Abstract:
A laminated wafer sensor structure includes a housing layer having pocket openings formed therein, a circuit layer having a sensor element and electronic components mounted for registration with the pocket openings in the housing layer, and a rigid back layer. The laminated structure is suitable for handling by conventional robotic wafer handling systems. The wafer sensor structure is adapted for electrical connection to a base station that is also adapted for connection to a host computer system to facilitate communication among the sensor structure, the base station and the host computer.

Description:
PRIORITY CLAIM 
   This application claims priority from U.S. Provisional Application No. 60/839,768, filed on Aug. 24, 2006, by Schloss et al., titled “Wafer Sensor System for UV Dose Measurement.” U.S. Provisional Application No. 60/839,768 is hereby incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention is directed to wafer sensors and, in particular, to a laminated wafer sensor system that is compatible with transfer by robotic wafer handling systems. 
   BACKGROUND OF THE INVENTION 
   For the last decade, pyroelectric sensors have been used as the primary standard for UV dose calibration of excimer laser based lithography tools. At regular calibration intervals, the lithography tool is opened, the pyroelectric sensor is inserted onto the tool&#39;s wafer stage and dose measurements are taken. While these measurements are very useful as a calibration procedure, significant down time is created by opening the tool to insert the sensor. 
   In conventional immersion lithography processes, de-ionized (DI) water covers the gap between the projection lens of the lithography tool and the wafer. Because of even greater contamination concerns, opening the stage for dose calibration becomes even less desirable for immersion lithography. 
   U.S. Pat. No. 6,889,568, which issued on May 10, 2005, discloses a measuring device that incorporates a substrate with sensors that measure the processing conditions that a wafer may undergo during manufacturing. The substrate can be inserted into a processing chamber by a robot head and the measuring device can transmit the conditions in real time or store the conditions for subsequent analysis. In the measuring device disclosed in the &#39;568 patent, the electronics platform is mounted on a recessed portion of the load bearing substrate. 
   U.S. Pat. No. 6,691,068, which issued on Feb. 10, 2004, discloses a sensor apparatus that is capable of being loaded into a process tool. From within the process tool, the sensor apparatus is capable of measuring, storing and transmitting data in near real time. As in the case of the &#39;568 patent, in the apparatus disclosed in the &#39;068 patent, the substrate is the load bearing foundation that carries the load of the sensor, the information processor and the power source. 
   SUMMARY OF THE INVENTION 
   The present invention provides a wafer sensor system that utilizes a laminated wafer sensor structure that includes a pyroelectric element bonded to a flex circuit, which is then bonded to a carrier “ring.” The UV dose sensor, which preferably has the same profile as a 200 mm or 300 mm silicon wafer, allows for measurements at the wafer stage of a lithography tool without opening the lithography system. The wafer-sized sensor is sent to the stage utilizing the tool&#39;s existing robotic wafer handling system. The sensor is wireless, low-outgassing and capable of storing more than one hundred dose measurements. After exposure on the stage, an external readout base station is used to download the dose measurements from the sensor to a host computer. Typically, it takes two to four minutes for a wafer to exit a lithography system via the robotic wafer handling system. Because of this handling time, the sensor system is capable of storing a dose measurement signal with minimal decay for at least two minutes. 
   Other features and advantages of the present invention will become apparent from a review of the specification, claims and appended drawings. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a plan view showing the upper surface of a wafer sensor in accordance with the present invention. 
       FIG. 1B  is an exploded perspective view illustrating the laminated structure of a wafer sensor in accordance with the present invention. 
       FIG. 2  is a block diagram illustrating an embodiment of measurement electronics utilizable in a wafer sensor in accordance with the present invention. 
       FIG. 3  is a partial cross section drawing illustrating an embodiment of a pyroelectric electrode pattern for a wafer sensor in accordance with the present invention. 
       FIG. 4  is a schematic drawing illustrating a wafer sensor and base station in accordance with the present invention. 
       FIG. 5  is a schematic drawing illustrating base station electronics. 
       FIG. 6  is a partial cross section drawing illustrating greater detail of the  FIG. 3  pyroelectric electrode pattern. 
