Abstract:
Disclosed is a focus detection apparatus for a projection lithography system. The apparatus includes: a laser; a focus optical unit configured to focusing the emitted laser beam; a force detection unit configured to reflect the focused laser beam at the backside; a position detection unit configured to detect variations in position of a light spot formed by the reflected laser beam, and output a strength signal indicating the strength of the interaction force between the force detection unit and the object; a differential amplifier configured to output a Z-direction differential signal based on the strength signal and a reference signal; a Z-direction feedback control unit configured to perform feedback control; and a scan signal generator configured to output a signal for controlling the movement of the stage in the XY plane. The focus detection apparatus has high precision, efficiency and process applicability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Chinese Patent Application Number 201110216470 8 filed on Jul. 29, 2011, the disclosure of which is hereby expressly incorporated by reference in its entirety and hereby expressly made a portion of this application. 
     TECHNICAL FIELD 
     The present disclosure relates to apparatuses used in projection lithography, in particular to focus detection apparatuses in projection lithography systems in the field of Super Large-Scale Integration Circuit fabrication and nano-device fabrication in optical Micro-Fabrication technology. 
     BACKGROUND 
     It is well known that an increase in resolution of optical projection lithography is achieved primarily by shortening exposure wavelength and increasing numerical aperture. However, as the exposure wavelength is shortened and the numerical aperture is increased, effective depth of focus (DOF) for a projection object lens will decrease dramatically. Although some resolution-enhancing techniques, such as off-axis lamination and phase-shift mask, have been used to alleviate the decrease of DOF, the trend towards a decreased DOF still dominate with the increase of the resolution. The DOF is typically around only 300 nm in the mainstream 193 nm lithography. The fact that the actual DOF does not reach the DOF tolerance required by the lithography process will adversely affect exposure line quality and IC yield. Accordingly, it is very important to make full use of the effective DOF for efficient exposure of a wafer, and to this end, various focus detection apparatuses have been proposed. The focus detection apparatus in a projection lithography machine is configured to measure height and inclination of a certain surface region of the wafer. Then, the wafer is subjected to leveling and focusing such that the exposure region of the wafer surface is positioned within the effective DOF range of the projection object lens, thereby achieving an efficient exposure of the wafer. 
     The existing focus detection apparatuses are divided, in terms of operation principle, into three categories including capacitive, optical and pneumatic focus detection. The capacitive focus detection technique was mainly used for stepping-type projection lithography machines in earlier days. Now such technique is no longer in use due to some problems with accuracy, process applicability and the like. The optical focus detection technique dominates in the current stepping scan lithography machines provided by major manufactures. The optical focus detection technique includes luminosity, CCD, laser interference and grating focus detections. The luminosity focus detection technique is simple in principle and easy in implementation, but has a low precision in the range of several hundred nanometers and significant non-linearity. The CCD focus detection technique can achieve a repetitive test precision up to about 30 nm by using an array of slits or apertures in cooperation with broadband illumination, and is widely used. The laser interference focus detection technique can achieve a repetitive test precision of 20 nm. However, this technique requires complex graphic processing algorithms and is not good in real-time application, and thus has a limited use. The grating focus detection technique enables measurement over a large area of wafer surface by using various gratings as focus detection marks. This can effectively smooth the effects caused by undulation of the wafer topography and variation in emissivity. Meanwhile, utilization of polarization modulation and Moire technique extends dynamic measurement range of a sensor and reduces influence of light intensity fluctuation. The grating focus detection technique can achieve a high precision. However, the precision suffers from multi-beam interference caused by a layer of photoresist mask, wavefront distortion caused by substrate dielectric medium and the like. The pneumatic focus detection is operated in an aerodynamic ranging principle, and can achieve a sensitivity of sub-nanometer order. This technique, however, has some disadvantages, such as inapplicability in a vacuum environment, direct reading conversion in which no air gap can pass through the airflow sensor, and slow-speed scanning, which limits application of the technique. 
