Abstract:
An inventive method for optically detecting a trench depth in a wafer is disclosed. The method includes detecting a first maxima in the intensity of a multi-wavelength light source, a portion of the light being reflected from the top trench surface of a wafer. A second maxima is then detected in the intensity of the multi-wavelength light source, a portion of which being reflected from the bottom trench surface of a wafer. The method further includes determining a maxima peak difference between the first maxima and the second maxima, wherein the trench depth corresponds to the maxima peak separation. The invention provides a robust, cost effective method for trench depth detection.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the fabrication of semiconductor integrated circuits (IC&#39;s). More particularly, the present invention relates to methods and apparatuses for determining wafer trench depth. 
     One of the operations in the fabrication of IC&#39;s is the etching of trenches into the surface of silicon wafers. This etch operation is typically performed using well known photolithography and plasma etch technology. Generally, the desired depth of these trenches ranges from about 0.1 microns to 3.0 microns with control of the precise depth of the trench being an important consideration. 
     During the plasma etch process, the etch rate can vary as a function of etching variables such as the chamber component temperature, the chamber conditioning, and the wafer resist age. Improved trench depth control can result if this “process drift” can be monitored and compensated. 
     Currently, there are several well know optical techniques used to measure the depths of trenches etched into silicon, such as spectral reflectometery, monochromatic interference, laser triangulation, confocal imaging, and phase contrast. 
     While these methods are potentially usable, each of them is limited in some way by the physics and specific limitations of the techniques. For example, the interpretation of spectral reflectometery data requires either a prior knowledge of: film thickness&#39;, materials, and refractive indexes, or, complex and error prone “fitting” techniques. The inaccuracies in the optical modeling techniques used are reflected as errors in the calculated etch depth. A change or drift in the refractive index of a film could be interpreted as an error in the trench depth and an erroneous control action taken. A trench depth monitoring capability that is excessively sensitive to extraneous variables may actually result in increased variability, and therefore error rate. 
     In view of the foregoing, what are needed are improved methods and apparatuses for detecting etch trench depth. Further, the methods should be sensitive only to distance, measure an average depth over a reasonable area, and be compact, robust, and cost effective. 
     SUMMARY OF INVENTION 
     The present invention addresses these needs by providing an optically based trench depth detection method. In one embodiment, a first maxima is detected in the intensity of a multi-wavelength light source, a portion of the light being reflected from the top trench surface of a wafer. A second maxima is then detected in the intensity of the multi-wavelength light source, a portion of which being reflected from the bottom trench surface of a wafer. The method further includes determining a maxima peak difference between the first maxima and the second maxima, wherein the trench depth corresponds to the maxima peak separation. 
     In another embodiment, a system for optically detecting a trench depth is disclosed. The optical trench depth detection system includes a multi-wavelength light source for providing multi-wavelength light to a wafer. The system further includes a light detector for detecting reflected multi-wavelength light. Preferably, the light detector is configured such that it will detect a first maxima in the light intensity from the multi-wavelength light source, a portion of which being reflected from the top trench surface of the wafer. In addition, the light detector is preferably configured such that it will detect a second maxima in the light intensity from the multi-wavelength light source, a portion of which is reflected from the bottom trench surface of the wafer trench. The system is further configured such that the separation between the first maxima and the second maxima corresponds to the trench depth of the wafer trench. 
     In yet another embodiment of the present invention a method for making an integrated circuit having an optically detectable trench depth is disclosed. The method comprises introducing a substrate into a processing chamber, and creating a plasma within the chamber. A first maxima is then detected in the light intensity of multi-wavelength light, wherein a part of the light is reflected from the top trench surface of a wafer. A second maxima is then detected in the light intensity of multi-wavelength light, a part of which is reflected from the bottom trench surface of a wafer. The method further includes determining a maxima peak difference between the first maxima and the second maxima, wherein the trench depth corresponds to the maxima peak separation. The substrate is etched until the maxima peak separation corresponding to a predetermined trench depth occurs. Thereafter, the substrate is processed through a series of semiconductor processes to form the integrated circuit. 
