Patent Publication Number: US-2022236198-A1

Title: Wafer inspection apparatus and method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of and claims the priority benefit of a prior application Ser. No. 16/805,849, filed on Mar. 2, 2020 and now allowed. The prior application Ser. No. 16/805,849 claims the priority benefit of U.S. provisional applications Ser. No. 62/880,668, filed on Jul. 31, 2019. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND 
     Fabricating semiconductor devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. The semiconductor fabrication processes may include, but are not limited to, implantation processes, deposition processes such as chemical vapor deposition (CVD), plasma-enhanced CVD (PECVD), physical vapor deposition (PVD), lithography and etching processes, grinding and polishing processes, and so on. 
     Inspection processes are performed at various steps during the fabrication of the semiconductor devices to detect defects on substrates (e.g. wafers) to promote higher yield in the fabricating process and thus higher profits. In the drive for greater efficiencies throughout the fabrication process, wafer inspection efficiency is a topic of interest. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a schematic view of an inspection apparatus in accordance with a first embodiment of the disclosure. 
         FIG. 2A  is a schematic view of an inspection apparatus in accordance with a second embodiment of the disclosure.  FIG. 2B  is a schematic view of the configuration of wafer stages and optical components of the inspection apparatus in accordance with the second embodiment of the disclosure.  FIG. 2C  is a perspective view of an optical splitting element of the inspection apparatus in accordance with the second embodiment of the disclosure. 
         FIG. 3  is a schematic view of an inspection apparatus in accordance with a third embodiment of the disclosure. 
         FIG. 4  is a schematic view of an inspection apparatus in accordance with a fourth embodiment of the disclosure. 
         FIG. 5  is a schematic view of an inspection apparatus in accordance with a fifth embodiment of the disclosure. 
         FIG. 6  is a flowchart illustrating an inspection method using the inspection apparatus in accordance with some embodiments of the disclosure. 
         FIG. 7  is a schematic cross-sectional view illustrating a wafer carried by a wafer stage according to some embodiments of the disclosure. 
         FIG. 8  is a schematic view illustrating an arrayed waveguide grating in accordance with some embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below for the purposes of conveying the present disclosure in a simplified manner. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a second feature over or on a first feature in the description that follows may include embodiments in which the second and first features are formed in direct contact, and may also include embodiments in which additional features may be formed between the second and first features, such that the second and first features may not be in direct contact. In addition, the same reference numerals and/or letters may be used to refer to the same or similar parts in the various examples the present disclosure. The repeated use of the reference numerals is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath”, “below”, “lower”, “on”, “over”, “above”, “upper” and the like, may be used herein to facilitate the description of one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Various embodiments of the disclosure are directed to provide inspection apparatus (e.g., wafer inspection apparatus) capable of implementing multi-wafer inspection, so as to improve inspection efficiency. 
       FIG. 1  is a schematic view of an inspection apparatus  100  in accordance with at least one embodiment of the disclosure. 
     Referring to  FIG. 1 , in some embodiments, the inspection apparatus  100  is configured to inspect at least two workpieces W 1 , W 2 . The inspection apparatus  100  may produce images of inspected workpieces W 1 , W 2  for determining whether either or both of the workpieces W 1 , W 2  has a defect. The workpieces W 1 , W 2  may be substrates. The substrate may include glass, silicon, ceramic, metal, stainless steel, plastic, resin, a composite material, tape, film, or other suitable materials. In some embodiments, the substrate is a semiconductor wafer. 
     In some embodiments, the inspection apparatus  100  includes an optical module OM 1 , a workpiece holder  160  for carrying a plurality of workpieces W 1 , W 2 , one-way mirrors  151 ,  152 , and optical sensors  171 ,  172 . 
     The optical module OM 1  is configured to emit a plurality of light beams for simultaneously inspecting the workpieces W 1 , W 2  carried by the workpiece holder  160 . In some embodiments, the optical module OM 1  includes a light source  110 , an optical amplifier  120 , an optical directional element  130 , and an optical splitting element  140 . The light source  110  is configured to emit a light L, such as a laser beam. 
