Abstract:
A plasma reactor for etching a workpiece such as a rectangular or square mask, includes a vacuum chamber having a ceiling and a sidewall and a workpiece support pedestal within the chamber including a cathode having a surface for supporting a workpiece, the surface comprising plural respective zones, the respective zones of the surface being formed of respective materials of different electrical characteristics. The zones can be arranged concentrically relative to an axis of symmetry of the wafer support pedestal.

Description:
BACKGROUND 
     Fabrication of photolithographic masks for use in processing of ultra large scale integrated (ULSI) semiconductor wafers requires a much higher degree of etch uniformity than semiconductor wafer processing. A single mask pattern generally occupies a four inch square area on a quartz mask. The image of the mask pattern is focused down to the area of a single die (a one inch square) on the wafer and is then stepped across the wafer, forming a single image for each die. Prior to etching the mask pattern into the quartz mask, the mask pattern is written in photoresist by a scanning electron beam, a time consuming process which makes the cost of the mask very high. The mask etch process is not uniform across the surface of the mask. Moreover, the e-beam written photoresist pattern is itself non-uniform, and exhibits, in the case of 45 nm feature sizes on the wafer, as much as 2-3 nm variation in critical dimension (e.g., line width) across the entire mask. (This variation is the 3σ variance of all measured line widths, for example.) Such non-uniformities in photoresist critical dimension typically varies among different mask sources or customers. In order to meet current requirements, the mask etch process must not increase this variation by more than 1 nm, so that the variation in the etched mask pattern cannot exceed 3-4 nm. These stringent requirements arise from the use of diffraction effects in the quartz mask pattern to achieve sharp images on the wafer. It is difficult to meet such requirements with current technology. It will be even more difficult for future technologies, which may involve 22 nm wafer feature sizes. This difficulty is compounded by the phenomenon of etch bias, in which the depletion of the photoresist pattern during mask etch causes a reduction in line width (critical dimension) in the etched pattern on the quartz mask. These difficulties are inherent in the mask etch process because the etch selectivity of typical mask materials (e.g., quartz, chrome, molybdenum silicide) relative to photoresist is typically less than one, so that the mask photoresist pattern is etched during the mask etch process. 
     Some mask patterns require etching periodic openings into the quartz mask by a precisely defined depth that is critical to achieving the extremely fine phase alignment of interfering light beams during exposure of the wafer through the mask. For example, in one type of phase shift mask, each line is defined by a chrome line with thin quartz lines exposed on each side of the chrome line, the quartz line on one side being etched to a precise depth that provides a 180 degree phase shift of the light relative to light passing through the un-etched quartz line on the other side of the chrome line. In order to precisely control the etch depth in the quartz, the etch process must be closely monitored by periodically interrupting it to measure the etch depth in the quartz. Each such inspection requires removing the mask from the mask etch reactor chamber, removing the photoresist, measuring the etch depth and then estimating the etch process time remaining to reach the target depth based upon the elapsed etch process time, depositing new photoresist, e-beam writing the mask pattern on the resist, re-introducing the mask into the mask etch chamber and restarting the etch process. The estimate of remaining etch time to reach the desired depth assumes that the etch rate remains stable and uniform, and therefore is an unreliable estimate. The problems of such a cumbersome procedure include low productivity and high cost as well as increased opportunity for introduction of contamination or faults in the photoresist pattern. However, because of the requirement for an accurately controlled etch depth, there has seemed to be no way around such problems. 
     The small tolerance in critical dimension variation requires extremely uniform distribution of etch rate over the mask surface. In masks requiring precise etch depth in the quartz material, there are two critical dimensions, one being the line width and the other being the etch depth. Uniformity of both types of critical dimension requires a uniform etch rate distribution across the mask. Non-uniformity in etch rate distribution can be reduced to some extent by employing a source power applicator that can vary the radial distribution of the plasma ion density, such as an inductive source power applicator consisting of inner and outer coil antennas overlying the wafer. Such an approach, however, can only address non-uniformities that are symmetrical, namely a center-high or a center-low etch rate distribution. In practice, non-uniformities in etch rate distribution can be non-symmetrical, such as a high etch rate in one corner of the mask, for example. A more fundamental limitation is that the mask etch process tends to have such an extremely center-low distribution of etch rate that a tunable feature, such an inductive power applicator having inner and outer coils, is incapable of transforming the etch rate distribution out of the center-low regime. 
     Another problem with non-uniform etch rate distribution is that the etch rate distribution tends to vary widely among different reactors of the same design and can vary widely within the same reactor whenever a key part or a consumable component is replaced, such as replacement of the cathode. The etch rate distribution appears to be highly sensitive to small variations in features of the replaced part, with unpredictable changes upon consumable replacement. 
