Abstract:
Methods and systems for control and monitoring processing of semiconductor materials with a focused laser beam. Laser light may be focused on a sample to excite optical emission at the sample surface during processing, which may include laser processing. Optical emission spectra produced may be analyzed for various properties effectively during the process. For example, process effects such as chemical composition analysis, species concentration, depth profiling, homogeneity characterization and mapping, purity, and reactivity may be monitored by optical spectral analysis. The wavelength may be selected to be appropriate for the process effect chosen.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 11/673,306, entitled “FOCUSED LASER BEAM PROCESSING”, filed on Feb. 9, 2007. 
    
    
     BACKGROUND 
     1. Field of Invention 
     This disclosure generally relates to process characterization, monitoring and control of semiconductor substrates with a focused laser beam in conjunction with laser and other processing methods. 
     2. Related Art 
     A number of techniques are available for chemical analysis of in situ semiconductor processes that incorporate some form of spectroscopy to assess species concentrations affecting the process. While each may have particular advantages, there are also disadvantages whereby incorporation of the method in situ is difficult or not possible. In many cases, evacuated chamber low pressure operation is required. In addition, many such processes are destructive of at least portions of the substrate, and therefore may not enable comprehensive mapping of process characterization. For example, inductively coupled plasma mass spectroscopy (ICP-MS) requires low pressure plasma discharge to enable mass spectroscopy. Glow discharge mass spectroscopy (GDMS) also requires low pressure. In addition, the sample must either be conductive or requires a conductive coating, which complicates process analysis and fabrication. Sputtering Optical emission spectroscopy (SOES) requires low pressure operation to sputter material for depth profiling. Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), and X-ray photoelectron spectroscopy (XPS) also all require low pressure for material sputtering and mass spectroscopy. The analytical sensor systems mentioned above all require complex low pressure environments and involve some form of mass transfer (i.e., sputtering, mass spectroscopy) or X-ray production equipment. All-optical techniques may be easier to implement, requiring access to the process environment only through transparent windows or via optical fiber, and therefore are not required to be vacuum-compatible. Additionally, an optical method of spectroscopy that does not also require some form of excitation beyond that which results in the normal course of processing would be advantageous. 
     Focused laser beams have found applications in drilling, scribing, and cutting of semiconductor wafers, such as silicon. Marking and scribing of non-semiconductor materials, such as printed circuit boards and product labels are additional common applications of focused laser beams. Micro-electromechanical systems (MEMS) devices are laser machined to provide channels, pockets, and through features (holes) with laser spot sizes down to 5 μm and positioning resolution of 1 μm. Channels and pockets allow the device to flex. All such processes rely on a significant rise in the temperature of the material in a region highly localized at the laser beam point of focus. 
     The foregoing applications, however, are all, to some degree, destructive, and relate generally to focused laser beams at power densities intended to ablate material. Thus, there is a need to provide and control laser beams to achieve process monitoring for electronic and/or optical device fabrication on semiconductor wafers that are non-destructive, and which do not interfere with, or are compatible with other laser-based and/or non-laser manufacturing processes. 
     SUMMARY 
     Methods and systems of characterization and/or monitoring semiconductor material and device processing with focused laser beams are disclosed. Specifically, in accordance with an embodiment of the disclosure, a method of monitoring the processing of semiconductor substrates, materials and devices includes providing a laser beam of a selected wavelength and a selected peak power. The laser beam may be continuous (CW) to a selected average power or may be modulated to provide pulses of a discrete time pulse width. The laser beam is focused at the surface plane of the semiconductor material. The total energy in each laser pulse is controlled to a selected value. The laser beam is scanned over the surface of the semiconductor material in a programmed pattern. The laser beam may be focused at a specific depth beneath the surface of the substrate to preferably monitor process effects at said depth. Process monitoring is accomplished by illuminating the substrate and collecting scattered optical emission that includes spectroscopic information descriptive of the composition of the substrate material. The method may be used to monitor and control the process. The method may further be integrated with laser-based processing methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of a galvanometric beam scanner, in accordance with an embodiment of the disclosure. 
         FIG. 2  is a diagram of the method of processing semiconductor materials and devices in accordance with an embodiment of the disclosure. 
         FIG. 3  is an illustration of the effects of laser pulse width in accordance with an embodiment of the disclosure. 
         FIG. 4  is an illustration of scanning and stepping in accordance with an embodiment of the disclosure. 