     The optical amplifier  120  is configured to amplify and control the intensity of the light L and thus control the intensities of the light beams L 1 , L 2  split from the light L. The light L emitted from the light source  110  may be further intensified efficiently by using the optical amplifier  120 , in order to provide desired optical intensity. In some embodiments, the optical amplifier  120  increases the intensity of the light L to twice or four times or more of its original intensity according to product design and requirement. In some embodiments, the optical amplifier  120  may be a solid-state amplifier, a doped fiber amplifier or a semiconductor optical amplifier, but the disclosure is not limited thereto. In some embodiments, the optical amplifier  120  is a laser amplifier, such as an RF pumped, fast axial flow, CO 2  laser amplifier. In some embodiments, the optical amplifier  120  is disposed immediately adjacent to the light source  110 , but the disclosure is not limited thereto. Alternatively or additionally, the optical amplifier(s) may be disposed at other positions to adjust the intensity of the light before the light is directed to the workpieces. For example, the optical amplifier may be disposed between the optical direction element  130  and the optical splitting element  140 , or disposed between the optical splitting element  140  and the workpiece holder  160 . However, the disclosure is not limited thereto. 
     In some embodiments, the optical module OM 1  may optionally include the optical directional element  130  for controlling the optical path of the light L. In some embodiments, the optical directional element  130  includes a reflector unit. The reflector unit may include a plurality of reflective mirrors for guiding the light L in an intended direction. The optical direction element  130  may include reflective mirrors  131 ,  132  and  133 , which may also be referred to as a trombone mirror unit (TMU). Number and configuration of the reflective mirrors included in the optical direction element  130  shown in the figure is merely for illustration, and the disclosure is not limited thereto. Embodiments including other suitable optical elements which can control the optical path of light are also contemplated herein. 
     The optical splitting element  140  is configured to split the light L into a plurality of light beams for inspecting multiple workpieces carried by the workpiece holder  160 . In some embodiments, the optical splitting element  140  may split the light L into two light beams L 1 , L 2  along two opposite vertical directions. The light L may be uniformly split into the two light beams, with the intensity of each of the two light beams being about half the intensity of the light L, but the disclosure is not limited thereto. In some embodiments, the optical splitting element  140  includes a beam splitter  142  and a total reflection mirror  144 . When a light is directed to the optical splitting element  140 , a portion of (e.g. half of) the incident light is reflected by the beam splitter  142 , and another portion of (e.g. half of) of the incident light transmits through the beam splitter  142  and is reflected by the total reflection mirror  144 . The beam splitter  142  may split an unpolarized light into two unpolarized lights, or split a polarized light (e.g. p-polarized light or s-polarized light) into two polarized lights, or split an unpolarized light into two polarized light (e.g. p-polarized light and s-polarized light). In some embodiments, the optical splitting element  140  is configured to be fixed during wafer inspection. 
     In some embodiments, the optical module OM 1  further includes one or more polarizers configured to polarize the lights to be directed to workpieces carried by the workpiece holder  160 , and the lights may respectively be polarized as p-polarized light or s-polarized light depending on the type of the defect to be detected. A polarizer  180  may be disposed along the light path of the light L at a position before the light L enters the optical splitting element  140  and configured to polarize the light L before being split by the optical splitting element  140 . The polarizer  180  may be disposed between the optical directional element  130  and the optical splitting element  140 . In some embodiments, the polarizer  180  is omitted, and one or more polarizers (not shown) are configured to polarize the light beam(s) L 1 , L 2  after the light L is split by the optical splitting element  140 . One or more polarizers may be disposed between the optical splitting element  140  and the workpiece W 1  and/or between the optical splitting element  140  and the workpiece W 2 , such as between the beam splitter  142  and the one-way mirror  151  and/or between the total reflection mirror  144  and the one-way mirror  152 . In some embodiments, the beam splitter  142  is a polarizing beam splitter for splitting an unpolarized light into two polarized lights. The beam splitter  142  may transmit p-polarized light and reflect s-polarized light. 
     In some embodiments, the workpiece holder  160  includes a plurality of workpiece stages  161 ,  162  for carrying a plurality of workpieces W 1 , W 2 . The workpieces W 1 , W 2  may be substrates, such as wafers. Accordingly, the workpiece holder  160  may include substrate holders, such as wafer holders, and the workpiece stages  161 ,  162  may be substrate stages, such as wafer stages. 
