflected radiation probe which results from the periodic heating. The photodetector output signals are processed in a processor (58) to detect the presence of thermal waves. The probe (52) passes through a polarising splitter (54) and a quarter wave plate (55) to a dichoic mirror (36) to be reflected through an objective (38) to the centre of the heated area. The reflected probe beam for the sample (22) returns to the splitter (54) where it is reflected to the photodetector (54). The heating beam (34) is modulated by a modulator (32) and the processor (58) includes filters to select the modulation frequency.

Description: The invention relates to apparatus for analyzing thermal parameters in a sample, the apparatus comprising means for generating thermal waves by periodic heating at a spot on a surface of the sample, a radiation source for emitting a probe beam of radiation, means for directing the probe beam to a spot within a portion of the periodically heated area on the sample surface in a manner such that the probe beam reflects off the sample surface, and means for detecting the reflected probe beam. Such an apparatus is known from EP-A-97 473. There is presently a significant amount of research being conducted in the field of thermal wave microscopy. In thermal wave microscopy, a periodic heat source is focused on the surface of a sample. The heat source is typically either an intensity modulated laser beam or a stream of particles, such as an electron beam. When the sample absorbs the incident energy at or near the sample surface, a periodic surface heating results which, in turn, generates thermal waves that propagate from the irradiated spot. These thermal waves have the same frequency as the beam modulation frequency. The wavelength of the thermal waves is determined both by the modulation frequency of the beam and by the thermal parameters of the sample. In a thermal wave microscope, thermal features beneath the sample surface are detected and imaged by sensing the thermal waves that scatter and reflect from these features. The thermal waves are so highly damped that they travel only one or two wavelengths before becoming too weak to detect. Nevertheless, several different methods have been developed capable of sensing and measuring the thermal waves generated in the sample. One known method of detection includes the sensing of acoustic waves which are generated by the thermal waves. Acoustic waves are generated because the thermal waves induce stress-strain oscillations in the heated region of the sample. These elastic waves are true propagating waves and can be detected with conventional ultrasonic transducers, as disclosed in US-A 4,255,971. This known technique utilizes a piezoelectric crystal and is a "contact" technique requiring the attachment of the transducer to the sample. The latter requirement is time-consuming and potentially contaminating and is not suitable for production situations encountered in the semiconductor industry. Accordingly, there has been significant work carried out in developing noncontact detection techniques. A noncontact detection technique is described in our European patent applications having publications nos 0102730 (published 14.3.84) and 0124224 (priority date 1.4.83; published 7.11.84), which disclose a method and apparatus for detecting thermal waves by monitoring the local angular changes occurring at the surface of the sample. More specifically, when thermal waves are generated in a localized region of the sample, the surface of the sample undergoes periodic angular changes within the periodically heated area because of local thermoelastic effects. These angular changes occur at a frequency equal to the modulation frequency of the heating beam. To monitor these changes, a beam of energy, such as a laser beam, is focused on the surface of the sample in a manner such that is reflected. Because of the local angular changes occurring at the surface of the sample, the reflected beam experiences angular displacements in a periodic fashion. By measuring the angular displacements, information about the thermal wave activity in the sample can be determined. The latter technique has proved to be a highly sensitive process for detecting thermal waves. Our copending application EP-A-163466 is concerned with the combination of detection of thermal waves that are primarily a function of the surface temperature with detection of thermal waves that are primarily a function of the integral of the temperature beneath the sample surface, and our copending application EP-A-162681 is concerned with an invention that is characterised by the combination of detection of the power of the reflected probe beam and determination of the periodic changes in optical reflectivity with focussing both a modulated laser irradiating beam and a radiation probe beam both to a spot size of about 1 micrometer and substantially in coincidence on the surface of the sample. In an article