       FIG. 7  is plot showing the spatial scan of three prototype apertures made in accordance with the  FIG. 6  structure. 
       FIG. 8  is a schematic drawing illustrating steps in the manufacture of a wafer sensor system in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of a wafer sensor system  100  in accordance with the present invention is shown in  FIGS. 1A and 1B . The wafer sensor system  100  can be utilized for both immersion and dry UV dose measurement in lithography tools. The structure  100  is preferably made with the same profile as a 200 mm or a 300 mm diameter semiconductor wafer of the type utilized in the manufacture of integrated circuits.  FIGS. 1A and 1B  show a 300 mm diameter wafer embodiment of the invention. 
   As shown in  FIGS. 1A and 1B , in accordance with the invention, the wafer sensor system  100  has a laminated construction. The outer “housing” is formed by laser cutting openings through a semi-standard silicon wafer  102 . A flexible or semi-rigid circuit board  104  populated with integrated circuits (ICs) and other electronic elements discussed in greater detail below is bonded to the silicon wafer housing  102  using an intermediate layer of thin film adhesive  106 . The integrated circuits on the circuit board  104  fit through the laser cut openings in the silicon wafer housing  102 . A pyroelectric sensor element  108  is first bonded to flex circuit wire  110 , then to a chemically etched heat sink shim (not shown), and finally to the flex circuit board  104  with epoxy. A thin film battery  112  and solar cells  114  are also bonded to the flex circuit board  104 . A stainless steel or plastic film back layer disk  116  is attached to the back of the flex circuit  104  via an intermediate layer of thin film adhesive  118 . The back layer disk  116  provides the additional flatness, smooth finish and strength required for handling by the robotic wafer handling system of a lithography tool. 
   The electronics, battery and solar cell pockets are potted using a low outgassing epoxy. The potting epoxy provides additional strength and protects the electrical connections from DI-water in an immersion measurement tool environment. 
   In a preferred embodiment of the invention, the final thickness of the wafer sensor system  100  is less than 1.35 mm. Recessing the electronics within the openings of the silicon housing  102  reduces the overall thickness of the assembly  100 . In a preferred embodiment, the warp of the wafer sensor system  100  over the full 300 mm diameter of the wafer is less than 100 um. The use of pressure sensitive thin film adhesive  106 ,  110  between the wafer housing  102  and the flex circuit  104  and between the flex circuit  104  and the back disk  116 , and room temperature cure epoxies for affixing the electronics components to the flex circuit  104 , minimizes internal stress in the assembly  100 . The low internal stress between the three laminated layers  102 ,  104 ,  116  helps minimize the warp of the sensor assembly  100 . The flatness, smooth finish and rigidity of the laminated structure  100  allow for robot handling performance that matches that of a standard silicon wafer. 
   A block diagram of an embodiment of measurement electronics  200  mounted on the flex circuit board  104  of the wafer sensor system  100  is shown in  FIG. 2 . An on board rechargeable battery  112  ( FIG. 1 ) powers the electronics  200  for up to about 2 hours. The battery  112  can be recharged via two solar cells  114  ( FIG. 1 ). An IR emitter and receiver pair  206  is used to communicate data from the wafer sensor assembly  100  to a base station, which is not shown in  FIG. 2 , but is discussed in greater detail below. A trans-impedance amplifier  208  receives the laser energy pulse signal from the pyroelectric sensor  108 . A fast peak detection sample-and-hold circuit  210  then captures the peak of the signal produced by each laser pulse via a delay line  211 . An on board microprocessor  212  then reads the peak via an analog-to-digital (A/D) converter, shown in  FIG. 2  as internal to the processor  212 . 
   The microprocessor  212  sums multiple laser pulse energy measurements to compute UV dose. In addition to dose measurements, the microprocessor  212  also controls the solar recharge of the battery  112 , a power saving sleep mode circuit  214 , IR communications link  206  and data storage (not shown in  FIG. 2 ). With this electronics approach, up to two hundred dose measurements can be taken during a single load onto the wafer stage. The IR link  206  also allows field upgrade of the firmware. Calibration and setup information is also stored in the read only memory (ROM) of the microprocessor  212 . 
