Abstract:
A processing apparatus is presented for applying to a substantially flat workpiece contained in a cassette, and a processing tool coupled to the housing. The processing tool is displaceable along three mutually perpendicular axes relative to the cassette for inserting the tool into the gap and moving the tool inside the gap relative to the workpiece. The processing apparatus can be utilized in chemical mechanical polishing arrangement, photolithography arrangement, and CVD arrangement.

Description:
FIELD OF THE INVENTION 
     This invention is generally in the field of optical monitoring techniques, and relates to an apparatus for monitoring parameters of workpieces being contained in a cassette. The invention is particularly useful in the manufacture of semiconductor devices for metrology/inspection of wafers progressing on a production line. 
     BACKGROUND OF THE INVENTION 
     The wafers&#39; manufacture always includes the inspection of wafers after various manufacturing steps, for example after photolithography and chemical mechanical planarization (CMP). Wafers progressing on a production line between various processing tools are located in load and unload cassettes, each cassette containing a number of wafers, usually 25 of 200 mm-diameter wafers. To inspect a specific wafer prior to or after its processing by a specific tool, the wafer should be taken out of, respectively, the load or unload cassette, and brought to an inspection tool, either by an operator or by a robot depending on the kind of the inspection tool used, namely a stand-alone station or an integrated tool (i.e., located within the processing environment). 
     FIGS. 1 a  and  1   b  illustrate (by the way of a block diagram) two different examples of inspecting wafers progressing on a production line, typically employed in FABs. 
     FIG. 1 a  exemplifies the part of a production line PL (A) , for example associated with photolithography environment containing a phototrack  2 , a load cassette  3   a,  an unload cassette  3   b  and a robot R which transfers wafers between the photolithography tools. An inspection tool  4   a,  such as a Scanning Electron Microscope (SEM), is accommodated outside the production line PL (A)  as a stand-alone machine. A wafer W to be inspected is brought (handled) to the machine  4  by an operator or robot P, and may then be returned to the production line PL (A) . 
     FIG. 1 b  illustrates another part of a production line PL (B) , for example associated with a CMP tool  6 . Here, an integrated inspection tool  4   b  is used, for example ITM NovaScan 210 model commercially available from Nova Measuring Instruments Ltd, Israel. A robot R supplies a specific wafer W to the tool  4   b  and brings the wafer to the unload cassette  3   b.    
     Thus, regardless of whether the stand-alone or integrated inspection tool is used, additional handling of the wafer to be inspected is typically required. Needless to say that the handling of such delicate workpieces as wafers May cause damage to the workpiece and should therefore be avoided. It is especially important in processing wafers of large size, like 300 mm-diameter wafers. 
     SUMMARY OF THE INVENTION 
     There is accordingly a need in the art to facilitate the inspection/measurement of workpieces progressing along a production line, by providing a novel monitoring apparatus capable of being applied to the workpiece contained in a cassette. 
     It is a major feature of the present invention to provide such an apparatus whose orientation with respect to the cassette is adjustable in a manner to apply the apparatus to a selected one in a plurality of workpieces contained in the cassette, and to enable scanning of the workpiece. 
     It is a further feature of the present invention to provide such an apparatus, whose dimensions allow its use as an integrated tool. 
     The main idea of the present invention is based on the following. Semiconductor wafers are typically kept in a cassette having a plurality of vertically aligned spaced-apart parallel slot-like compartments. Each compartment contains a corresponding one of the wafers in a lot, and a certain gap is typically provided in the slot-like compartment above the upper surface of the wafer contained therein. Thus, in an inspection tool according to the present invention, a probe, which is to be positioned above the wafer during inspection, is designed like an arm insertable into the gap and driven for movement relative to the wafer under inspection. To this end, the probe supporting an optical head is mounted for movement along three mutually perpendicular axes. 
     Generally, such a monitoring apparatus can be applied to any substantially flat workpiece contained in a cassette, provided a certain gap exists inside the cassette above the workpiece. Moreover, such an apparatus may be any processing tool, e.g., cutting tool or marker, provided the gap size is sufficient for processing the workpiece with this tool. 
     There is thus provided according to one aspect of the present invention a processing apparatus for applying to a substantially flat workpiece contained in a cassette having a gap thereinside above said workpiece, the apparatus comprising a housing located outside said cassette and a processing tool coupled to the housing, wherein the processing tool is displaceable along three mutually perpendicular axes relative to the cassette for inserting the tool into the gap and moving the tool inside the gap relative to said workpiece. 