entitled "Thermal-wave detection and thin-film thickness measurements with laser beam deflection, at pages 3169 to 3176 in issue No. 20 of Volume 22, Applied Optics published in 1983 in New York, USA, J. Opsal, A. Rosencwaig and D.L. Willenborg describe the apparatus and the theory for thermoelastic deformation of the surface of a sample resulting from focussing of a acousto-optically modulated Argon laser beam to a 2 to 4 micrometer spot on the sample surface. Measurements are made of the deflection of an He-Ne laser probe beam focussed to a spot having its centre about 2 micrometers from the centre of the heating beam spot. The apparatus described is an example of the kind defined hereinbefore at the beginning. The periodic deflections of the probe beam are measured by a bicell photodetector. Another example of the use of a laser to heat a sample in order to obtain information about the sample is described at page 42 of issue No. 6, Volume 16 (1980) of Laser Focus, under the heading "Yag-Laser inspection of solder joints would speed processing of pc boards". This known technique makes use of radiation emitted by the joint after it has been heated by the laser. The radiation is detected by an infra-red sensor that generates a thermal signature signal for analysis to determine whether the joint is good. It is implied that the thermal signature depends on the reflection, absorption, and emission characteristics of the sample, and on its thermal mass. According to the present invention, apparatus of the kind defined hereinbefore at the beginning is characterised in that the detecting means measures changes in the optical reflectivity of the sample surface by detecting variations in the power of the reflected probe beam resulting from, and at the frequency of, the said periodic heating, and means are provided for so processing the measured power variations of the reflected probe beam, which are induced by the said thermal waves, as to analyze thermal parameters in the sample. The invention also provides a method of analyzing thermal parameters in a sample, the method comprising the steps of generating thermal waves in the sample by periodic heating at a spot on a surface of the sample, emitting a probe beam from a radiation source, directing the probe beam to a spot within a portion of the periodically heated area on the sample surface in such a manner that the probe beam reflects off the sample surface, and detecting the reflected probe beam, characterised in that the step of detecting the reflected probe beam includes measuring changes in the optical reflectivity of the sample surface by detecting variations in the power of the reflected probe beam resulting from, and at the frequency of, the said periodic heating, and so processing the measured power variations of the reflected probe beam, which are induced by the said thermal waves, as to analyze thermal parameters in the sample. The invention is based on the principle that the changes in optical reflectivity of a sample, occurring as it is periodically heated, will vary, depending on the thermal characteristics of the sample. It has been known that optical reflectivity is dependent, to some extent, on temperature. This dependence is defined by the following equation: (1) R T = R o + (∂R/∂T) (ΔT) In this equation, R o represents the reflectivity at a set temperature and the second term in the equation gives the change of reflectivity resulting from the change in surface temperature. The term (∂R/∂T) is the temperature coefficient of reflectivity which represents the rate of change in reflectivity with respect to the change in temperature. The term ΔT is the change in temperature at the sample surface. The first term R o is at least four orders of magnitude greater than the second term for temperature changes, ΔT of less than 100 o . Furthermore, the noise level associated with R o as measured with a photodetector, is on the order of √ R o . The latter value is still 100 times greater than the second term of the equation which makes measurement of the second term quite difficult. In absolute terms, the value of the ratio (∂R/∂T) (ΔT/R o is on the order of 10⁻⁴ to 10⁻⁵ and, therefore, has not been used as a measurement parameter. In embodiments of the present invention, this difficulty is overcome by modulating the heating source. Periodic changes in reflectivity which are occurring at the frequency of the modulation beam are then monitored. In a preferred embodiment this information is processed by passing the signal through narrow bandwidth filters. The result is that only the periodic reflectivity signal ΔR T , as a result of the periodic temperature variations ΔT, is measured, rather than absolute reflectivity R T . The periodic reflectivity signal Δ R T is defined by the following equation: (2) Δ R T = (∂R/∂T) (ΔT) As seen from the above equation, the periodic reflectivity signal Δ R T is dependent of the temperature coefficient of reflectivity (∂R/∂T) times