   Pyroelectric sensors measure laser energy. For calibration of lithography systems, the dose in units of energy per unit area (fluence) is required. Thus, to measure fluence with a pyroelectric sensor, energy is measured over a known aperture area (NA). For an immersion system, a physical aperture would limit the maximum NA that could be measured with high accuracy. To avoid using a physical aperture, in a preferred embodiment of the invention, the pyroelectric sensor  108  ( FIG. 1 ) is patterned as shown in  FIG. 3 . As shown in  FIG. 3 , a palladium electrode  300  is formed around the pyroelectric  302  with a 3 mm gold electrode  304  formed at the bottom of the pyroelectric  302 . This assembly is connected to the flex wire  306 . A chemically etched heat sink  308  is provided between the flex wire  310  and the flex circuit board  116 . 
   With the  FIG. 3  electrode pattern, the pyroelectric  302  is sensitive to the laser energy only in the 3 mm diameter patterned disk  304 . Thus, for a 26 mm diameter pyroelectric  302 , only a 3 mm diameter disk  304  in the center responds to the laser energy. The fluence incident on the pyroelectric  302  is then calculated via the measured energy value divided by the area of the 3 mm disk  304 . 
   Referring back to  FIG. 2 , arrival of a signal from the sensor element  108  causes three events to happen. One, the wake-up integrator  214  analyzes the accumulated energy and, if a certain wake-up threshold is exceeded, the microcontroller  212  is awakened from the “sleep” mode. Two, a trigger integrator  216  analyzes the accumulated energy and, if a certain trigger threshold is exceeded, the present value of the peak detector  210  is latched as an analog value. Three, the signal is passed through the delay line  211  to the peak detector  210 . This delay is designed to match the latency of the trigger circuit  216 , with its time constant of integration. Without the delay line  211 , the peak value would pass before a triggered capture could occur. 
   The NA of the light is varied typically over the range 0.2 to 1.4 in a lithography tool. The angular response of the wafer sensor system must be flat over this range. Preferably, a diffuse surface is used for the pyroelectric. This diffuse surface provides a maximum measurement error of less than 5% over the NA range. 
     FIG. 4  shows an embodiment of a wafer sensor system  100  in a base station  400 . A standard wafer carrier  402  known as a FOUP (Front Opening Unified Pod) is used in the base station  400 . In the illustrated embodiment, a FOUP with a clear rear window is required. An electronics unit is mounted on the FOUP adjacent the clear rear window. 
     FIG. 5  shows the details of the base station electronics unit  500 . An array  502  of red high power LEDs is used to recharge the wafer sensor battery  112  ( FIG. 1 ) via the two solar cells  114  ( FIG. 1 ). An IR emitter and receiver pair  206  ( FIG. 2 ) communicates data to the wafer sensor system  100 . The IR communications and optical recharging prevents possible damage to the wafer sensor system  100  that would be caused by mechanical contacts. It also allows the “clean” (low particle count) wafer within the FOUP to be isolated from the “dirty” (high particle count) electronics unit. Two proximity sensors  504 ,  506  detect the presence of the wafer sensor assembly  100  in the FOUP and the state of the FOUP door (open or closed), respectively. An RS-232 interface connects the base station to an external host computer. A microprocessor in the base station electronics unit controls the communication with host computers, optical charging of the wafer sensor battery  112 , data exchange between the base station  400  and the wafer sensor system  100 , and health and status diagnostics. On the front panel of the base station electronic unit  400 , indicator lights showing battery and communication status are provided. Also buttons to reset the unit and to start the charging cycle are present. 
   As noted above, a novel aperture technology is required to meet the NA requirements of the immersion stepper. Further details are shown in  FIG. 6 . This aperture includes an active area defined in the pyro element by laser-machining 3 mm diameter cuts in the top electrically conductive chrome layer on top of the pyro element and 3 mm diameter cuts into the pyro crystal in the bottom side. After further investigation, it was found that it was not necessary to machine the top surface. Further, only the electrode needs to be machined. 
   In both the  FIG. 3  and the  FIG. 6  approach, the patterning defines an effective cylinder that constitutes the effective volume of the pyroelectric element and electrically isolates the active region from the remaining bulk material. This limits the sensitivity of the element to the aperture size of 3 mm diameter. 