     SUMMARY OF THE INVENTION 
     A plasma reactor for etching a workpiece such as a rectangular or square mask is provided. In one aspect, the reactor includes a vacuum chamber having a ceiling and a sidewall and a workpiece support pedestal within the chamber including a cathode having a surface for supporting a workpiece. The surface comprises plural respective zones, each being formed of respective materials of different electrical characteristics. The zones can be arranged concentrically relative to an axis of symmetry of the wafer support pedestal. In one embodiment, an inner zone comprises a conductor material and an annular outer zone comprises an insulator. In another embodiment, the zones are of different insulator materials of different electrical permittivities. 
     In another aspect, the cathode and an underlying facilities plate are formed of a metal. The cathode has a bottom surface and the facilities plate has a top surface facing the bottom surface of the cathode, and they are fastened together by screws of a dissimilar metal. In order to reduce RF non-uniformities at the screw heads, a thin ring layer is provided between the cathode and the facilities plate formed of the dissimilar metal and being located at the periphery of the cathode and facilities plate. In order to improve uniformity of conductivity between the cathode and the plate, a highly conductive coating is provided on the periphery of facing surfaces of the cathode and plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  depicts a plasma reactor for carrying out a mask etch process. 
         FIG. 2A  depicts a lower portion of the reactor of  FIG. 1 . 
         FIG. 2B  illustrates a mask support pedestal of the reactor of  FIG. 1  in a raised position. 
         FIG. 3  is a top view of a cathode of the reactor of  FIG. 1 . 
         FIGS. 4 and 5  are top and side views of one alternative embodiment of the cathode. 
         FIGS. 6 and 7  are top and side views of another alternative embodiment of the cathode. 
         FIG. 8  is a simplified diagram of a plasma reactor having a backside end point detection apparatus. 
         FIGS. 9 and 10  are graphs of an optical end point detection signal obtained from the front side and back side, respectively, of the mask. 
         FIGS. 11 and 12  are graphs of an interference fringe optical signal obtained from the front side and back side, respectively, of the mask. 
         FIG. 13  is a graph of a multiple wavelength interference spectrum signal obtained in one embodiment of the reactor of  FIG. 8 . 
         FIG. 14  illustrates an embodiment of the reactor of  FIG. 8  with backside end point detection based upon overall reflected light intensity, corresponding to  FIG. 10 . 
         FIG. 15  illustrates an embodiment of the reactor of  FIG. 8  with backside endpoint detection based upon interference fringe counting, corresponding to  FIG. 12 . 
         FIG. 16  illustrates an embodiment of the reactor of  FIG. 8  with backside endpoint detection based upon multiple wavelength interference spectrometry. 
         FIG. 17  illustrates an embodiment of the reactor of  FIG. 8  with backside endpoint detection based upon optical emission spectrometry (OES). 
         FIG. 18  illustrates a working example having both OES and interference-based backside endpoint detection. 
         FIGS. 19 and 20  are perspective view of the cathode and facilities plate, respectively, of the embodiment of  FIG. 18 . 
         FIG. 21  is a cross-sectional view of the cathode of  FIG. 19 . 
         FIGS. 22A and 22B  depict a sequence of steps in a quartz mask etch process employing backside endpoint detection. 
         FIGS. 23A ,  23 B,  23 C,  23 D and  23 E depict a sequence of steps in a chrome-molysilicide-quartz mask etch process employing backside endpoint detection. 
         FIGS. 24A ,  24 B,  24 C,  24 D and  24 E depict a sequence of steps in a chrome-quartz mask etch process employing backside endpoint detection. 
         FIGS. 25 and 26  are side and top views, respectively, of an embodiment in which real time etch rate distribution is continuously measured from the mask backside. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Cathode with Enhanced RF Uniformity: 
     We have discovered that one source of non-uniform etch rate distribution in mask etch processes is the existence of RF electrical non-uniformities in the support pedestal or cathode holding the mask in the plasma reactor in which the mask etch process is carried out. RF bias power is applied to the pedestal to control plasma ion energy at the mask surface, while RF source power is applied to an overhead coil antenna, for example, to generate plasma ions. The RF bias power controls the electric field at the mask surface that affects the ion energy. Since the ion energy at the mask surface affects the etch rate, RF electrical non-uniformities in the pedestal create non-uniformities in the distribution of etch rate across the mask surface. We have discovered that there are several sources of RF non-uniformity in the pedestal. One is the titanium screws that fasten the aluminum pedestal (cathode) and aluminum facilities plate together. The screws create nodes in the electric field pattern across the surface of the pedestal (and therefore across the surface of the mask because their electrical properties differ from that of the aluminum cathode. Another is the non-uniform distribution of conductivity between the cathode and the facilities plate. Electrical conduction between the facilities plate and the cathode is confined primarily to the perimeter of the plate and cathode. This can be due at least in part to bowing of the cathode during plasma processing induced by vacuum pressure. The conduction around this perimeter can be non-uniform due to a number of factors, such as uneven tightening of the titanium screws and/or surface finish variations around the perimeter of either the plate or the pedestal. We have solved these problems by the introduction of several features that enhance RF electrical uniformity across the pedestal. First, the non-uniformities or discontinuities in the RF field arising from the presence of the titanium screws in the aluminum cathode are addressed by providing a continuous titanium ring extending around the perimeter of the top surface of the cathode that encompasses the heads of all the titanium screws. Variations in conductivity due surface differences or uneven tightening of the titanium screws are addressed by providing highly conductive nickel plating on the facing perimeter surfaces of the facilities plate and the cathode, and by the introduction of an RF gasket between the facilities plate and the cathode that is compressed between them at their perimeter. 