the periodic surface temperature ΔT. The periodic reflectivity signal ΔR T thus provides a measure of the periodic surface temperature ΔT. The periodic surface temperature, in turn, provides information about thermal wave propagation and interaction in the material. Thus, with suitable mathematical equations, one can determine the thermal wave activity based on the measured changes in reflectivity. Calculation of the thermal parameters is carried out by normalizing the signals against a known reference sample. The invention will now be described by way of example with reference to the accompanying drawings in which:- Brief Description of the Drawings Figure 1 is a composite block and schematic diagram of the apparatus for carrying out the detection of thermal waves in accordance with the subject invention. Figure 2 is a graphical representation comparing the available signal strength in both reflectivity and deflection-type thermal wave detection systems, measured as a function of the distance on the sample surface from the heating source. Best Mode For Carrying Out The Invention Referring to Figure 1, there is illustrated the apparatus 20 for carrying out the method of the subject invention. A sample 22 is shown resting on a platform 24. Platform 24 is capable of movement in two orthogonal directions in a manner such that the sample can be rastered with respect to the heating and probe beams of the subject invention. Controllable stages are well known in the art and also disclosed in U.S. Patent No. 4,255,971, cited above. As illustrated in Figure 1, a means is shown for generating thermal waves. This means is defined by laser 30 which is intensity modulated by modulator 32. In the preferred embodiment, beam 34 is focused on the surface of the sample by a microscopic objective 38. Beam 34 is intended to create a periodic surface heating at the sample surface. This periodic heating is the source of thermal waves that propagate outwardly from the center of the beam. The thermal waves interact with thermal boundaries and barriers in a manner that is mathematically equivalent to scattering and reflection of conventional propagating waves. Any features on or beneath the surface of the sample that have thermal characteristics different from their surroundings will reflect and scatter thermal waves and thus become visible to these thermal waves. The intensity modulated heating source could be supplied by electromagnetic radiation at various wavelengths, including X-rays, gamma rays, infrared, ultraviolet, visible light, microwaves or radio frequencies. The intensity modulated source can also be generated through thermal excitations arising from the interaction of the sample with an intensity modulated stream of particles, such as a beam of electrons, protons, neutrons, ions or molecules. However, because of the ease of directing and focusing a laser beam, it is believed that the illustrated embodiment is preferable. The intensity modulated beam 34 is passed through dichroic mirror 36 prior to passing through the microscopic objective 38. In the preferred embodiment, the heating beam is an argon ion laser and the dichroic mirror is transparent to argon ion radiation. As will be discussed below, the dichroic mirror functions to reflect the measuring laser beam, which is preferably generated by a helium-neon laser. In accordance with the subject invention, a new and improved method and apparatus is provided for detecting the thermal waves which are being generated in the sample. The detection system includes a light probe which is directed on the surface of the sample that has been periodically heated by the modulated energy beam 34. In the illustrated embodiment, the light probe is generated by helium-neon laser 50. Various other sources of electromagnetic radiation may be used for the probe beam as long as the beam reflectivity is affected by the temperature changes on the sample surface in a manner which can be measured. Probe beam 52, emanating from the helium-neon laser 50, is then passed through a polarizing splitter 54. The polarizing splitter is oriented in a manner such as to let the coherent light emanating from laser 50 to pass freely therethrough. The splitter will, however, deflect all light whose phase has been rotated through 90° relative to beam 52. The reason for this arrangement will become apparent below. Light probe 52 is then passed through a 1/4λ - waveplate 55. Waveplate 55 functions to rotate the phase of the probe beam by 45°. As can be appreciated, on the return path of the beam, the waveplate will rotate the phase of the beam another 45° so that when it reaches splitter 54 the phase of the beam will have been rotated a total of 90° from the incoming orientation. By this arrangement, the splitter 54 will deflect the retro-flected light beam up to detector 56, as discussed in more detail below. After the probe beam 52 initially passes through waveplate 55, it is reflected downwardly