     SUMMARY 
     The present disclosure provides a focus detection apparatus for a projection lithography machine, comprising: 
     a laser configured to emit a laser beam; 
     a focus optical unit configured to focus the emitted laser beam; 
     a force detection unit configured to receive the focused laser beam at the backside of the force detection unit and reflect the received laser beam; 
     a position detection unit configured to detect variations in position of a light spot formed by the reflected laser beam so as to obtain information about the topography of the object, and to detect and output a strength signal indicating the strength of an interaction force between the force detection unit and an object under detection disposed on a wafer, the wafer being carried on a stage of the projection lithography machine; 
     a differential amplifier configured to have one of its two input connected to the output of the position detection unit to receive the strength signal, and the other input connected to receive a reference signal, and to perform differential calculation between the strength signal and the reference signal to obtain and output a Z-direction differential signal; 
     a Z-direction feedback control unit configured to perform feedback control such that a spacing between the force detection unit and the object is maintained within a specified range, wherein the Z-direction feedback control unit has one of its two inputs connected to the output of the differential amplifier to receive the Z-direction differential signal, and the other input connected to receive a predetermined gain signal, and the Z-direction feedback control unit outputs a Z scan drive signal for controlling the movement of the stage in the Z direction; and 
     a scan signal generator configured to connect to the stage and output to the stage a XY 2D scan drive signal for controlling the movement of the stage in the XY plane, the Z direction being perpendicular to the XY plane. 
     The apparatus according to embodiments of the present disclosure has advantages of high precision, efficiency and process applicability. In the apparatus, the weak interaction force between atoms of the photoresist coated over the wafer surface and atoms of the probe tip of the force detection unit can cause deflection of the force detection unit. The position detection unit can detect such deflection to obtain information about the distribution of the interaction force. That is, the 3D topography information of the photoresist over the wafer surface can be obtained with a resolution of the nanometer order by detecting the interaction force between the probe tip and the photoresist. In this way, it is possible to solve the low-resolution problem with the convention optical focus detection approach and provide data basis for subsequent wafer leveling and focusing action. Further, in the apparatus, a plurality of force detection units can be used simultaneously for scanning and measurement. This addresses the low-efficiency problem with the single-spot measurement of the pneumatic focus detection, and improves the operating efficiency. In addition, the apparatus according to embodiments of the present disclosure performs topography measurement by detecting the inter-atomic force, and thus the measurement is not influenced by process conditions, such as photoresist type, property, reflectivity, thickness and substrate dielectric medium. Accordingly, the apparatus can be applied to various process conditions. In the apparatus according to embodiments of the present disclosure, a coarse/fine movement driving unit composed of a linear motor and a Lorentz motor can be employed, which enables a large travel and high precision movement of a six-freedom stage. This solves some problems with the atomic force microscope, such as narrow scanning range and deviation of leveling reference. The apparatus according to embodiments of the present disclosure is suitable for surface topography measurement of wafers having a diameter equal to or larger than 300 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present disclosure will be more apparent from the following detailed description in conjunction with accompanying drawings in which: 
         FIG. 1  is a schematic diagram showing a focus detection apparatus having a single probe according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic diagram showing in principle micro displacement detection with a light spot deflection method according to an embodiment of the present disclosure; 
         FIG. 3  is a simplified diagram showing a Z-direction feedback control unit in the focus detection apparatus having a single probe according to an embodiment of the present disclosure; and 
         FIG. 4  is a schematic diagram showing the scanning operation of a focus detection apparatus having multiple probes according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, the present disclosure will be described by embodiments with reference to figures. Any parameter given in the embodiments should be construed for illustrating the present disclosure. One skilled in the art will appreciate that the present disclosure is not limited to the specific elements in the given embodiments. Rather, the present disclosure is defined by the appended claims. 
       FIG. 1  is a schematic diagram showing a focus detection apparatus having a single probe according to an embodiment of the present disclosure. The focus detection apparatus may include a laser  101 , a focus optical unit  102 , a force detection unit  103 , a position detection unit  104 , a differential amplifier  107 , a Z-direction feedback control unit  108 , a scan signal generator  109  and a 3D topography data storage unit  110 . 