         FIG. 5  is an illustration of a substrate processing station in accordance with an embodiment of the disclosure. 
         FIG. 6  is an illustration of a laser optical spectroscopy system in accordance with an embodiment of the disclosure. 
         FIG. 7  is an illustration of a laser optical spectroscopy system in accordance with an embodiment of the disclosure. 
         FIG. 8  is a diagram of the method of controlling the processing of semiconductor materials and devices on the basis of optical emission spectroscopy in accordance with an embodiment of the disclosure. 
         FIG. 9  is an example of optical emission spectra obtained in accordance with an embodiment of the disclosure. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  is a drawing of a galvanometric beam substrate scanning system  100  for directing a focused laser beam at a semiconductor substrate during a processing step of device manufacturing. A collimated laser beam  10  is directed to a first mirror galvanometer  20  configured to scan laser beam  10 , for example, in an axial direction about a first axis. Laser beam  10  is then directed toward a second mirror galvanometer  30  configured to scan laser beam  10 , for example, in an axial direction about a second axis, which is perpendicular to the first axis. The effect of the two galvanometer mirrors  20  and  30  is to scan laser beam  10  in perpendicular X and Y directions in the plane of a semiconductor substrate  60 . Laser beam  10  is directed by the combination of mirror galvanometers  20  and  30  through a flat field focusing lens  40 . The function of flat field lens  40  is to bring laser beam  10  to a focused spot  50  at the surface of semiconductor substrate  60  with minimum distortion of the focused beam across the entire area to be scanned. Lens  40  may be a single lens or, alternatively a compound system of lenses configured to accomplish the same objective. Programmable controls (not shown) of mirror galvanometers  20  and  30  then position focused spot  50  at specified locations on substrate  60 . 
       FIG. 2  is a diagram of the method of laser processing  200  semiconductor materials and devices in accordance with an embodiment of the disclosure. Laser beam  10 , as provided in block  210 , may be selected to have a wavelength appropriate to the process application. For many such possible applications, the wavelength may range, for example, from 140 nanometers to 3 microns; however, wavelengths beyond this range may be useful for some processes. Laser beam  10  may be continuous or, alternatively, it may be pulsed. Regardless, laser beam  10  may require a selected peak power to meet the requirements of a particular process application. 
     For the required application, laser beam  10  may be appropriately modulated (block  220 ). Modulation may include providing pulses of laser light where the pulse width may range from approximately 10 femtoseconds to approximately 100 milliseconds, depending on the process application. A pulse repetition rate may be selected to provide laser energy to the surface of the semiconductor substrate at a selected average power and peak pulse power. The selected average power is generally the product of the pulse width times the peak power times the fraction corresponding to a selected duty cycle, assuming the peak power is constant over the length of the pulse. The duty cycle is the percentage corresponding to the fraction of the pulse width divided by the period corresponding to the pulse repetition rate, where the period is the inverse of the pulse repetition rate. 
     Laser beam  10  may then be focused (block  230 ) to a preferred beam diameter at a focal plane containing semiconductor substrate  60  with flat field lens  40 . Depending on the application, the preferred beam diameter may range from approximately 0.1 micron to 1 millimeter. The “spot” size is dependent on the wave length, the lens aperture, and the optical configuration of the lens relative to the substrate. Lens  40 , or in the case of a compound lens optical system, is shown disposed between the system of mirror galvanometers  20  and  30 , but may also be disposed elsewhere in the optical beam system. 
     The beam diameter may be defined in a variety of ways, all of which may substantially serve as definitions of beam diameter. For a circular beam having a Gaussian profile of intensity, one typical definition specifies the beam diameter according to the radial distance from the beam center at which the power density drops to 1/e 2  of the power density at the beam center, where e is the natural logarithm base. Another definition, for example, where the intensity of a circular beam is substantially constant over the aperture of the spot size, is the radial distance at which the power density drops to a given percentage of the central power density, such as 50% or 10%. Other definitions of beam diameter may also be acceptable, in accordance with the embodiment of the disclosure. The ultimate requirement is to provide sufficient thermal heating in a highly localized region of the semiconductor substrate or sample, at the surface and to a controlled but sufficient depth, to produce the desired process effect. 