     Referring to  FIG. 1 , a plasma reactor for etching patterns in a mask includes a vacuum chamber  10  enclosed by a side wall  12  and an overlying ceiling  14  and is evacuated by a vacuum pump  15  that controls chamber pressure. A mask support pedestal  16  inside the chamber  10  supports a mask  18 . As will be described later in this specification, the mask typically consists of a quartz substrate and can further include additional mask thin film layers on the top surface of the quartz substrate, such as chrome and molybdenum silicide. In addition, a pattern-defining layer is present, which may be photoresist or a hardmask formed of the chrome layer. In other types of masks, the quartz substrate has no overlying layers except for the photoresist pattern. 
     Plasma source power is applied by overlying inner and outer coil antennas  20 ,  22  driven by respective RF source power generators  24 ,  26  through respective RF impedance match circuits  28 ,  30 . While the sidewall  12  may be aluminum or other metal coupled to ground, the ceiling  14  is typically an insulating material that permits inductive coupling of RF power from the coil antennas  20 ,  22  into the chamber  10 . Process gas is introduced through evenly spaced injection nozzles  32  in the top of the side wall  12  through a gas manifold  34  from a gas panel  36 . The gas panel  36  may consist of different gas supplies  38  coupled through respective valves or mass flow controllers  40  to an output valve or mass flow controller  42  coupled to the manifold  34 . 
     The mask support pedestal  16  consists of a metal (e.g., aluminum) cathode  44  supported on a metal (e.g., aluminum) facilities plate  46 . The cathode  44  has internal coolant or heating fluid flow passages (not shown) that are fed and evacuated by supply and drain ports (not shown) in the facilities plate  46 . RF bias power is applied to the facilities plate by an RF bias power generator  48  through an RF impedance match circuit  50 . The RF bias power is conducted across the interface between the facilities plate  46  and the cathode  44  to the top surface of the cathode  44 . The cathode  44  has a central plateau  44   a  upon which the square quartz mask or substrate  18  is supported. The plateau dimensions generally match the dimensions of the mask  18 , although the plateau  44   a  is slightly smaller so that a small portion or lip  18   a  of the mask perimeter extends a short distance beyond the plateau  44   a , as will be discussed below. A pedestal ring  52  surrounding the plateau  44   a  is divided (in wedge or pie section fashion as shown in  FIG. 2B  or  FIG. 7 ) into a cover ring  52   a  forming about two-fifths of the ring  52  and a capture ring  52   b  forming the remaining three-fifths of the ring  52 . The capture ring  52   b  has a shelf  54  in which the lip  18   a  of the mask  18  rests. Three lifts pins  56  (only one of which is visible in the view of  FIG. 1 ) lift the capture ring  52   b , which raises the mask  18  by the lip  18   a  whenever it is desired to remove the mask  18  from the support pedestal  16 . The pedestal ring  52  consists of layers  53 ,  55  of materials of different electrical characteristics selected to match the RF impedance presented by the combination of the quartz mask  18  and the aluminum plateau  44   a , at the frequency of the bias power generator  48 . (Both the cover and capture rings  52   a ,  52   b  consist of the different layers  53 ,  55 .) Moreover, the top surface of the capture ring  52  is coplanar with the top surface of the mask  18 , so that a large uniform surface extending beyond the edge of the mask  18  promotes a uniform electric field and sheath voltage across the surface of the mask  18  during plasma processing. Typically, these conditions are met if the lower ring layer  55  is quartz and the upper ring layer  53  is a ceramic such as alumina. A process controller  60  controls the gas panel  36 , the RF generators  24 ,  26 ,  48 , and wafer handling apparatus  61 . The wafer handing apparatus can include a lift servo  62  coupled to the lift pins  56 , a robot blade arm  63  and a slit valve  64  in the side wall  12  of the chamber  10 . 