     Advantageously, the use of a direct separation measurement technique by the present invention provides a more accurate and robust measurement than techniques using parameter sensitive models. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is an illustration showing a light model of two light paths, in accordance with an embodiment of the present invention; 
     FIG. 2 is a graph showing the intensity of monochromatic light as a function of the path length difference between the first and second light paths, in accordance with an embodiment of the present invention; 
     FIG. 3 is a graph showing the intensity of white light as a function of the path length difference between the first and second light paths, in accordance with another embodiment of the present invention; 
     FIG. 4 is an illustration showing trench depth detector system, in accordance with an embodiment of the present invention; 
     FIG. 5 is a graph showing the intensity of the white light detected by the light detector as a function of the position of the movable mirror, in accordance with an embodiment of the present invention; 
     FIG. 6A is an illustration showing a stepper motor system, in accordance with one embodiment of the present invention; 
     FIG. 6B is an illustration showing a sensor piezoelectric motor system, in accordance with another embodiment of the present invention; 
     FIG. 6C is a illustration showing a piezoelectric motor system, in accordance with another embodiment of the present invention; 
     FIG. 7 is a flowchart showing a method for optically detecting a trench depth on a wafer, in accordance with one embodiment of the present invention; and 
     FIG. 8 is a flowchart showing a method for aligning a movable mirror to maximize light intensity maxima peaks, in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     An invention is disclosed for optically detecting a trench depth in a wafer using white light. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order not to unnecessarily obscure the present invention. 
     FIG. 1 is an illustration showing a light model  10  of two light paths, in accordance with an embodiment of the present invention. The light model  10  includes an initial light path  12 , a first light path  14 , a second light path  16 , and a light detector  18 . Also included in the light model  10  are a beam splitter  20 , a first reflector  22 , and a second reflector  24 . In operation, light along the initial light path  12  is divided into light paths  14  and  16  by beam splitter  20 . Light along the first light path  14  travels to detector  18  utilizing the first reflector  22 , while light along the second light path  16  travels to detector  18  using the second reflector  24 . 
     Depending on the type of light used, light detector  18  will generate different light intensities as the lengths of the first and second light paths  14  and  16  are varied in relation to one another. FIG. 2 is a graph  30  showing the intensity of monochromatic light as a function of the path length difference between the first and second light paths  14  and  16 , in accordance with an embodiment of the present invention. Included in graph  30  are the intensities of red monochromatic light  32 , the intensities of blue monochromatic light  34 , a maxima of the intensities of the red monochromatic light  36 , and a maxima of the intensities of the blue monochromatic light  38 . 
     As shown in FIG. 2, the intensity of monochromatic light varies as the path length difference varies. However, at the point of zero path difference  40 , by definition there is a maxima in monochromatic light intensity. For monochromatic light, this maxima may be repeated at other path length differences, for example whenever the wavelength period of each path is in phase. However, only at the point of zero path difference  40  is there a maxima at all light wavelengths. 
     FIG. 3 is a graph  50  showing the intensity of white light as a function of the path length difference between the first and second light paths  14  and  16 , in accordance with an embodiment of the present invention. Included in graph  50  are the intensities of the white multi-wavelength light  52 , and a maxima of the white multi-wavelength light  54 . Since, as described above, all light wavelengths have a maxima at the point of zero path difference  40 , they all constructively interfere at the point of zero path difference  40 , thus producing maxima  54 . At any other point of path difference, some of the wavelengths will constructively interfere, while others will destructively interfere at the detector  18 . Thus, there is only one maxima  54  for the white multi-wavelength light, and it occurs at the point of zero path difference  40 . All other points along graph  50  are substantially lower in magnitude than maxima point  54 . Therefore, maxima  54  appears as an easily distinguishable sharp peak on graph  50 . 
     Because multi-wavelength white light produces an easily distinguishable sharp peak on an intensity graph when two recombined light path lengths are equal, the present invention uses multi-wavelength white light to determine trench depth. FIG. 4 is an illustration showing a trench depth detector system  60 , in accordance with an embodiment of the present invention. The trench depth detector system  60  includes a white light source  62 , a light detector  64 , a motor system  66 , a beam splitter  68 , and a movable mirror  70 . Further included in the trench depth detector system  60  is an electrostatic chuck  71 , a trench depth detector  72 , an etch control system  74 , and an etch machine  76 . Disposed above the electrostatic chuck  71  is a wafer  78  including trenches  80 . The wafer  78  further includes two surfaces, a top trench surface  82 , and a bottom trench surface  84 . 