     The total energy in a single laser pulse is generally the product of the peak power and the pulse width, assuming the power is constant over the pulse width. The total energy in a single laser pulse may be controlled (block  240 ) by selecting a combination of peak power and pulse width. A typical range of total pulse energy may extend from approximately 1 micro-Joule to 1 Joule, but various process applications may require higher or lower total impulse energy. It is worth noting, as a matter of practicality, when peak laser energy is too low, the thermal conductivity of the semiconductor substrate and any fixture supporting it may result in a negligible rise in temperature. Therefore, peak laser power must be able to overcome thermal conductivity effects to the extent sufficient for the process application. A discussion of the effect of laser pulse width is included below. 
     Laser beam  10 , is directed to scan (block  250 ) substrate  60  by actuation of mirror galvanometers  20  and  30 . The area scanned may range from 7840 nanometer 2 —on the order of a single focused laser spot  50 —to about 400 cm 2 , potentially the entire area of substrate  60 . Scanning may occur over one section of substrate  60  at a time, and may be repeated as necessary, or it may occur over the entire substrate in a single programmed scanning path. It may be advantageous to scan a limited segment area of substrate  60  and then reposition substrate  60 , with the aid of a substrate processing station (described below) adapted to translate the location of substrate  60  for a successive scan of another area. In this way, distortions of the optical beam, and consequent degradation of focused spot  50  resulting from large angular excursion that may be required of mirror galvanometers  20  and  30  may be avoided by restricting the scanned field of view required, thereby improving accuracy and uniformity of process performance. Between scanning segments, laser beam  10  may be blocked or otherwise terminated so as not to cause any process effects to occur on substrate  60  in undesirable locations. Alternatively, an entire substrate may be scanned by simultaneous combination of beam scanning and substrate translation. 
     A large variety of process effects may be accomplished (block  260 ) using laser beam  10  as focused spot  50 . These may include annealing, implant activation, dopant diffusion control, deposition, thin film formation, chemical reaction, curing, baking, and other forms of material modification. The spatial extent to which these effects are achieved may be critically controlled by the size of focused spot  50 . 
     As an example of the effect of focused laser scanning,  FIG. 3  is an illustration of the effects of laser pulse width processing  300  on substrate  60 , and any devices being fabricated therein, in accordance with an embodiment of the disclosure. Substrate  60  may be subjected to laser pulses of various pulse widths. As an example, we consider that each pulse has the same total energy in a given repetition period and the same spot size  50 , so that longer pulses have low peak power and density and, conversely, shorter pulses have higher peak power and density. Considering that the pulse width can potentially vary by as much as 11 orders of magnitude, a considerable range of processing possibilities may exist. 
     At one extreme, the laser may be operating in continuous wave (c.w.) mode  310 . Therefore, the peak and average power may be quite low. In this case, a thermal impulse may result in a certain degree of thermal heating that may range from having a negligible effect to being sufficient to cause a process such as annealing or local melting. In the case of a laser pulse of nanosecond (ns) duration  320 , the peak power may be correspondingly higher, under the exemplary conditions being assumed. The thermal impulse produced may result in ablative removal, for example, of photo-resist or other deposited material, such as a metallic trace. In addition, the pulse width may be such that thermo-elastic effects result in producing a shock wave that couples to elastic wave generation into substrate  60  as a further means of dissipating the energy deposited by beam  10 . In the case of picosecond (ps) or femtosecond (fs) pulses  330 , the peak power may be so high and the pulse width so narrow that energy is dissipated in processes such as via drilling of narrow holes with production of ablative material, for example, to enable contact between subsurface layers of substrate  60  and top surface layers currently existing or layers deposited on substrate  60  in subsequent steps. In this case the pulse width may be too short to effectively couple significant energy into elastic waves, and the efficiency of the process effect may thereby be improved. 
       FIG. 4  is a drawing illustrating scanning and stepping  400  in accordance with an embodiment of the disclosure. Substrate  60  may contain a plurality of segments  410 . Within a single segment  410 , substrate scanning system  100  may generate a scanning path  420 , such as a raster scan, for focused spot  50  to follow, as well as control various other parameters introduced above, such as the size of focused spot  50 , pulse width, duty cycle, peak power, and total pulse energy under the direction of a processor and controller (both not shown, discussed below). Upon completion of scanning path  420  in a first one of segment  410 , substrate  60  may be repositioned, i.e., stepped, to locate a second one of segment  410  in the optimal center of the field of view of scanning system  100  and the process repeated, until all selected segments  410  have been scanned and the process effect achieved in each selected segment  410 . A stepping path  430  may be provided by the substrate processing station to position substrate  60  accordingly. Stepping path  430  may be an approximate raster scan, as shown, or other suitable stepping or scanning pattern. 