     A series of evenly spaced titanium screws  70  fasten the cathode  44  and facilities plate  46  together along their perimeters. Because of the electrical dissimilarities between the aluminum cathode/facilities plate  44 ,  46  and the titanium screws  70 , the screws  70  introduce discrete non-uniformities into the RF electrical field at the top surface of the cathode  44 . Variations in the opposing surfaces of the cathode  44  and facilities plate  46  create non-uniformities in the conductivity between the cathode  44  and facilities plate  46  along their perimeter, which introduces corresponding non-uniformities in the RF electrical field. Because the cathode  44  tends to bow up at its center during plasma processing (due to the chamber vacuum), the principal electrical contact between the cathode  44  and the facilities plate  46  is along their perimeters. In order to reduce the sensitivity of the electrical conductivity between the cathode  44  and facilities plate  46  to (a) variations in tightness among the various titanium screws  70  and (b) variations in surface characteristics, an annular thin film  72  of a highly conductive material such as nickel is deposited on the perimeter of the bottom surface  44   b  of the cathode  44 , while a matching annular thin film  74  of nickel (for example) is deposited on the perimeter of the top surface  46   a  of the facilities plate  46 . The nickel films  72 ,  72  are in mutual alignment, so that the two annular nickel thin films  72 ,  74  constitute the opposing contacting surfaces of the pedestal  44  and facilities plate  46 , providing a highly uniform distribution of electrical conductivity between them. Further improvement in uniform electrical conductivity is realized by providing an annular groove  76  along the perimeter of the bottom surface of the cathode  44  and placing a conductive RF gasket  80  within the groove  76 . Optionally, a similar annular groove  78  in the top surface of the facilities plate  46  may be provided that is aligned with the groove  76 . The RF gasket  80  may be of a suitable conventional variety, such as a thin metal helix that is compressed as the cathode  44  and facilities plate  46  are pressed together and the screws  70  tightened. In order to reduce or eliminate the point non-uniformities in electrical field distribution tending to occur at the heads of the titanium screws  70 , a continuous titanium ring  82  is placed in an annular groove  84  in the perimeter of the top surface of the cathode  44 . 
       FIG. 2A  depicts the mask support pedestal  16  and its underlying lift assembly  90 . The lift assembly  90  includes a lift spider  92  driven by a pneumatic actuator or lift servo  94  and the three lift pins  56  resting on the lift spider  92 . The lift pins  56  are guided in lift bellows  96  that include ball bearings  98  for extremely smooth and nearly frictionless motion (to reduce contamination arising from wear).  FIG. 2B  depicts the cathode  44  with the capture ring  52   b  and mask  18  in the raised position. The void formed by separation of the cover and capture rings  52   a ,  52   b  when the mask is raised permits access by a robot blade to the mask  18 . 
     The problem of an extremely center-low etch rate distribution across the surface of the mask  18  is solved by altering the distribution of the electrical properties (e.g., electrical permittivity) of the cathode plateau  44   a . This is achieved in one embodiment by providing, on the top surface of the plateau  44   a , a center insert  102  and a surrounding outer insert  104 , the two inserts forming a continuous planar surface with the pedestal ring  52  and being of electrically different materials. For example, in order to reduce the tendency of the etch rate distribution to be extremely center-low, the center insert  102  may be of a conductive material (e.g., aluminum) while the outer insert  104  may be of an insulating material (e.g., a ceramic such as alumina). This conductive version of the center insert  102  provides a much lower impedance path for the RF current, boosting the ion energy and etch rate at the center of the mask  18 , while the insulating outer insert  104  presents a higher impedance, which reduces the etch rate at the periphery of the mask  18 . This combination improves the etch rate distribution, rendering it more nearly uniform. With this feature, fine tuning of the etch rate distribution can be performed by adjusting the relative RF power levels applied to the inner and outer coil antennas  20 ,  22 . The change in radial distribution of plasma ion density required to achieve uniform etch rate distribution is reduced to a much smaller amount which is within the capability of RF power apportionment between the inner and outer coils  20 ,  22  to attain uniform etch rate distribution.  FIG. 3  is a top view of the inner and outer inserts  102 ,  104 . In an alternative embodiment, the inserts  102 ,  104  may be insulators having different dielectric constants (electrical permittivities).  FIGS. 4 and 5  depict an elaboration upon this concept, in which four concentric rings  102 ,  104 ,  106 ,  108  of progressively different electrical properties are employed to render the etch rate distribution more uniform.  FIGS. 6 and 7  depict an alternative embodiment that provides real-time tunability of distribution of RF electrical properties of the cathode  44 . A plunger  110  controls the axial position of a movable aluminum plate  112  within a hollow cylinder  114  in the center interior of the cathode  44 . The aluminum plate  112  is in electrical contact with the remainder of the aluminum plateau  44   a . An insulator (e.g., ceramic) top film  116  can cover the top of the cathode  44 . As the aluminum plate  112  is pushed closer to the top of the cylinder  114 , the electrical impedance through the center region of the cathode  44  is reduced, thereby raising the etch rate at the center of the mask  18 . Conversely, the etch rate at the mask center is reduced as the aluminum plate  112  is moved downward in the cylinder  114  away from the mask  18 . An actuator  118  controlling axial movement of the plunger  110  can be governed by the process controller  60  ( FIG. 1 ) to adjust the etch rate distribution to maximize uniformity or compensate for non-uniformities. 