     Distance D 1  is the distance between the beam splitter  68  and an initial location of the mirror  70 . Distance ΔD 1  is the distance between D 1  and a second location of the movable mirror  70 . Thus, distance ΔD 1  is a measure of the movement of the movable mirror  70 . Trench top distance D 2  is the distance between the beam splitter  68  and the top trench surface  82 , while trench bottom distance D 2 ′ is the distance between the beam splitter and the bottom trench surface  84 . Thus, the difference between distance D 2 ′ and distance D 2  is the trench depth. 
     In use, the white light source  62  applies white light to the wafer  78  through the beam splitter  68 . The white light then travels along two separate paths to the light detector  64 . A first path travels from the wafer  78  to the light detector  64 , while a second path travels from the mirror  70  to the light detector  64 . 
     The first and second light paths recombine at the beam splitter  68  before traveling to the light detector  64 . Thus, the distance between the beam splitter  68  and the light detector  64  is the same for both light paths. 
     When distance D 1 +ΔD 1  is essentially equal to trench top distance D 2  light at all wavelengths will constructively interfere at the light detector  64  producing a first easily distinguishable sharp peak on an intensity graph. Similarly, when D 1 +ΔD 1 ′ is essentially equal to trench bottom distance D 2 ′ light at all wavelengths will also constructively interfere at the light detector  64  producing a second easily distinguishable sharp peak on an intensity graph. Thus, by moving the mirror  70  across a range greater than or equal to the trench depth, two maximas in the intensity of the reflected white light will be detected by light detector  64 . A first maxima occurring when D 1 +ΔD 1  equals D 2 , and a second maxima occurring when D 1 +ΔD 1 ′ equals D 2 ′. 
     FIG. 5 is a graph  100  showing the intensity of the white light  101  detected by the light detector  64  as a function of the position of the movable mirror  70 , in accordance with an embodiment of the present invention. The graph  70  includes various intensities of white light  101 , a first and second intensity maxima  102  and  104 , and a peak distance  106 . Intensity maxima  102  occurs when distance D 1 +ΔD 1  essentially equals distance D 2 , while intensity maxima  104  occurs when distance D 1 +ΔD 1 ′ essentially equals distance D 2 ′. It should be borne in mind that the order of occurrence of the maxima may be interchanged, depending on the initial mirror placement. The peak distance  106  is the difference between the intensity maxima  102  and  104 . 
     The peak distance  106  corresponds to the trench depth. More particularly, the distance traveled by the mirror  70  between maxima peaks  102  and  104  is the trench depth. Thus, the trench depth can be monitored by calculating the difference between the location of the mirror  70  when maxima peak  102  occurs, and the location of the mirror  70  when maxima peak  104  occurs. Referring back to FIG. 4, the trench detector system  60  may be utilized to monitor of trench depth post etch, or to control etch time during etch. If used as to control etch time, the movement of the movable mirror  70  is monitored and analyzed by a trench depth detector  72 . Analyzed data from the trench depth detector  72  is then transmitted to an etch control system  74 , which controls an etch machine  76 . In this manner, the etch time can be automated to end when the trench depth reaches a predetermined depth. The trench depth detector  72  typically receives mirror movement data from the motor system  66 . 
     Preferably, the motor system is calibrated by measuring the white light peak difference of a known sample step height as measured in step pulses, capacitance difference, or voltage difference. The type of motor used determines which calibration measurement to use, for example, stepper motors use step pulses as a measurement of mirror movement. 
     FIG. 6A is an illustration showing a stepper motor system  66   a , in accordance with one embodiment of the present invention. The stepper motor system  66   a  includes a stepper motor  110  and rotation to translation mechanism  111 . In use, the stepper motor  110  moves a given distance, or rotates a given angle, for each input pulse, thus moving the mirror  70 . As will be apparent to those skilled in the art, the number of input pulses used between the white light intensity peaks  102  and  104  on graph  100  will correspond to the trench depth. 