       FIG. 5  is an illustration of an exemplary substrate processing station  500  in accordance with an embodiment of the disclosure. Substrate processing station  500  includes substrate scanning system  100  and a sample handling system  501 . Substrate processing station  500  further includes a sample handler  570 , such as a robot arm, for example, a sample delivery cassette system  580 , and a sample retrieval cassette system  590 . Sample handler  570  acquires substrate  60  from delivery cassette system  580  and places substrate  60  on a substrate stage  505 . Substrate stage  505  may be enabled to align substrate  60 , or alternatively, an additional substrate alignment/orientation stage (not shown) may be provided separately in sample handling system  501 . Alignment/orientation comprises X and Y translation in the plane of substrate stage  505 , Z translation normal to the plane of stage  505 , and rotation θ about the axis perpendicular to the X-Y plane. Z translation provides motion to enable change of position of substrate  60  relative to lens  40 , such as for positioning focus spot  50  above, at or below the surface of substrate  60 . Alternatively, lens  40  may be translatable in the Z direction, by means of piezo-motor or other well known mechanical translation stages. Sample handler  570  may also provide for transferring substrate  60  from the alignment stage to substrate stage  505 . After substrate processing, substrate  60  is transferred by sample handler  570  from stage  505  to retrieval cassette system  590 . Sample handling system  501  components including sample handler  570 , delivery cassette system  580 , and retrieval cassette system  590 , may further be coupled to, and supervised by, a processor  573  and a controller  575 . Alignment of substrate  60  may be performed on stage  505 , or, alternatively, on a separate sample aligner (not shown) included in sample handling system  501 . Sample handler  570  performs sample transport operations, including moving substrate  60  from delivery cassette system  580  to substrate scanning system  100 , and then to retrieval cassette system  590 . Details of a related sample handling system may be found in commonly-owned U.S. Pat. No. 6,568,899, entitled “Wafer Processing System Including a Robot”, which is incorporated by reference in its entirety. 
       FIG. 6  is an illustration of a laser optical spectroscopy system  600  in accordance with an embodiment of the disclosure. System  600  includes laser beam  10  directed to substrate  60  via flat field lens  40  or similar objective focusing optics to provide focused spot  50 . Substrate  60  is disposed on a substrate stage  505 , which is adapted to provide X, and Y movement in the plane of stage  505 , Z translation normal to stage  505 , and angular rotation θ of substrate  60  about the axis normal to the substrate. Alternatively, lens  40  ( FIG. 1 ) may be translated by means of a piezo-motor or other well know types of mechanical translation stages to provide the same effective Z relative translation between substrate  60  and focus spot  50 . In addition, goniometric orientation capability (not shown), may be provided to permit orientation of substrate  60  to be either normal or non-normal to laser beam  10 . 
     Z translation enables positioning of substrate  60  relative to focus spot  50  above, at or below the surface of substrate  60 . For example, if beam  10  is of sufficient energy, and focused to spot  50  at a depth beneath the surface of substrate  60  that gradually increases, material from substrate  60  may be ablated as a function of depth. A portion of light emission from focused spot  50  on substrate  60  resulting from illumination from laser beam  10  is directed by collecting optics  693  to the input of a spectrometer  695  for spectral analysis of the emitted light components. If, for example, the collected light results from optical emission as focused spot  50  from light beam  10  ablates material at increasing depth beneath the surface of substrate, optical emission spectra may be obtained as a function of depth, which provides subsurface material characterization information. 
     Collecting optics  693  may include a combination of lenses and mirrors (not shown) and may further include filters and other standard optical components to provide the collected light emission to the spectrometer. Collecting optics  693  may further include optical fiber (not shown) and associated optical components to perform the same or equivalent functions. System  600  may further include interfaces between controller  575 , processor  573 , and spectrometer  695 . 