     In some embodiments, the wafer holder  160  includes a first wafer stage  161  for carrying a first wafer W 1 , a second wafer stage  162  for carrying a second wafer W 2 , and a connecting element  165  for connecting the first and second wafer stages  161  and  162 . In some embodiments, the first wafer stage W 1  and the second wafer stage W 2  are connected to opposite ends of the connecting element  165 , and are fixed on the connecting element  165 . Herein, the wafer stages being fixed on the connecting element means that the relative positional relation between the wafer stages and the connecting element are fixed. The connecting element  165  may be connected to sidewalls of the wafer stages  161  and  162 , but the disclosure is not limited thereto. In some embodiments, the connecting element  165  is connected to back surfaces of the wafer stages  161  and  162 . Herein, the back surface of the wafer stage refers to the surface opposite to the front surface  10  (i.e. receiving surface  10 ) of the wafer stage for receiving a wafer. In some embodiments, the wafer stages  161 ,  162  are symmetric about the connecting element  165 , but the disclosure is not limited thereto. In some embodiments, the wafer stages  161 ,  162  are configured as face to face, such that the front surfaces F 1  of wafers W 1 , W 2  to be inspected are configured as face to face. In some embodiments, the wafer stages  161 ,  162  are disposed at opposite sides of the optical splitting element  140 . 
     In some embodiments, the wafer holder  160  is operable. For example, the wafer holder  160  is able to rotate about a rotation axis RA. The rotation axis RA may be along the horizontal direction X which passes through the center of the connecting element  165 . The rotation axis RA is along a direction parallel with the receiving surfaces  10  of the wafer stages  161  and  162  or the front surfaces F 1  of the wafers W 1 , W 2  carried by the wafer stages. In some embodiments, the wafer holder  160  rotates about the rotation axis RA in a clockwise or a counterclockwise direction by any reasonable degree (e.g. 0 to 360 degrees). That is to say, the wafer stages  161 ,  162  of the wafer holder  160  are able to rotate about the rotation axis RA, such that the wafers W 1 , W 2  carried by the wafer stages  161 ,  162  of the wafer holder  160  rotate as the wafer holder  160  rotates. In some embodiments, during wafer inspection, the wafer holder  160  stops rotating and is located at the inspection position as shown in  FIG. 1 . In some embodiments, the wafer stages  161 ,  162  are located at opposite vertical sides of the optical splitting element  140  and are overlapped with the optical splitting element  140  in the vertical direction Z during inspection. In some embodiments, the first wafer stage  161  is disposed below the optical splitting element  140 , and the second wafer stage  162  is disposed over the optical splitting element  140 . The wafer holder  160  is movable along the horizontal directions, such as the directions X, Y. 
     In alternative embodiments, the wafer holder  160  is movable in the horizontal directions, such as the directions X, Y, but is not rotatable. For example, the wafer holder  160  is configured to be fixed at the inspection position as shown in  FIG. 1  and is not rotatable. 
     In the present embodiments, when the wafer holder  160  moves along the horizontal direction (e.g. direction X or direction Y), the wafers W 1 , W 2  carried by the wafer holder  160  simultaneously move along a same direction as the wafer holder  160  moves. 
     In some embodiments, the wafer stages  161 ,  162  are or include an electrostatic chuck (E-chuck), respectively. The E-chucks use an electric force to secure the wafers W 1 , W 2 . In other embodiments, the wafer stages  161 ,  162  respectively include a chuck that uses clamps to secure the wafers W 1 , W 2 . In alternative embodiments, the wafer stages  161 ,  162  respectively include a vacuum chuck that generates vacuum pressure through vacuum ports in the chuck to hold the wafers W 1 , W 2  thereon. Combinations of the above chucks may also be used. However, the disclosure is not limited thereto. The wafers W 1 , W 2  may be carried by the wafer stages  161 ,  162  through any appropriate mounting force. 
       FIG. 7  is a schematic cross-sectional view illustrating a wafer W carried by a wafer stage WS including an E-chuck. The wafer stage WS may be one of the wafer stages  161 ,  162 , and the wafer W may be the corresponding one of the wafers W 1 , W 2 . 