     In some embodiments, the focus optical unit  102  may be provided between the laser  101  and the force detection unit  103 . The position detection unit  104  may be provided in a path along which light is reflected from the force detection unit  103 . A laser beam emitted by the laser  101  may be focused by the focus optical unit  102  onto the backside of the force detection unit  103 , and then reflected to the position detection unit  104 . The force detection unit  103  may have a probe which has a spacing of nanometer order from a photoresist  111  disposed on a wafer  106 , which may be carried on a six-freedom stage  105 . 
     A signal may be generated with atomic interaction force between the probe and the photoresist  111  to indicate the strength of the interaction force. The wafer  106  may be vacuum-absorbed onto the stage  105 . The atomic interaction force between the probe tip and the photoresist  111  causes deflection of the force detection unit  103  in accordance with the topography of the photoresist  111 . Along with the deflection, the reflected beam is deflected, and the position detection unit  104  may detect change in the position of a light spot. This enables the position detection unit  104  to obtain the topography information of the photoresist  111 . The 6-freedom stage  105  may be driven by a coarse/fine movement driving unit (not shown) consisting of a linear motor and a Lorentz motor (for more details, reference may be made to patent document U.S. Pat. No. 5,969,441), and thus can achieve a large travel and high precision movement. 
     The stage  105  may be connected to the driving output of the scan signal generator  109 . During a XY 2D scanning process, the wafer  106  may be moved along with the movement of the stage  105  driven by a driving signal from the scan signal generator  109 . The position detection unit  104  may detect and output a probe signal indicating the strength of the interaction force between the probe and the photoresist  111 . The Z-direction feedback control unit  108  may calculate the spacing between the probe and the photoresist  111 , and accordingly change a Z-direction driving voltage for driving the stage along the Z direction. The differential amplifier  107  may have one of its inputs connected to the output of the position detection unit  104 . In this way, the differential amplifier  107  may receive the probe signal, perform a differential calculation between the probe signal and a reference signal, and output a Z-direction differential signal to be used for moving the stage  105 . The Z-direction feedback control unit  108  may have one of its input connected to the output of the differential amplifier  107  to receive the Z-direction differential signal. The Z-direction feedback control unit  108  may also receive a predetermined gain signal and output a Z scan driving signal. The output of the Z-direction feedback control unit  108  may be connected to the 3D profilometry data storage unit  110  and the Z scan voltage input of the stage  105 . Under the Z scan voltage, a Z-direction leveling and focusing mechanism (not shown) provided for the stage  105  moves and thus brings the photoresist  111  to rise or fall. In this way, the spacing between the probe and the photoresist  111  may be adjusted and always maintained in the nanometer order. This may achieve feedback control by the Z-direction feedback control unit  108  and facilitate the 3D topography measurement of the photoresist  111  disposed on the wafer  106 . The 3D topography data storage unit  110  may also be connected to the scan signal generator  109  to receive the output (x, y) coordinates, and to the Z-direction feedback control unit  108  to receive the outputted z coordinate. Then the storage unit  110  may store the (x, y, z) coordinates as the 3D topography data for the photoresist  111 . Since multiple exposure fields (DIE) on the wafer  106  are subjected to a series of processes, like exposure, development, etching, etc., in exactly the same process condition, the topography and structure of these exposure fields are also uniform. Accordingly, it is possible to improve the efficiency with an operating mode in which scanning and measurement of the multiple exposure fields may be simultaneously performed by using multiple respective micro suspension arms. 
     The force detection unit  103  may include a micro suspension arm  201 . The micro suspension arm  201  may be form by a silicon or silicon nitride wafer typically having a length of 100-500 μm and a thickness of 500 nm-500 μm. The micro suspension arm  201  may have a probe provided at one of its ends. The probe may have a sharp tip which may be used to detect the atomic interaction force between the probe and the photoresist  111 . The position detection unit  104  may include a position-sensitive photoelectric detector  203  which may be a quadrant detector or a CCD image sensor. The 6-freedom precision stage  105  may be provided with a Z-direction leveling and focusing mechanism (not shown) which may be composed of three piezoelectric ceramics arranged in equilateral. The 3D topography data storage unit  110  may associate the (x, y) coordinates obtained by the scan signal generator with the z coordinate obtained by the Z-direction feedback control unit  108  and store them as 3D topography data (x, y, z). 