     FIG. 6B is an illustration showing a sensor piezoelectric motor system  66   b , in accordance with another embodiment of the present invention. The sensor piezoelectric motor system  66   b  includes a piezoelectric motor  112 , a capacitive sensor  114 , and a voltage source  116 . The movable mirror  70  utilized with the piezoelectric motor system  66   b  is preferably a metallic front reflective movable mirror. The capacitive sensor  114  includes a capacitive plate  118  disposed behind mirror  70 , creating a mirror capacitor with the metallic front reflective movable mirror  70 . In use, the capacitive sensor  114  senses the capacitance of the mirror capacitor at varying locations of the movable mirror  70 . Thus, the difference in capacitance between the white light intensity peaks  102  and  104  on graph  100  will correspond to the trench depth. 
     FIG. 6C is a illustration showing a piezoelectric motor system  66   c , in accordance with another embodiment of the present invention. The piezoelectric motor system  66   c  includes a piezoelectric motor  112 , and a voltage source  116 . In use, the driving voltage produced by the voltage source  116  is correlated to the mirror position. More particularly, the difference between in driving voltage between the white light intensity peaks  102  and  104  on graph  100  will correspond to the trench depth. 
     FIG. 7 is a flowchart showing a method  200  for optically detecting a trench depth on a wafer, in accordance with one embodiment of the present invention. In an initial operation  202 , the wafer is prepared for trench depth detection. Wafer preparation may include, for example, placing the wafer onto an electrostatic chuck, and other pre-process operations, as will be apparent to those skilled in the art. 
     In an alignment operation  204 , the movable mirror, which is used to vary the second light path length, is aligned. The movable mirror is preferably positioned normal to the wafer surface. Mirror alignment includes rotating the movable mirror through a range to determine the best angle for light path reflection. In this manner, the white light peak maxima may be increased in magnitude to a desired level. 
     Next, in a mirror oscillation operation  206 , the movable mirror is oscillated over a range. The larger the range of movement, the larger the placement tolerance of the wafer. However, the larger the range of movement, the more movement there is to resolve into sub-micron distances, thus the detection problem increases in complexity. Thus, the range of movement is determined by how accurate the placement of the wafer may be performed without undue complexity and expense. 
     Preferably, the range of movement is at least one order of magnitude greater than the trench depth, and more preferably at least is two orders of magnitude, and most preferably at least three orders of magnitude greater than the trench depth. Generally, not more than 1 mm of movement range is required. 
     In a capture operation  208 , white light intensity maxima peaks are captured. For each oscillation of the moveable mirror, two intensity maxima peaks will occur. As described with reference to FIG. 5, one intensity maxima occurs when the path length of the light path from the mirror to the light detector essentially equals the path length of the light path from the top trench surface to the light detector. A second intensity maxima occurs when the path length of the light path from the mirror to the light detector essentially equals the path length of the light path from the bottom trench surface to the light detector. The two maxima peaks are used to calculate trench depth, subsequently. 
     The mirror is preferably moved over the range at a relatively fast speed to reduce inaccuracies do to machinery vibration. However, only movement occurring between the two maxima peaks will affect the measurement of trench depth. Movement occurring at any other time will only cause the maxima peaks to move together, and therefore the relative distance between the peaks will remain constant. Thus, if the mirror is moved at a relatively fast speed, little vibration motion will occur between the two maxima peaks. Preferably, the oscillations are greater than 50 Hz, more preferably greater than 100 Hz, and most preferably the oscillations are greater than 1000 Hz. 
     In a calculation operation  210 , the difference between the two maxima peaks, as measured in mirror movement, is calculated. The distance the movable mirror travels between the two maxima peaks directly corresponds to the trench depth. Thus, by calculating the position of the mirror at each maxima peak, the distance the mirror traveled can be determined, and therefore the trench depth can be determined. As noted above, preferably, the motor system is calibrated by measuring the white light peak difference of a known sample step height. This measurement is typically measured in step pulses, capacitance difference, or voltage difference, the motor type determining which calibration measurement to use. Thus, by pre-process calibrating of the motor system, the maxima peak difference may be more easily correlated into mirror movement. 