     To provide the above movements of the tool, the apparatus comprises at least one translation assembly. Preferably, the translation assembly guides the movement of the tool along the three axes relative to the housing. However, the construction can be such that a first translation assembly guides the movement of the tool along the two axes relative to the housing, and a second translation assembly guides the movement of the tool along the third axis relative to the cassette, or, alternatively, guides the movement of the housing together with the tool. 
     Preferably, the processing tool is displaceable between its inoperative position projecting from the housing and an operative position being retracted towards the housing. In the inoperative position of the processing tool, it may be located substantially inside the housing or be folded so as to extend along an outer surface of a sidewall of the housing. 
     The construction may be such that the processing tool is supported on a single-arm structure projecting from the housing, and this arm is displaceable along the three axes. Alternatively, the processing tool may be supported on a multiple-link kinematic structure, a so-called “frog-legs”, pivotal movements of one or more links providing the displacement of the tool along one or two axes. These pivotal movements may provide the displacement of the tool between its operative position projecting from the housing and an inoperative position being retracted towards the housing. 
     According to another aspect of the present invention, there is provided an optical monitoring apparatus for monitoring parameters of a substantially flat workpiece contained in a cassette having a gap thereinside above said workpiece, the apparatus comprising a measuring unit located outside said cassette and an optical head carried by a conveyor arm coupled to the measuring unit, the conveyor arm conveying the optical head along three mutually perpendicular axes relative to the cassette so as to insert the optical head into the gap and moving the head inside the gap relative to said workpiece. 
     The term “monitoring” used herein signifies determining predetermined characteristics of the workpiece by measurements, inspection or metrology technique. 
     According to yet another aspect of the present invention, there is provided a processing station for processing a substantially flat workpiece which arrives to or leaves the station in a cassette having a gap thereinside above the workpiece, the station comprising processing environment and an optical monitoring apparatus for monitoring the workpiece whilst inside the cassette. 
     According to some more aspects of the present invention, there are provided an CMP arrangement, a photolithography arrangement and an CVD arrangement, respectively, for processing semiconductor wafers, which utilize a monitoring apparatus for monitoring parameters of the wafers being contained in a cassette having a gap thereinside above the wafer. 
     More specifically, the present invention is used for monitoring semiconductor wafers (constituting a substantially flat workpiece) with an optical monitoring apparatus (constituting a processing tool) for determining thickness parameters of one or more wafer&#39;s layers, and is therefore described below with respect to this application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
     FIGS. 1 a  and  1   b  schematically illustrate two conventional systems used in the manufacture of semiconductor devices, which utilize, respectively, a stand-alone monitoring device and an integrated monitoring tool; 
     FIGS. 2 a  and  2   b  schematically illustrate the main principles of monitoring apparatus according to the invention, showing the apparatus in its operative and inoperative positions, respectively; 
     FIG. 2 c  more specifically illustrates an optical arm of the monitoring apparatus in its operative position; 
     FIG. 3 schematically illustrates the main components of a measuring unit in the apparatus of FIG. 2; 
     FIG. 4 more specifically illustrates a translation assembly suitable to be used in the monitoring apparatus of FIGS. 2 a  and  2   b;  and 
     FIGS. 5 a  and  5   b  illustrate another embodiment of an optical arm in operative and inoperative positions, respectively, of a monitoring apparatus. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 a  illustrates a conventional photolithography tools arrangement and a stand-alone SEM associated with this arrangement. FIG. 1 b  illustrates a conventional CMP tools arrangement utilizing an integrated inspection tool such as ITM NovaScan 210 model commercially available from Nova Measuring Instruments Ltd., Israel. As indicated above, the conventional inspection tools, either for off-line or for integrated application, always require additional handling of wafers, in addition to that required for translating the wafers between the tools of a specific processing station. 
     Referring to FIGS. 2 a  and  2   b,  there are illustrated the main constructional features of a monitoring apparatus  10  according to the invention. The apparatus  10  is intended for monitoring the parameters of wafers W contained in a cassette  12  (such as cassettes  3   a  and  3   b  in FIGS. 1 a - 1   b ). As shown, the cassette  12  has several spaced-apart slot-like compartments—six slots in the present example, generally at  14 , each containing the corresponding wafer W. A certain small gap  16  exists between two adjacent wafers (typically 10 mm for a 300 mm-diameter wafer). 