     Etch Rate Monitoring and End Point Detection Through the Mask Backside: 
     The high production cost of periodic interruptions of the etch process to measure the etch depth or critical dimension on the mask is reduced or eliminated using optical sensing through the cathode  44  and through the backside of the mask or substrate  18 . It has been necessary to interrupt the etch process to perform such periodic measurements because of the poor etch selectivity relative to photoresist: in general, the mask materials etch more slowly than the photoresist. This problem is typically addressed by depositing a thick layer of photoresist on the mask, but the high rate of etching of the resist renders the photoresist surface randomly uneven or rough. This roughness affects light passing through the photoresist and so introduces noise into any optical measurement of critical dimension or etch depth. Therefore, the photoresist is temporarily removed for each periodic measurement to ensure noise-free optical measurements, necessitating re-deposition of photoresist and re-writing of the reticle pattern into the photoresist before re-starting the interrupted mask etch process. 
     The mask etch plasma reactor depicted in  FIG. 8  avoids these difficulties and permits continuous observation of critical dimensions or measurement of etch depth during the entire etch process while the mask or substrate  18  is left in place on the mask support pedestal  16  using backside optical measurement apparatus provided within the cathode  44 . The backside measurement apparatus takes advantage of the optically transparent nature of the mask substrate  18 , which is typically quartz. The thin films that may be deposited over it (such as chrome or molybdenum silicide) may be opaque, but the formation of patterned openings defining the reticle pattern of the mask  18  can be sensed optically. The change in light intensity reflected by such layers or transmitted through such layers may be observed at the mask back side through the cathode  44 . This observation may be used to perform etch process end point detection. When etching the quartz material, optical interference observed at the mask back side through the cathode  44  may be sensed to perform etch depth measurements in real time during the etch process. One advantage is that the images or light signals sensed from the mask backside are not affected by photoresist noise, or at least are affected very little compared with attempts to perform such measurements from the top surface (photoresist side) of the mask  18 . 
     For these purposes, the reactor of  FIG. 8  includes a recess  120  within the top surface of the cathode  44  that accommodates a lens  122  whose optical axis faces the backside of the mask or substrate  18 . A pair of optical fibers  124 ,  126 , whose diameters are small relative to the lens  122 , have ends  124   a ,  126   a  close to or contacting the lens  122  and both are aligned next to each other at the optical axis of the lens  122 . Each of the optical fibers  124 ,  126  depicted in  FIG. 8  may actually be a small bundle of optical fibers. The optical fiber  124  has its other end  124   b  coupled to a light source  128 . The light source emits light of a wavelength at which the mask  18  is transparent, typically visible wavelengths for a quartz mask. In the case of interference depth measurements, the wavelength spectrum of the light source  128  is selected to facilitate local coherence in the reticle pattern of the mask  18 . For periodic features in the etched mask structure on the order of about 45 nm (or periodic feature sizes below one micron), this requirement is met if the light source  128  radiates in the visible light spectrum. The optical fiber  126  has its other end  126   b  coupled to a light receiver  130 . In the case of simple end point detection, the light receiver  130  may simply detect light intensity. In the case of critical dimension (e.g., line width) measurements, the light receiver  130  may sense the image of etched lines within the field of view of the lens  122 , from which the line width can be determined. In the case of etch depth measurements, the light receiver  130  may detect an interference pattern or interference fringes, from which the etch depth may be determined (i.e., inferred from the interference or diffraction pattern or computed from the counting of interference fringes). In other embodiments, the light receiver  130  may include a spectrometer for performing multiple wavelength interference measurements, from which etch depth may be inferred or computed. For such determinations, the process controller  60  includes an optical signal processor  132  capable of processing the optical signal from the light receiver. Such optical signal processing may involve (depending upon the particular implementation) one of the following: performing etch process end point detection from ambient light intensity changes; measuring critical dimensions from two-dimensional images sensed by the optical receiver  130 ; computing etch depth by counting interference fringes; determining etch depth from the multiple wavelength interference spectrum, in which case the optical receiver  130  consists of a spectrometer. Alternatively, such a spectrometer may be employed to perform etch process end point detection by optical emission spectrometry from the wafer backside, using light emitted by the plasma and transmitted through the transparent mask  18 , in which case the light source  128  is not employed. 