     To obtain a more accurate trench depth measurement, more than one measurement of trench depth is preferably obtained. Thus, a decision is then made as to whether more trench depth measurements are required, in operation  212 . If more trench depth measurements are required, the method  200  continues to operation  214 . If not, the method  200  continues to operation  216 . 
     In an averaging operation  214 , the calculated differences are averaged. The current calculated difference from operation  210  is averaged with any previous calculated differences. In this manner, a more accurate determination of trench depth may be determined because measurement inaccuracies are generally averaged out over a large sample size. The method  200  then continues with the oscillation operation  206 . 
     When there are no more trench depth measurements to be made, the method  200  continues with a final trench depth calculation operation  216 . The final trench depth is calculated to be the average trench depth when multiplied by an appropriate calibration factor, as determined by the calibration of the instrument. 
     Finally, in operation  218 , The final calculated trench depth can be used to monitor trench depth in wafer production. The final calculated trench depth can also be used to control etch time by transmitting the trench depth data to an etch control system controlling an etch machine. In this manner, the etch time can be automated to end when the trench depth reaches a predetermined depth. 
     FIG. 8 is a flowchart showing a method  204  for aligning a movable mirror to increase light intensity maxima peaks, in accordance with an embodiment of the present invention. In an initial operation  300 , the mirror and wafer are prepared for trench depth detection. Mirror and wafer preparation may include determining an initial placement of the mirror, placing the wafer onto an electrostatic chuck, and other pre-process operations, as will be apparent to those skilled in the art. 
     In a mirror rotation operation  302 , the movable mirror is rotated over a range. The rotation is performed to determine what mirror angle will result in the largest magnitude of the white light intensity maxima peaks. Higher intensity maxima peaks allow for easier trench depth detection. Thus, to increase performance of the trench depth detection system, the movable mirror is aligned to maximize the magnitude of the intensity maxima peaks. 
     In a capture operation  304 , the magnitude of a white light intensity maxima peak is captured. At this point in the trench depth detection, only the magnitude of an intensity maxima peak is required. The delta between the intensity maxima peaks is not needed at this point because the trench depth is not determined until later in the trench depth detection process, see FIG.  7 . 
     A decision is then made as to whether the latest captured peak is greater than the previous captured peak, in operation  306 . If the latest captured peak is greater than the previous captured peak, the method  204  continues with mirror rotation operation  302 . In this case, the magnitude of the intensity maxima is still increasing, indicating the maximum magnitude may not yet have been reached. If the latest captured peak is less than the previous captured peak, the method  204  continues with operation  308 . 
     In a reversing operation  308 , the direction of mirror rotation is reversed from the previous rotation direction. If, in operation  306 , the latest captured peak is less than the previous captured peak, the maximum magnitude for the intensity maxima peak has been reached. Thus, the direction of mirror rotation is reversed to re-capture the maximum magnitude for the intensity maxima peak. 
     Next, in a count operation  310 , a counter is increased by one. Every time operation  308  occurs, the maximum magnitude for the intensity maxima peak has been reached. During operation of method  204 , the mirror rotation travels back and forth across the intensity maxima peak. When this occurs a predetermined number of times, the method  204  determines that the best alignment for the moveable mirror has been reached. Thus, the method  204  tracks the number of times the intensity maxima peak has been encountered. 
     A decision is then made as to whether the counter is greater than a maximum count number, in operation  312 . If the counter is less than the maximum count number, the method  204  continues with mirror rotation operation  302 . In this case, more mirror adjustment may be necessary for to attain the maximum magnitude for the intensity maxima peak. If the counter is greater than or equal to the maximum count number, the method  204  continues with operation  314 . 
     Finally, the aligned mirror may be used for trench depth detection, in operation  314 . Once the maximum count number has been reached, essentially the best alignment for the moveable mirror has been reached, and the method  204  is complete. As mentioned previously, higher intensity maxima peaks allow for easier trench depth detection. Thus, to increase performance of the trench depth detection system, the movable mirror is aligned to maximize the magnitude of the intensity maxima peaks. 
     While the present invention has been described in terms of several preferred embodiments, there are many alterations, permutations, and equivalents which may fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.