     The monitoring apparatus  10  includes such main constructional parts as a measuring unit  18  and a control unit  20 , whose input is coupled (by wires or wireless) to the output of the measuring unit  18 . The measuring unit  18  is contained in a housing  22  formed with a substantially flat, arm-like member  24  (constituting a conveyor arm). It should be noted, although not specifically shown, that the measuring and control units  18  and  20  could be accommodated in the common housing  22 . 
     As shown in FIG. 2 a,  in an operative position of the apparatus  10 , the arm  24  projects from the housing  22  and is insertable into the gap  16 . When in an inoperative position of the apparatus  10 , the arm  24  is retracted towards the housing, for example to be located inside the housing  22  (FIG. 2 b ). 
     FIG. 2 c  illustrates the selected wafer W to be monitored and the arm  24  positioned above the wafer W in the X-Y plane. The arm  24  serves as an “optical arm” carrying an optical head OH in a manner allowing the illumination of a region on the wafer W with incident radiation and collection of radiation returned (reflected) from the illuminated region. 
     The optical arm  24  is mounted for movement along X-, Y- and Z-axes relative to the cassette by means of a suitable translation assembly, as will be described more specifically further below with reference to FIGS. 4 and 5. The displacement of the optical arm  24  along the Z-axis enables to adjust the vertical position of the arm  24  to be applied to a selected wafer, namely to be inserted into the gap  16  above the selected wafer W. By displacing the optical arm  24  in the X-Y plane, measurements of successive regions (measurement sites) on the selected wafer can be performed. Thus, the optical arm conveys the optical head towards the selected wafer, and, after being inserted into the gap inside the corresponding slot, conveys the optical head within an inspection (monitoring) plane above the wafer. 
     Turning now to FIG. 3, the construction of the measuring unit  18  is more specifically illustrated. The measuring unit  18  provides spectrophotometric measurements, which are commonly used for measuring the thickness and optical properties of thin films in different processes of semiconductors&#39; manufacturing. The measuring unit  18  includes an optical system  26  and a detection assembly  28 . The optical system  26  is composed of a radiation source  29 , a radiation directing arrangement and an optical head  32 . As shown, the detection assembly  28  and almost all the elements of the optical system  26  are accommodated in the housing  22 , except for the optical head  32  which is mounted on the arm  24 . 
     It should be noted that, in the present example, the optical head  32  (or a corresponding part thereof) is mounted on an outer surface of the optical arm  24 . The optical head, however, may be mounted inside the arm  24 , provided the latter is formed with an appropriate optical window. 
     The optical system  26  and the detection assembly  28  are constructed generally similar to those of the above-mentioned metrology tool ITM NovaScan  210 , except for additional movable mirrors, which convey incident and returned light between the optical elements accommodated in the housing  22  and the optical head accommodated in the arm  24 . Light from the light source  29  passes through an optical fiber  34  (the provision of which is optional) and a condenser  36  that directs the light onto a beam splitter  38 , which, in turn, reflects the light onto a tube lens  40  and a mirror  42 . The optical head  32  is formed by an objective lens  44  and a mirror  46  that directs incident light onto the surface region of the wafer W, and directs light returned from this region towards the objective lens  44 . Additional movable mirrors  48  and  49  are provided for conveying incident and returned light between the optical head  32  and the mirror  42 . Since the operation of each of these optical elements is known per se, the light propagation is shown here schematically in order to facilitate illustration of the main components of the apparatus. 
     The light beam reflected from the illuminated region and collected by the optical head  32  propagates towards the beam splitter  38 , and, after passing therethrough, impinges onto a pinhole mirror  50 , which directs light to the detection assembly  28 . More specifically, the pinhole mirror  50  reflects the periphery region of the light beam towards a CCD camera  28   a  through an imaging lens  52   a,  and transmits the central region of the light beam towards a spectrophotometer  28   b  through a lens  52   b.  The provision of the pinhole mirror  50  divides the entire optical path of the collected light into measurement and positioning channels C m  and C p  associated, respectively, with the spectrophotometer and CCD camera. 