     The process controller  60  reacts to the process end point detection information (or the etch depth measurement information) from the optical signal processor  132  to control various elements of the plasma reactor, including the RF generators  24 ,  26 ,  48  and the wafer-handling apparatus  61 . Typically, the process controller  60  stops the etch process and causes removal the mask  18  from the pedestal  16  when the etch process end point is reached. 
       FIG. 9  is a graph depicting ambient reflected light intensity sensed from the top (photoresist-coated) side of the mask as a function of time during a chrome etch process (in which a chrome thin film on the quartz mask surface is etched in accordance with a mask reticle pattern). The large swings in intensity depicted in the graph of  FIG. 9  represent noise induced by roughness in the top surface of the photoresist layer. The dashed line represents a step function signal hidden within the noise, the step function coinciding with the chrome etch process end point.  FIG. 10  is a graph of the same measurement taken from the wafer backside through the cathode  44  in the reactor of  FIG. 8 , in which the light receiver  130  senses the reflected light level. The photoresist-induced noise is greatly reduced, so that the end-point defining step function is clearly represented in the optical data. The edge of the step function depicts a transition point at which reflected light intensity drops upon the etch process reaching the bottom of the chrome thin film, at which point the reflective surface area of the chrome is abruptly reduced. 
       FIGS. 11 and 12  are graphs of light intensity over time (or, equivalently, over space), and, in  FIG. 12 , as sensed by the optical receiver  130 , in which the periodic peaks in light intensity correspond to interference fringes whose spacing determines the etch depth, or difference in thickness between different surfaces of closely periodically spaced features etched in the transparent quartz mask substrate  18 .  FIG. 11  depicts the intensity sensed through the photoresist from the top side of the mask, with a heavy photoresist-induced noise component that impairs interference fringe detection.  FIG. 12  depicts the intensity sensed through the mask backside by the optical receiver  130  of  FIG. 8 , in which photoresist-induced noise is virtually absent. 
       FIG. 13  is a graph representing light intensity as a function of wavelength for the case in which the light receiver  130  consists of a spectrometer and the light source  128  produces a spectrum of wavelengths. The behavior of the intensity spectrum of the graph of  FIG. 13  is typical of a situation in which interference effects occur between light reflected from surfaces of different depths in sub-micron features that are periodically spaced in the transparent mask  18 . At the lower wavelengths, the peaks are fairly periodic and even spaced, the predominant optical effect being interference. At the higher wavelengths, local coherence among the periodic features in the mask  18  is not as strong, so that diffraction effects become increasingly significant with increasing wavelength, causing the intensity behavior at the higher wavelengths to be less evenly spaced and more complex, as depicted in  FIG. 13 . The spacing of the peaks in  FIG. 13 , particularly at the lower wavelengths, is a function of the etch depth, which may be inferred from the peak-to-peak spacing. 
       FIG. 14  illustrates an embodiment of the reactor of  FIG. 8 , in which the light receiver  130  is an ambient light intensity detector and the optical signal processor  132  is programmed to look for a large inflection (step function) in the overall reflected light intensity, corresponding to the end point detection graph of  FIG. 10 . The light source  128  in this embodiment can be any suitable light source. Alternatively, the light source  128  can be eliminated, so that the light sensor  130  simply responds to light from the plasma transmitted through the transparent mask or substrate  18 . 
       FIG. 15  illustrates an embodiment of the reactor of  FIG. 8  in which the light receiver  130  is an interference fringe detector sufficiently focused by the lens  122  to resolve interference fringes, and the optical signal processor  132  is programmed to count interference fringes (e.g., from intensity versus time data of the type illustrated in  FIG. 12 ) in order to compute etch depth in the transparent quartz mask  18 . This computation yields a virtually instantaneous etch depth, which is compared by logic  200  with a user-defined target depth stored in a memory  202 . The logic  200  can use a conventional numerical match or minimization routine to detect a match between the stored and measured depth values. A match causes the logic  200  to flag the etch end point to the process controller  60 . 
       FIG. 16  illustrates an embodiment of the reactor of  FIG. 8  which employs the interference spectroscopy technique of  FIG. 13  to measure or determine etch depth in the transparent quartz mask or substrate  18 . In this case, the light source  128  emits multiple wavelengths or a spectrum in the visible range (for periodic mask feature sizes on the order of hundreds of nanometers or less). The light receiver  130  is a spectrometer. A combination signal conditioner and analog-to-digital converter  220  converts the spectrum information collected by the spectrometer  130  (corresponding to the graph of  FIG. 13 ) into digital data which the optical signal processor  132  can handle. One mode in which end point detection can be performed is to compute the etch depth from the spacing between the periodic peaks in the lower wavelength range of the data represented by  FIG. 13 , as mentioned above. Comparison logic  200  can compare the instantaneous measured etch depth to a user-defined target depth stored in memory  202  to determine whether the etch process end point has been reached. In another mode, the comparison logic  200  is sufficiently robust to compare the digitally represented wavelength spectrum (corresponding to the graph of  FIG. 13 ) representing the instantaneous output of the spectrometer  130  with a known spectrum corresponding with the desired etch depth. This known spectrum may be stored in the memory  202 . A match between the measured spectrum and the stored spectrum, or an approximate match, detected by the comparison logic  200  results in an etch process end point flag being sent to the process controller  60 . 