   The electrical contacts are such that the anode is connected only to the bottom side of the 3 mm active area. The cathode is continuous throughout the entire surface of the wafer, with the exception of the anode. A voltage forms across the z-axis of the pyro crystal corresponding to the dT/dt of absorbed laser energy and cooling. The surface of the pyro element is electrically common except for the active area. The voltage can only form over the anode of the active 3 mm area and the cathode along the z-axis of the crystal. The rejection of dT/dt outside the active area is relative to the uniformity of the crystal lattice. As can be seen from the  FIG. 7  plots, the pyroelectric element is only significantly responsive in the active area. 
   The plot in  FIG. 7  shows the spatial scan of three prototype apertures made in accordance with  FIG. 6 . Each aperture is cut to different depths: 100 um, 50 um, and 1 um. Beam size for these scans was 200 um diameter. Data points were taken every 50 um across the diameter of the aperture. 
   A main feature of a wafer sensor system in accordance with the invention is its ability to handle like a standard silicon wafer. Specifically, the sensor system needs to be sufficiently flat on the bottom surface to allow handling with the relevant vacuum arms and stages. The sensor needs to be light enough to avoid overloading the vacuum arms. Finally, the sensor needs to be sufficiently thin to allow it to travel within the target equipment like a standard silicon wafer. 
   The rigid flex material is stiffer, thinner, and lighter than standard flex material. The enhanced stiffness improves the flatness of the bottom surface, particularly in areas where the flex is not directly supported by the silicon frame. This can significantly improve robotic handling of the sensor. 
   The flexible printed circuit design is, thus, optimized for thinness and dimensional stability in the following ways: a single layer with copper applied to opposing sides provides a symmetrical design that in inherently thinner, stronger and more warp-resistant than conventional multi-layer designs with asymmetrically placed areas of copper. In pursuit of this objective, the thinnest available polyimide material, having the DuPont trademark Kapton®, is a preferred embodiment of the substrate used in the present invention. As discussed herein, minimal etching is performed, resulting in large areas of non-functional copper opposing a monolithic ground plane. 
   The goal is to laminate two materials (substrates A and B) together using Pressure Sensitive Adhesive (PSA), with the requirement that the two substrates are aligned with each other and that minimal air bubbles are trapped between the two substrates. At least one of the two substrates is flexible, although the degree of flexibility need not be much (elastic deformation with a radius of curvature less than 5 m). 
   With reference to  FIG. 8 , a substrate  800  is held flat and immobile by vacuum chuck  802 . A PSA sheet  804 , packaged between two backing layers, is place onto and aligned to the substrate  800 . While aligned, the PSA sheet  804  is fixed to the flexible arm  806  with removable tape or other means (vacuum pickup, etc.). The arm  806  is attached to a bearing block  808  pivots around a shaft  810 . In this configuration, the arm  806  can rotate out of the plane of the vacuum chuck  802 , but will always return to the exact same location when moved back into contact with the vacuum chuck  802 . The PSA sheet  804 , along with arm  806 , can now be moved off of the substrate  800 , allowing the bottom backing layer from PSA  804  to be removed. PSA  804  is now lowered back onto substrate  800  at an angle, so that the portion of PSA  804  attached to the arm  806  makes contact with substrate  800  first. A roller  812  is rolled across arm  806 , onto PSA  804  and across the entirety of PSA  804 , allowing the bond line between substrate  800  and PSA  804  to move sequentially from the initial contact area underneath arm  806  to the far end of substrate  800  (i.e., to the right in  FIG. 8 ). Consequently, air is expelled between the layers  804  and before the layers adhere to each other. Arm  806  can now be removed from PSA  804 . 
   In one embodiment of the invention, substrate  802  is then placed back onto vacuum chuck  802 , with PSA  804  facing up (not in contact with chuck  802 ). A second substrate (not shown) is placed onto PSA  804  and aligned to relevant features on substrate  800  and PSA  804 . With alignment achieved, arm  806  is attached to the second substrate which is rotated off PSA  804 . The remaining backing layer is then removed from PSA  804 , and the second substrate is rolled onto PSA  804  in the same manner as described above. In this embodiment, substrate  800  may be rigid or flexible, but the second substrate, must be flexible. 