     The ability for translation of the optical arm  24  along the Z-axis enables to bring the optical arm to any selected wafer, while the ability for translation of the arm along the X- and Y-axes enables to bring the optical head to a desired location on the wafer inside the cassette  12 . It should be noted that, generally, a pivotal movement of the arm within the X-Y-plane can also be provided, for example to reduce the footprint of the system along at least one of the X- or Y-axis. 
     FIG. 4 illustrates one possible example of a translation assembly, generally designated  60 , suitable to be used in the monitoring apparatus  10  for guiding the movements of the optical arm along the X-, Y- and Z-axes. The translation assembly  60  actually presents X-Y-Z stage, or the so-called “Cartesian Supporting System”. In the present example, the translation assembly  60  is composed of three single axis motion systems  64 ,  66  and  68  responsible for the movements of the optical arm  24  along the X-, Y- and Z-axes, respectively. As shown, the systems  64 ,  66  and  68  include base-and-guide arrangements (stages), associated with drives D X , D Y  and D Z , respectively. The arm  24  (carrying the mirror  46 ) is mounted on a base  64   a  for movement along a guide  64   b  (X-axis), which is coupled to a base  66   a.  The latter carries the mirror  48  and is driven for sliding movement along a guide  66   b  (Y-axis). The guide  66   b  is, in turn, coupled to a base  68   a  carrying the mirror  49  and mounted for movement along a guide  68   b  (Z-axis) towards and away from the stationary mounted mirror  42 . 
     FIGS. 5 a  and  5   b  illustrate operative and inoperative positions, respectively, of a monitoring apparatus  100 , which is constructed generally similar to the apparatus  10 , but has somewhat different design of an optical arm  124  and a translation assembly (not shown). To facilitate understanding, same reference numbers are used for identifying those components, which are identical in the apparatuses  10  and  100 . Here, the optical arm  124  is in the form of a so-called “frog-legs” structure having four links  124   a,    124   b,    124   c  and  124   d.  The optical arm  124  is operated by a driving mechanism  70  that drives it between its operative, extended (“open”) position (FIG. 5 a ) and an inoperative, retracted (“closed”) position (FIG. 5 b ). In the present example, the structure  124 , when in the inoperative position, is located inside the housing  22 . It should, however, be noted that the structure  124 , whilst being in its “closed” position may be located outside the housing  22  being retracted to its outer surface  22   a.  The structure  124  is movable along the surface  22   a  (along the Y-axes), and by “opening” such a frog-legs structure, the movement of the optical head  32  along the X-axis is provided, thereby enabling the penetration of the optical head  32  into the gap inside the cassette  12 . The movement along the Z-axis can be achieved either by providing a translation means for guiding the movement of the entire structure  124  relative to the housing  22 , or by providing the vertical movement of the entire housing  22  relative to the cassette. It should be understood that the provision of the “frog-legs” structure may reduce the system&#39;s footprint along the X-axis, and could therefore meet the requirements of a specific application. 
     Turning back to FIG. 3, the penetration depth L d  of the optical arm ( 24  or  124 ) between the two adjacent wafers is determined by the linear field of view F of the objective lens  44  for a given value of the gap  16  between the adjacent wafers. It is known that the beam&#39;s width D (i.e., the height of the optical arm), after passing the distance L d  from the objective lens  44  (keeping in mind that the object plane, i.e., the wafer&#39;s surface, is located in the focal plane of the objective lens  44 ) is determine as follows:        D   ≈       2   ·   f   ·   NA     +       F   ·     L   d       f                              
     wherein NA is the numerical aperture of the objective lens, and f is the focal length of the objective lens. Since the height D of the optical head should be less than the distance d between two vertically adjacent wafers, the penetration depth L 4  of the optical head should satisfy the following condition:        D   =       (       2   ·   f   ·   NA     +       F   ·     L   d       f       )     ≤   d                            
     From the above equation, the penetration depth L d  can be determined for the actual values of the distance d and linear field of view F. For example, let us consider the following conditions: d 1 =6 mm and d 2 =10 mm for, respectively, 200 mm and 300 mm wafers; f=20 mm; F=0.5 mm and NA=0.1 (a sufficient value for pattern recognition and spectrophotometric measurements in the case of thin films measurements). The corresponding values of the penetration depth are: L d   (l) =80 mm and L d   (2) =120 mm. Hence, different measuring sites within the measured wafer can be accessed. In many cases, e.g., after CVD or other deposition process, access to part of the wafer may be sufficient for the in-line process control. However, for some processes, e.g., CMP, access to the entire wafer&#39;s surface is needed. 