     In operation, a laser beam emitted by the laser  101  is focused by the focus optical unit  102  onto the backside of the micro suspension arm  201 , and then reflected from the backside to the position-sensitive photoelectric detector  203 . During the XY 2D scanning process, the wafer  106  is moved along with the movement of the stage  105 , and the micro suspension arm  201  is deflected in accordance with the topography of the photoresist  111  due to the interaction force between the atoms of the photoresist  111  surface and the probe tip of the micro suspension arm  201 . The reflected beam is deviated with the deflection of the micro suspension arm  201 . As a result, the detection result of the position-sensitive photoelectric detector  203  varies. The topography information of the photoresist  111  can be obtained by detecting variations in the detection result. Such obtained measurement may have a precision of sub-nanometer order. Meanwhile, the detection result from the position-sensitive photoelectric detector  203  may be differentiated with a reference signal and amplified, and the amplified result may be inputted to the Z-direction feedback control unit  108 . The Z-direction feedback control unit  108  may in turn adjust the Z-directional height of the 6-freedom stage  105 , such that the photoresist  111  may be lifted or lowered to maintain the spacing between the probe and the photoresist  111 . For example, the spacing may be maintained in a specified range, such as 1˜100 nanometers. 
       FIG. 2  shows in principle micro displacement detection with a light spot deflection method according to an embodiment of the present disclosure. A laser beam emitted by the laser  101  is focused by the focus optical unit onto the backside of the micro suspension arm  201 , and then reflected from the backside to the position-sensitive photoelectric detector  203 . The micro suspension arm  201  is deflected with, for example, the fluctuation of the topography of the photoresist  111 , leading to a deflection angle Δα. The reflected beam is thus deflected by an angle of 2Δα on the position-sensitive photoelectric detector  203 . The resulting displacement may be detected using the following equations: 
                   {             Δ   ⁢           ⁢   s     =         (     L   -     Δ   ⁢           ⁢   z   ⁢           ⁢     cos   ⁡     (     2   ⁢           ⁢   β     )           )     ×     sin   ⁡     (     2   ⁢           ⁢   Δ   ⁢           ⁢   α     )         +     Δ   ⁢           ⁢   z   ×     sin   ⁡     (     2   ⁢           ⁢   β     )                           Δ   ⁢           ⁢   s     ≈     L   ×   2   ⁢           ⁢   Δ   ⁢           ⁢   α       =       L   ×   2   ×       Δ   ⁢           ⁢   z     l       =         2   ⁢           ⁢   L     l     ×   Δ   ⁢           ⁢     z   (   2   )                         (   1   )               
wherein Δs denotes the displacement of a detected light spot, L denotes the optical path of the reflected light, Δz denotes the deviation of the probe, β denotes the initial position angle of the probe, Δα denotes the deflection angle of the micro suspension arm  201 , and l denotes the length of the micro suspension arm  201 . The relation between the probe deviation Δz and the photoelectric detector output may be derived in connection with the following equations (3) and (4):
 
                   {           X   =       k   ⁡     (       I   A     +     I   C     -     I   B     -     I   D       )           I   A     +     I   B     +     I   C     +     I   D                     Y   =       k   ⁡     (       I   A     +     I   B     -     I   C     -     I   D       )           I   A     +     I   B     +     I   C     +     I   D                         (   3   )                 Δ   ⁢           ⁢   s     =         X   2     +     Y   2                 (   4   )               
wherein I A             I B           I C           I D  denotes four current outputs from the photoelectric detector  203 , X, Y denotes the detected x- and y-direction displacement of a light spot on the photoelectric detector  203 , respectively, and k is a correction factor.