by dichroic mirror 36. As pointed out above, the dichroic mirror is transparent to argon ion light but will reflect the light rays in the helium-neon frequencies. In the preferred embodiment, the heating beam and the probe beam are aligned in such a manner that they are directed in a coincident manner down through lens 38 and focused at the same spot on the surface of the sample. By focusing the probe beam and the heating beam at the same spot, the maximum signal output can be achieved. It is to be understood that the reflectivity signals of interest exist at any areas on the surface of the sample which has been periodically heated by the beam 34. Therefore, the probe beam does not have to be directly coincident with the heating beam 34 to detect the signals of interest. Accordingly, a microscope objective is not necessary for focusing either the heating beam 34 or the probe beam 52. Rather, it is only necessary to direct the probe beam within at least a portion of the area periodically heated by beam 34. A discussion and equations for calculating the size of the periodically heated area are set forth in EP-A-102730. Briefly, the diameter of the heated area, which extends radially away from the center of the heating beam, is a function of the modulation frequency and the diameter of the heating beam and of the thermal parameters of the sample. Because the signals to be measured are so small, on the order of 10⁻⁵ of the DC level of the probe beam, every effort should be made to maximize the output for detection. Accordingly, it is desirable to direct the probe beam essentially coincident with the heating beam. Direction of the probe beam can be accomplished by movement of mirror 36. The alignment of the probe beam with the heating beam should be contrasted with the measuring technique described in EP-A-102730. wherein the probe beam is preferably directed off center from the heating beam but within the periodically heated area. As set forth in detail in the latter specification, the probe beam is intended to measure angular changes in the surface of the sample. However, the surface of the sample at the center of the heating beam undergoes only vertical movements. The angular surface changes occur in areas on the surface spaced from the center of the heating beam. A graphical representation of the signal strength available for detection by these two techniques, as a function of the distance on the sample surface from the heating source, is illustrated in Figure 2. In that figure, the horizontal axis indicates the distance away from that central heating point C on the surface of the sample. The vertical axis is a measure of available signal strength. Curve 70 represents signals available in the deflection detection technique, while curve 72 illustrates signal strength available with the subject reflectivity detection technique. As will be seen from curve 70, the maximum output signals measurable in the deflection technique are at a minimum adjacent the center of the heating beam. The signals increase at positions located radially outwardly from the center and then taper off towards the border of the periodically heated area. The actual dimensions of the periodically heated area can be calculated by the equations set forth in the prior application. In contrast, curve 72 indicates that the reflectivity output signal is maximized when the probe beam is centered on the heating beam. From Figure 2 it should be apparent that in the subject technique, maximum signal output can be achieved by focusing the probe beam to be coincident with the heating laser beam. As the probe beam is reflected off the surface of the sample, it interacts with the electrons and thus with the lattice structure of the sample at its surface. The lattice structure of the sample will undergo periodic changes as the temperature of the sample changes periodically. The probe beam essentially "sees" the changes of this lattice structure and the level of the intensity of the beam changes along with the changing thermal conditions of the sample surface. The probe beam is then reflected back up to the dichroic mirror where it is, in turn, reflected back along the incoming path and through the 1/4λ-waveplate 55. As discussed above, waveplate 55 rotates the phase of the probe beam by another 45° such that when the beam reaches splitter 54, its phase has been rotated 90° with respect to the original beam. Accordingly, this splitter will deflect the retro-reflected probe beam upwardly towards detector 56. Since power variations of a radiation beam are to be detected, a standard photodetector may be employed as a sensing mechanism. The power variations which are measured are then supplied as an output signal to a processor for deriving the data on the thermal waves based on the changing surface temperature conditions as indicated by the changing output signal. The operation of processor 58 is dependent on the type of testing configuration which is