   In an alternate embodiment of the invention, PSA  804  is rolled onto substrate  800  as described previously. Substrate  800  is removed from vacuum chuck  802  and the second substrate is placed onto the vacuum chuck  802  and immobilized. Substrate  800 , with PSA  804  facing down, is placed onto and aligned to the second substrate. Arm  806  is attached to substrate  800  once alignment is achieved. Substrate  800  can now be rotated off the second substrate, allowing the final backing layer to be removed from PSA  804 . Substrate  800  is now rolled onto the second substrate, as described previously. In this embodiment, the second substrate maybe rigid or flexible, but substrate  800  must be flexible. 
   Ideally, the openings in the silicon housing  102  ( FIG. 1 ) should be as small as possible, circular, and with maximum spacing between holes and a somewhat larger distance from holes to the wafer edge. If circular holes are not possible, then all corners should be radiused to minimum of 5 mm ( 3/16 in). 
   The spacing of the openings is dependent on the wafer diameter and thickness. For the standard 300 mm×0.775 mm silicon wafer, hole-to-wafer edge spacing should not be less than 30 mm, while the spacing between holes should not be less than 20 mm. For the standard 200 mm×0.725 mm silicon wafer, hole-to-wafer edge spacing should not be less than 15 mm, while the spacing between holes should not be less than 10 mm. For both the 300 mm and 200 mm wafers, the total hole area should not exceed 30% of the wafer area. 
   For alignment purposes, the holes should be sized so that the minimum spacing between the hole edge and any component or pad on the flex attached to the silicon housing should be 1 mm or twice the component height, whichever is larger. 
   The wafer sensor firmware upgrade or reprogramming utilizes the same physical IR link as that for normal sensor to base station communications. The reprogramming demands three unique pieces of software/firmware that reside in the wafer sensor, the base station and a host computer, respectively. The software in the host computer will initiate an upgrade process by sending special commands to the wafer sensor and the base station. These commands will place the sensor and base station in the upgrade mode (as opposed to normal application mode). After the commands are sent, the host software will upload the new code, typically one section at a time, to the base station via a standard serial cable. The base station processes the code then passes it to the wafer sensor via the IR link. The wafer sensor and the base station will be restored to the normal application mode automatically after the sensor is successfully reprogrammed. 
   Special algorithms and processes are developed and built in the aforementioned software and/or firmware to ensure a high degree of reliability and consistence for the reprogramming. Segments of the firmware can be reprogrammed separately or independently. 
   The firmware includes a battery conserving sleep mode where the “wake up” into measurement mode is caused by the signal from the sensor. For a pyroelectric sensor “wake up” can be caused by a specific laser pulse temporal pattern fired onto the sensor. 
   When a prototype wafer sensor is loaded onto the wafer stage, a specific timed laser pulse pattern is used to wake up the sensor from sleep. 
   With reference back to  FIG. 2 , the preferred embodiment of the present invention also includes a remotely activated ON/OFF switch whereby the electronics may be effectively disconnected from the battery and, thus, extend the operational life between recharge or replacement of the batteries. Since the thinness and hermetic isolation of the wafer sensor system precludes mechanical switching, an electromagnetic or optical switch is utilized. Energy sensors are placed in a different configuration such that equal stimulation by an electromagnetic source has no effect. The on/off switching is thus achieved by irradiation on one sensor (the “on” element) or the other of the pair (the “off” element). In addition, it is advantageous to have the microcontroller  212  capable of activating the “off” state and thus disconnect itself, and all other electronics the ON/OFF switch, from the battery. This is preferable enacted by the microcontroller  212  after a user-selected period of inactivity. Restoration of the “on” state is readily achieved in the base station electronics by means of a different irradiation preferring the “on” sensor. 
   It should be understood that the particular embodiments of the invention described above have been provided by way of example and that other modifications may occur to a person skilled in the art without departing from the scope and spirit of the invention as expressed in the appended claims and their equivalents.