       FIG. 3  is a simplified diagram showing the control configuration of the Z-direction feedback control unit  108  of  FIG. 1 . The feedback control unit  108  may include a forward amplifier  303 , a decoder  302 , a scaling unit  304 , an integration unit  305  and a voltage amplification unit  306 . 
     The Z-direction feedback control unit  108  may use a digital feedback control loop. The input and output of the decoder  302  may be connected with the input of the forward amplifier  303  and the output of the position-sensitive photoelectric detector  301 , respectively. A current signal outputted from the position-sensitive photoelectric detector  301  may be decoded by the decoder  302  and then amplified by the forward amplifier  303 , which may have its output connected with one of the inputs of the differential amplifier  107 . The differential amplifier  107  may receive the amplified current signal and a reference signal and output a differential signal. The output of the differential amplifier may be connected to the scaling unit  304  which is connected in series with the integration unit  305  and the voltage amplification unit  306 . 
     In some embodiments, the differential signal may be scaled by the scaling unit  304  with a factor Kp, adjusted by the integration unit  305  with an integration time factor Ki, and then adjusted by the voltage amplification unit  306  with an amplification coefficient K. As such, the feedback loop may control to drive the Z-direction leveling and focusing mechanism of the stage  105  such that the spacing between the probe and the photoresist  111  is maintained. More specifically, during the scanning process where the wafer  106  is moved along with the movement of the stage  105 , the strength of the interaction force between the probe and the photoresist  111  may be derived from the output of the position-sensitive photoelectric detector  301 . Accordingly, the Z-direction height information may be obtained for the photoresist  111  surface, and such information may be used to change the Z-direction driving voltage for moving the stage  105 . In this way, the spacing between the probe and the photoresist  111  may be adjusted by the Z-direction leveling and focusing mechanism formed of piezoelectric ceramics (PZT) and maintained in the nanometer order. This achieves the desired feedback control. 
     To increase the yield in the industry, a wafer having a diameter of about 300 mm is commonly used. Currently preparation for using a wafer of an about 400 mm diameter is undergoing. Such large-area wafer typically includes a plurality of exposure fields. These exposure fields (DIE) on the wafer are subjected to a series of processes, like exposure, development, etching, etc., in exactly the same process condition, and thus the topography and structure of these exposure fields are also uniform. Accordingly, it is possible to improve the efficiency with an operating mode in which scanning and measurement of the multiple exposure fields may be simultaneously performed by using multiple respective micro suspension arms. 
       FIG. 4  is a schematic diagram showing the scanning operation of a focus detection apparatus having multiple probes according to an embodiment of the present disclosure. As shown in  FIG. 4 , first to fifth position-sensitive photoelectric detectors  403 - 1  to  403 - 5 , first to fifth micro suspension arms  402 - 1  to  402 - 5 , and first to fifth laser beams  404 - 1  to  404 - 5 . The numbers 1, 2, . . . , n denote how many exposure fields  401  are provided in the wafer  106 . The number of the micro suspension arms  402 - 1  to  402 - 5  may be the same as the number of columns of the exposure fields  401 , and they may be fixedly connected at the same height. The first to fifth laser beams  404 - 1  to  404 - 5  may be incident onto the backsides of the first to fifth micro suspension arms  402 - 1  to  402 - 5  respectively, and reflected to the first to fifth position-sensitive photoelectric detectors  403 - 1  to  403 - 5  respectively. The wafer  106  may be moved with the movement of the stage along the thin solid line in  FIG. 4  during the XY 2D scanning process. The multiple probes of the micro suspension arms  402 - 1  to  402 - 5  may operate simultaneously to efficiently measure the 3D topography of the photoresist over the wafer  106 . The measurements may be provided as data basis for leveling and focusing action in subsequent exposure process. 
     The foregoing description is only made to the embodiments of the present disclosure, and the present disclosure is not limited thereto. It should be noted to those ordinarily skilled in the art that various modifications and refinements can be made within the principle of the present disclosure and should be encompassed by the scope of the present disclosure.