       FIG. 17  illustrates an embodiment of the reactor of  FIG. 8  in which the optical receiver  130  is an optical emission spectrometer capable of differentiating emission lines from optical radiation emitted by the plasma in the chamber, to perform optical emission spectrometry (OES). The processor  132  is an OES processor that is programmed to track the strength (or detect the disappearance) of selected optical lines corresponding to chemical species indicative of the material in the layer being etched. Upon the predetermined transition (e.g., the disappearance of a chrome wavelength line in the OES spectrum during a chrome etch process), the processor  132  sends an etch process end point detection flag to the process controller  60 . 
       FIG. 18  depicts an embodiment that we have constructed, having a pair of lenses  230 ,  232  in respective spaced recesses  231 ,  233  in the surface of the cathode  44 , the lenses  230 ,  232  being focused to resolve interference fringes, the focused light being carried by respective optical fibers  234 ,  236  facing or contacting the respective lenses  230 ,  232 . The optical fibers  234 ,  236  are coupled to an interference detector  238  (which may be either a fringe detector or a spectrometer), the detector  238  having an output coupled to the process controller  60 . The lenses  230 ,  232  receive light from a light source  240  through optical fibers  242 ,  244 . This light is reflected from the top surface of the mask  18  back to the lenses  230 ,  232  and carried by the optical fibers  234 ,  236  to the detector  238 . In addition, the embodiment of  FIG. 18  has a third recess  249  in the cathode surface accommodating a third lens  250  coupled through an optical fiber  252  to the input of an OES spectrometer  254 . An OES processor  256  processes the output of the OES spectrometer  254  to perform end point detection, and transmits the results to the process controller  60 . The cathode  44  of the embodiment of  FIG. 18  is depicted in  FIG. 19 , showing the three recesses  231 ,  233 ,  249  accommodating the respective lenses  230 ,  232 ,  250 .  FIG. 20  illustrates the corresponding holes  260 ,  261 ,  262  for accommodating within the facilities plate  46  optical apparatus (not shown) supporting the lenses  230 ,  232 ,  250 .  FIG. 21  is a cross-sectional view showing the coupling of the optical fibers to the lenses inside the pedestal  16 . 
     While the reactors of  FIGS. 16 ,  17  and  18  have been described as employing spectrometers  130  ( FIGS. 16 and 17 ) and  254  ( FIG. 18 ), the spectrometer  130  or  254  may be replaced by one or more optical wavelength filters tuned to predetermined wavelengths. Each such optical wavelength filter may be combined with a photomultiplier to enhance the signal amplitude. 
     Backside End Point-Detected Mask Etch Processes: 
       FIGS. 22A and 22B  depict a process for etching a reticle pattern in the quartz material of a mask. In  FIG. 22A , a quartz mask substrate  210  has been covered with a photoresist layer  212  having a periodic structure of spaced lines  214  and openings  216  defined in the photoresist layer  212 . In the reactor of  FIG. 15  or  16 , a quartz-etching process gas of CHF3+CF4+Ar is introduced into the chamber  10 , power is applied by the RF generators  24 ,  26  and  48  and the quartz material is etched within the openings  216  formed in the photoresist layer  212 . The etch depth in the quartz is continually measured by interference between light  218  reflected from an etched top surface and light  219  reflected from an unetched top surfaces of the quartz substrate  210 . The etch process is halted as soon as the desired etch depth is reached ( FIG. 22A ). The photoresist is then removed to produce the desired mask ( FIG. 22B ). 
       FIGS. 23A through 23E  depict a process for etching a three-layer mask structure consisting of the underlying quartz mask substrate  210 , a molybdenum silicide layer  260 , (containing molybdenum oxy-silicon nitride), a chrome layer  262 , a chromium oxide anti-reflective coating  264  and a photoresist layer  266 , with openings  268  formed in the photoresist layer  266  ( FIG. 23A ). In the step of  FIG. 23B , the chrome layer  262  and the anti-reflection coating  264  are etched in a plasma reactor chamber having simple reflectance end point detection (the chamber of  FIG. 14 ) or having OES end point detection (the chamber of  FIG. 17 ) using a chrome etch process gas such as C12+O2+CF4. The photoresist layer  266  is removed ( FIG. 23C ). The molybdenum silicide layer  260  is then etched as shown in  FIG. 23D , using a process gas which is an etchant of molybdenum silicide, such as SF6+C12, and using the chrome layer  262  as a hard mask. This step is carried out in a plasma reactor having end point detection by simple ambient reflectance or by OES end point detection, such as the chamber of  FIG. 14  or  FIG. 17 . In  FIG. 23E , the chrome layer  262  and the chromium oxide anti-reflection coating  264  are removed using a chrome etching process gas such as CH3+CF4+Ar. This step can be carried out using the reactor of  FIG. 14  or  17  having simple end point detection without etch depth measurement. This leaves a quartz mask substrate with an overlying layer of molybdenum silicide defining the reticle pattern. 