     Referring to  FIG. 7 , in some embodiments, the E-chuck of the wafer stage WS includes an electrode EC embedded near the receiving surface  10  for receiving the wafer W, and the receiving surface  10  is directly over and overlaps the electrode EC. The electrode EC may be covered by a dielectric material such as an oxide or a ceramic, or the like, so as to separate the electrode EC from the wafer W. In other words, the receiving surface  10  is a surface of the dielectric material. In some embodiments, the electrode EC is electrically coupled to a power source (not shown). A voltage provided by the power source may be applied to the electrode EC. In some embodiments, the power source may be configured to provide a direct current (DC) or alternating current (AC) power to the electrode EC. In some other embodiments, the power source is configured to provide radio frequency (RF) power to the electrode EC. In a chucking mode, the power source is turned on, and a high voltage is provided by the power source and applied to the electrode EC. The electrode EC is then charged to generate an electrostatic force to attract the wafer W, such that the wafer W is secured by the wafer stage WS. In some embodiments, the wafer W is in contact with the receiving surface  10  of the dielectric material, but the disclosure is not limited thereto. In alternative embodiments, a supporting element (not shown) may be disposed between the wafer W and the receiving surface  10 , such that the wafer W is not in direct contact with the receiving surface  10  and a gap exists between the wafer W and the receiving surface  10  of the wafer stage WS. The wafer W has a front surface F 1  and a back surface F 2 . Throughout the specification, the front surface F 1  of the wafer refers to the surface to be inspected, and the back surface F 2  of the wafer refers to the surface opposite to the front surface and facing the receiving surface of the wafer stage. That is, the back surface F 2  of the wafer W may be in contact with or separate from the receiving surface  10  of the wafer stage WS. In a de-chucking mode, the power source is turned off, and the electrostatic force is eliminated, such that the wafer W may be removed from the wafer stage WS. 
     Referring back to  FIG. 1 , in some embodiments, the optical splitting element  140  is located between the wafer stage  161  and the wafer stage  162 . In some embodiments, the distance between the optical splitting element  140  and the wafer stage  161  and the distance between the optical splitting element  140  and the wafer stage  162  are the same or different. 
     The one-way mirrors  151  and  152  are disposed between the optical splitting element  140  and the wafer stages  161  and  162 , respectively. In some embodiments, the one-way mirror is configured to transmit the light incoming from a first side, and reflect the light incoming from a second side opposite to the first side. 
     The optical sensors  171 ,  172  are configured to receive the light beams reflected by the wafers W 1 , W 2 , and generate the inspection results (e.g. images) of the wafers W 1 , W 2 . In some embodiments, the optical sensors  171 ,  172  may include time delay and integration (TDI) sensors, but the disclosure is not limited thereto. Other suitable optical image capturing components may also be used. 
       FIG. 6  is a flowchart of an inspection method using the inspection apparatus described herein. Although the method is illustrated and/or described as a series of acts (processes) or events, it will be appreciated that the method is not limited to the illustrated ordering or acts. Thus, in some embodiments, the acts may be carried out in different orders than illustrated, and/or may be carried out concurrently. Further, in some embodiments, the illustrated acts or events may be subdivided into multiple acts or events, which may be carried out at separate times or concurrently with other acts or sub-acts. In some embodiments, some illustrated acts or events may be omitted, and other un-illustrated acts or events may be included. 
     The inspection method may be used to inspect any kind of workpiece, such as a substrate. In some embodiments, the substrate is a semiconductor wafer, and an illustrative embodiment of a wafer inspection method is described as below with reference to  FIG. 1  and  FIG. 6 . 
     Referring to  FIG. 1  and  FIG. 6 , in some embodiments, the wafers may be loaded to the inspection apparatus before or after the processing of wafer(s). For example, after the wafer(s) is/are subjected to deposition process(es), such as CVD, PECVD to deposit a dielectric layer or a polymer layer, lithography and etching processes, chemical mechanical polishing (CMP) processes and/or any other appropriate semiconductor fabrication processes, the wafers W 1 , W 2  are loaded to the inspection apparatus to be inspected, so as to determine whether the wafers have defect. In some embodiments, the wafer holder includes a plurality of wafer stages, and multiple wafers carried by multiple wafer stages are inspected simultaneously. 
     In some embodiments, at act  610 , a plurality of wafers are loaded to the wafer stages of the wafer holder. For example, the wafer W 1  is loaded to the wafer stage  161  of the wafer holder  160 , and the wafer W 2  is loaded to the wafer stage  162  of the wafer holder  160 . The wafers W 1 , W 2  may be carried by the wafer stages  161 ,  162  by electrostatic force and/or mechanical force. In some embodiments in which the wafer holder  160  is rotatable, the loading of the each of the wafers W 1 , W 2  is performed at a loading position. The loading position refers to a position where the receiving surface  10  of the wafer stage  161  or the wafer stage  162  faces upward, such as the position of the wafer stage  161  shown in  FIG. 1 . In some embodiments, before the wafers W 1 , W 2  are loaded, the wafer stage  162  rotates to the loading position (i.e. the position where the wafer stage  161  is located shown in  FIG. 1 ) such that the receiving surface  10  of the wafer stage  162  faces upward. The wafer stage  162  is at a lower position for loading, and the wafer stage  161  is at an upper position over the wafer stage  162  (not shown). Thereafter, the wafer W 2  is loaded onto the receiving surface  10  of the wafer stage  162  with the front surface F 1  of the wafer W 2  facing upward. In some embodiments, the wafer W 2  is placed on the wafer stage  162  by a mechanical arm (not shown), with the front surface F 1  thereof facing up. Thereafter, in some embodiments in which the wafer stage  162  includes an E-chuck as described in  FIG. 7 , the power source of the E-chuck is turned on, and the electrode EC is charged to generate an electrostatic force to attract the wafer W 2 , such that the wafer W 2  is secured by the wafer stage  162 . 