       FIG. 7  is an illustration of another laser optical spectroscopy system in accordance with an embodiment of the disclosure. System  700  is generally similar to system  600  with the following exceptions. A laser  701 , which may be single wavelength or multi-wavelength, may be CW. Alternatively, laser  701  may be pulsed, with a specific pulse width and duty cycle. Pulse formation may be achieved using a Q switch  702  configured with laser  701 . System  700  further differs from system  600  in at least that laser beam  10  is provided to substrate  60  by means of a beam splitter/combiner  703 , such as a cube prism or a partially reflecting mirror, to direct beam  10  through objective (or flat field) lens optics  40  (which may be equipped with Z translation, as described above) to focused spot  50  at or relative to the surface of substrate  60 . In system  700 , in contrast to system  600 , light emitted from substrate  60  is collected through the same optics  40 , is at least partially transmitted through beam splitter/combiner  703 , and is directed to spectrometer  595  by means of at least one mirror  704  and focusing optics  705 . This arrangement may be beneficial as a more compact arrangement of optical components. Note that substrate  60  can be or comprise a semiconductor material, a conductor, or an insulator. 
       FIG. 8  is a diagram of the method of controlling the processing of semiconductor materials and devices on the basis of optical emission spectroscopy in accordance with an embodiment of the disclosure. Method  800  is generally similar to method  200  with the following exceptions. Light spectra emitted during a beam scanning  850  are collected  870  and analyzed  880  in spectrometer  595 . Based on the analysis obtained, processor  573  and controller  575  may adjust process parameters  890  accordingly to optimize the desired results. This process can be iterative or continuous to continually adjust parameters or monitor and maintain desired settings. The laser can be adjusted such that spectra from a desired depth of the substrate or device is analyzed. As a result, properties at specific locations and depths of the substrate can be analyzed and process parameters adjusted if needed. Note that in the embodiments of  FIGS. 6-8 , the laser beam does not need to be scanned in a pattern or otherwise. A simple spot illumination can be used to obtain emission spectra from a single spot on the substrate. 
     An example of optical emission spectroscopy is shown in  FIG. 9 . Optical spectra of samples of Al, Si, and AlSi are superimposed in the same graph of normalized intensity (where each trace is offset for viewing convenience. Measurements were made under ambient atmospheric conditions. Laser beam  10  was initially CW, but an optical modulator provided a delivered beam of 1064 nm wavelength light with pulse widths approximately from 2 to 50 nanoseconds, at a duty cycle approximately between 5% and 50%. Laser pulse power was between 50 and 100 kW. The spot size may vary from approximately 1 to 40 micrometers. Since the measurement was for the purpose of depth profiling, there was no scanning of the substrate area, and the spot location was stationary. The focal length varied from 100 mm to 400 mm, with a shorter focal length producing a smaller spot size and shorter depth of field focus. Signal-to-noise in the spectral emission signal is improved by integrating the signal over time, particularly with modulation spectroscopy, which is the case here. Typical integration times may be from approximately 1 ms to 100 ms. 
     Referring to  FIG. 9 , as material is removed from the surface, the spectral emission will change as the atomic or molecular species that emit radiation changes. For example, the lower trace of  FIG. 9  provides the spectrum of Al, which may be the result of an ablative removal of aluminum from a portion of a substrate. When the ablative removal of Al on a Si substrate is complete, the spectrum obtained will look like the middle trace, i.e., a Si spectrum. Detection of the interface between the two materials is evident in the upper trace, where the spectrum descriptive of AlSi show features of both atomic species, as well as possible additional, more complex structure. 
     In implementations, the above described techniques and their variations may be implemented at least partially as computer software instructions operational in processor  573  and controller  575 . Such instructions may be stored on one or more machine-readable storage media or devices and are executed by, e.g., one or more computer processors, or cause the machine, to perform the described functions and operations. Processor  573  may generate scripts to control all components of exemplary substrate processing station  500 , optical emission system  600  or  700 . For example, the script may generate a set of scanning path  420  commands within segment  410 , and an X-Y translation and/or θ rotation commands to substrate stage  505 , typically via controller  575 . Furthermore, focused laser beam processing may be accomplished by a simultaneous combination of operations of substrate scanning system  100  and substrate stage  505  under direction from processor  573 . Additional control processes may be contemplated within the scope of the disclosure. The laser beam can be from a conventional laser scriber or any suitable laser system. 
     Also, only those claims which use the word “means” are intended to be interpreted under 35 USC 112, sixth paragraph. Moreover, no limitations from the specification are intended to be read into any claims, unless those limitations are expressly included in the claims. Accordingly, other embodiments are within the scope of the following claims. For example, the above describes certain methods for laser scanning; however, any suitable laser beam scanning mechanism may be used including vibrating mirrors, rotating mirrors, galvo mirror systems, and/or piezo micro position control systems.