utilized. In all cases, the processor is designed to evaluate the power changes of the incoming probe beam which are the result of the periodic reflectivity changes caused by the periodic heating on the sample. These periodic power changes are filtered to produce a signal which may be evaluated. The derivation of thermal wave signals from the periodic reflectivity signal is carried out by normalizing either the phase or magnitude of the measured signal. These normalized values are then compared to normalized values taken from a known reference sample. Calculations of this general type are discussed in "Thermal Wave Depth Profiling: Theory" by Jon Opsal and Allan Rosencwaig, Journal of Applied Physics , June, 1982. The calculations set forth in the latter article are based on detection techniques which measure an output signal that is a function of the integral of the temperature beneath the sample surface. As discussed above, the subject detection system measures an output signal which is primarily a function of the surface temperature and therefore the calculations must be modified accordingly. Once the thermal wave information is derived, an analysis can be performed to provide significant information about a sample. Various types of thermal wave analysis are set forth in EP-A-97473. For example, an evaluation can be made of the thickness of thin film layers. In addition depth profiling varying thermal parameters is possible. Referring again to Figure 1, a controllable stage 24 is provided to simplify the movement of the sample with respect to the heating and probe beams. By this arrangement, a two-dimensional thermal wave image may be readily generated. In the alternative, in a manufacturing setting, point testing may be utilized to evaluate if a particular fabrication step has been successful. Thermal wave analysis is particularly suited for the evaluation of integrated circuits. In summary, there has been disclosed a new and improved method and apparatus for detecting thermal waves which have been generated in a sample by a periodic localized heating at a spot on the sample surface. The subject invention includes a radiation probe which is directed onto a portion of the area which has been periodically heated, in a manner such that it reflects off the surface of the sample. A means is provided for measuring the power variations of the reflected radiation probe resulting from the periodic heating. A processing means is provided for analyzing the power variations of the reflected radiation probe to detect the presence of thermal waves. While the subject invention has been described with reference to a preferred embodiment, various other changes and modifications could be made therein, by one skilled in the art, without varying from the scope of the subject invention as defined by the appended claims.

Claims: Apparatus for analyzing thermal parameters in a sample, the apparatus comprising means (30, 32) for generating thermal waves by periodic heating at a spot on a surface of the sample (22), a radiation source (50) for emitting a probe beam (52) of radiation, means (36, 38) for directing the probe beam (52) to a spot within a portion of the periodically heated area on the sample surface in a manner such that the probe beam (52) reflects off the sample surface, and means (54, 56) for detecting the reflected probe beam, characterised in that the detecting means (54, 56) measures changes in the optical reflectivity of the sample surface by detecting variations in the power of the reflected probe beam resulting from, and at the frequency of, the said periodic heating, and means (58) are provided for so processing the measured power variations of the reflected probe beam, which are induced by the said thermal waves, as to analyze thermal parameters in the sample (22). A method of analyzing thermal parameters in a sample, the method comprising the steps of generating thermal waves in the sample (22) by periodic heating at a spot on a surface of the sample (22), emitting a probe beam (52) from a radiation source (50), directing (36, 38) the probe beam (52) to a spot within a portion of the periodically heated area on the sample surface in such a manner that the probe beam (52) reflects off the sample surface, and detecting (54, 56) the reflected probe beam, characterised in that the step of detecting the reflected probe beam includes measuring changes in the optical reflectivity of the sample surface by detecting variations in the power of the reflected probe beam resulting from, and at the frequency of, the said periodic heating, and so processing (58) the measured power variations of the reflected probe beam, which are induced by the said thermal waves, as to analyze thermal parameters in the sample (22).

Inventors: OPSAL, JON, ROSENCWAIG, ALLAN, SMITH, WALTER LEE, WILLENBORG, DAVID L.
Applicants: THERMA-WAVE INC.
Application year: 1985
Main classification: G01N25
IPC codes: G01N29, G01N21, G01N25, G01J5