       FIGS. 24A through 24E  depict a process for fabricating a binary mask consisting of periodic chrome lines on a transparent quartz mask flanking periodic spaces of exposed quartz, alternate ones of the exposed quartz spaces being etched to a depth at which transmitted light is phase-shifted by a desired angle (e.g., 180 degrees).  FIG. 24A  depicts the initial structure consisting of a quartz mask substrate  300 , a chrome layer  302 , a chromium oxide anti-reflection coating  304  and a photoresist layer  306 . In the step of  FIG. 24B , the chrome and chromium oxide layers  302 ,  304  are etched in a process gas of C12+O2+CF4 in a reactor chamber such as the chamber of  FIG. 14  or  17 . In the step of  FIG. 24C , the photoresist layer  306  is removed, after which the exposed portions of the quartz mask substrate  300  are etched as shown in  FIG. 24D  in a quartz-etching process gas of CHF3+CF4+Ar. The quartz etch step of  FIG. 24D  is carried out in a reactor chamber capable of sensing or monitoring the etch depth in the quartz mask substrate  300 , such as the chamber of  FIG. 15  or  16 . During the etch process, the instantaneous etch depth is continually monitored, and the etch process is halted as soon as the target etch depth is reached on the mask  300 . The final result is depicted in  FIG. 24E . 
     Continuous Monitoring of Etch Rate Distribution Across the Mask Surface: 
       FIGS. 25 and 26  illustrate an embodiment of the wafer support pedestal  16  of  FIG. 1  with a matrix of backside etch depth sensing elements (lenses and optical fibers) in the top surface of the cathode  44 , continuously providing an instantaneous image or sample of the etch rate distribution or etch depth distribution across the entire surface of the mask or substrate during the etch process without interrupting the etch process or otherwise disturbing the mask substrate. The aluminum plateau  44   a  has a matrix of openings  320  in its top surface, each opening holding a lens  322  facing the backside of the mask substrate  300 . A light source  324  provides light through output optical fibers  326  coupled to the respective lenses  322 . The lenses  322  provide sufficient focusing to resolve interference fringes. An interference detector  328 , which may be either a sensor that facilitates fringe counting or a spectrometer, is coupled to input optical fibers  330  coupled to the respective lenses  322 . A switch or multiplexer  332  admits light to the detector  328  from each of the input optical fibers  330  sequentially. There are three modes in which the apparatus of  FIGS. 25 and 26  may operate. In a first mode, the etch depth in the field of view of a given one of the lenses  322  is computed from the interval between interference fringes. In a second mode, the detector  328  is a spectrometer and the etch depth in the field of view of a given one of the lenses  322  is computed from the lower wavelength peak interval of the multiple wavelength interference spectrum (corresponding to  FIG. 13 ). In a third mode, the multiple wavelength interference spectrum is detected at a given instant of time and compared with a library  340  of spectra for which the corresponding etch depths are known. The etch rate distribution is computed from the etch depth and the elapsed time. This distribution records the etch nonuniformity of the process and is fed to the process controller  132 . The controller  132  can respond by adjusting tunable features of the reactor to reduce non-uniformity in the etch rate distribution. 
     While the embodiment of  FIGS. 25 and 26  is depicted as having a 3-by-3 matrix of etch depth sensors or lenses  322  in the top surface of the plateau  44   a , any number of rows and columns in the matrix of such sensors may be employed so that the matrix is an n-by-m matrix, where m and n are suitable integers. 
     In one embodiment, the process controller  132  may be programmed to deduce (from the etch rate distribution information supplied by the spectrometer or sensor  130 ) whether the etch rate distribution is center high or center low. The process controller  60  can respond to this information by adjusting certain tunable features of the reactor to decrease the non-uniformity. For example, the process controller  60  may change the RF power apportionment between the inner and outer coils  20 ,  22 . Alternatively or in addition, the process controller  60  may change the height of the movable aluminum plate  112  in the reactor of  FIGS. 6 and 7 . Feedback from the array or matrix of etch depth sensing elements in the plateau  44   a  allows the process controller  60  to improve uniformity of etch rate distribution by continuous trial and error adjustments of the reactor tunable elements. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.