     The two main functions of the measuring unit includes positioning of the optical head  32  (including focusing, image acquisition and image processing) by the positioning channel C p , and measuring (including illumination and detection) by the measuring channel C m . The operation of the positioning channel is aimed at identifying the exact location of the selected wafer and the measurement site thereon to be currently measured. After the completeness of this operational step, the optical arm  24  is brought into such a position that enables the measuring channel to take measurements, after the required focus is achieved. Alternatively, the pattern recognition can first be performed with the field of view F=0.5 mm at a region on the wafer close to its edge. This area is limited to the depth of penetration L d , and is predetermined by the wafer&#39;s specific pattern. For spectrophotometric measurements, the field of view not exceeding 20 μm is typically needed, which results in the penetration depth being less than 500 mm. Once the pattern recognition is complete, measurements can be taken at any site on the wafer, as far as the relative location of the site relative to the identified pattern can be determined. 
     Thus, the monitoring apparatus constructed according to the invention operates in the following manner. First, either a load/unload cassette is placed (by a robot or by an operator) in a measuring position with respect to the monitoring apparatus, or the apparatus is brought to the measuring position. For example, the housing of the monitoring apparatus may be provided with a translation means, e.g., wheels. Then, the respective drive mechanism is operated for moving the optical arm so as to penetrate inside the gap between the adjacent wafers, wherein the lowermost of these wafers is a selected one to be measured. The positioning of the arm is performed in accordance with a recipe design and known pattern recognition algorithms, for example in a manner disclosed in U.S. Pat. No. 5,682,242 assigned to the assignee of the present application. Alternatively, an alignment feature can be chosen during the recipe preparation. This should be any asymmetrical feature, which is always found in the area that can be scanned by the optical arm, irrespectively of the orientation of the wafer. The position of a measurement site is determined relative to the alignment feature. Then, the optical arm scans a portion on the wafer and a processor of the control unit “looks” for the alignment feature within this portion. This can be done by any known suitable technique such as correlation, by which various orientations of the image of the alignment feature stored in the control unit are compared to the image from the scanned portion. If the alignment feature was not found, the optical arm moves to a next portion and the process is repeated until the alignment feature is identified on the wafer. At this point, the control unit operates the optical head movement to a predetermined location relative to the alignment feature in order to reach the measurement site. By this, the positioning is complete, and measurements are taken and processed. It should be noted that, in this specific example, where the processing of workpieces is optical monitoring and the workpieces are semiconductor wafers, the measurement unit as well as the optical arm should not interfere with the wafers and with the load/unload cassette or introduce any potential risk of contamination. 
     Moreover, different techniques of dynamic or static auto-focusing may be applied during the wafer imaging, for example in a manner disclosed in U.S. Pat. No. 5,604,344 assigned to the assignee of the present application. After measuring of the actual out-of-focus distance, it may be adjusted for example, either by moving the lens  40 , acting simultaneously for C p  and C m  channels, or by moving sensors  28   a  and  28   b  separately. 
     The advantages of the present invention are thus self-evident. The monitoring apparatus can be tiny, having sufficiently small footprint to be physically installed inside the processing environment, when its use as an integrated inspection tool is required. The apparatus has a narrow optical arm that can be freely inserted into the gap between two adjacent wafers and measure and/or scan at least one die of the wafer. Thus, the wafer is monitored whilst being located inside the cassette, thereby avoiding additional handling of the wafer. This facilitates the installation, integration and operation of the apparatus and of that station of the production line (processing environment) with which the apparatus is used. 
     The monitoring apparatus according to the invention can be used with any processing tool arrangement, either as an integrated tool or as a stand-alone review station, as well as with CVD or PVD processing environment for thickness measurements. The films on CVD/PVD wafers are usually highly uniform. Thus, the thickness measurement in a single site of such wafers is sufficient to estimate the layer thickness on the wafer. To this end, the optical arm is designed so as to be capable of arriving to a first die proximate to the wafer&#39;s edge. However, multiple sites over the wafer can be measured as well providing mapping of the entire wafer. 
     Those skilled in the art will readily appreciate that many modifications and changes may be applied to the invention as hereinbefore exemplified without departing from its scope, as defined in and by the appended claims.