     After the wafer W 2  is held by the wafer stage  162 , the mechanical arm moves away. The wafer holder  160  then rotates around the rotation axis RA by 180 degrees along clockwise or counterclockwise direction. That is, the wafer holder  160  is flipped upside down, such that the wafer stage  161  rotates to the loading position with the receiving surface  10  thereof facing upward for loading a wafer, while the wafer W 2  carried by the wafer stage  162  is moved to the upper position as shown in  FIG. 1  and the front surface F 1  of the wafer W 2  faces downward. In some embodiments, the electrostatic force generated by the E-chuck of the wafer stage is strong enough to avoid the wafer W 2  detaching from the wafer stage  162  when the wafer stage  162  rotates to the upper position. 
     Thereafter, the wafer W 1  is loaded to the wafer stage  161  by the loading method similar to that of the wafer W 2  as described above. The wafer W 1  is placed on the wafer stage  161  by the mechanical arm (not shown) with the front surface F 1  of the wafer W 1  facing up. In the embodiments is which the wafer stage  161  includes a E-chuck ( FIG. 7 ), the power source of the S-chuck is turned on, and the electrode EC is charged to generate an electrostatic force to attract the wafer W 1 , such that the wafer W 1  is secured by the wafer stage  161 . 
     After the wafers W 1 , W 2  are secured by the wafer stages  161 ,  162 , the wafer holder  160  may rotate to the inspection position and stop rotating for wafer inspection. An inspection position refers to the position of the wafer holder  160  during inspection. In some embodiments, the wafer stages  161 ,  162  are located at opposite vertical sides of the optical splitting element  140 , as shown in  FIG. 1 . 
     In some embodiments in which the wafer holder  160  is not rotatable, the wafer stages  161 ,  162  are designed and configured to be at the inspection position. The wafers W 1 , W 2  are respectively loaded to the wafer stages  161 ,  162  at the position shown in  FIG. 1 . In some embodiments, the first wafer stage  161  is at the lower position and the second wafer stage  162  is at the upper position. The receiving surfaces  10  of the wafer stages  161 ,  162  are in a face-to-face configuration. The receiving surface  10  of the wafer stage  161  faces upward, while the receiving surface  10  of the wafer stage  162  faces downward. The wafer W 1  is placed on the wafer stage  161  by a mechanical arm with the front surface F 1  of the wafer W 1  facing up, and the wafer W 1  is then secured by the wafer stage  161  through electrostatic force and/or mechanical force, for example. The wafer W 2  is moved to the wafer stage  162  at the upper position by a mechanical arm, such that the front surface F 1  of the second wafer W 2  faces downward. In some embodiments, the wafer stage  162  includes an E-chuck to attract and secure the wafer W 2  by electrostatic force. In some other embodiments, the wafer stage  162  includes both a clamp and an E-chuck. The clamp provides initial force to hold the wafer W 2  by mechanical force, then the E-chuck attracts the wafer W 2  by electrostatic force, so as to hold the wafer W 2  firmly. In some embodiments, the electrostatic force is strong enough to prevent detachment of the wafer W 2  from the wafer stage  162 . After the wafers W 1 , W 2  are held by the wafer stages  161 ,  162 , the mechanical arms move away. In some embodiments, one mechanical arm is used to load the wafers W 1 , W 2  to the wafer stages  161 ,  162  sequentially. In alternative embodiments, two mechanical arms are used to load the first wafer W 1  and the second wafer W 2  simultaneously. 
     In some embodiments, after the wafers are loaded to the wafer stages, an alignment is executed, as shown in the act  620  of  FIG. 6 . In some embodiments, the inspection method adopts a wafer coordinate system for determining and recording the positions of a point (such as a defect) of the wafer. The coordinate system includes an x coordinate and a y coordinate, and a coordinate of the point may be expressed as (x value, y value). In some embodiments, the alignment is executed to determine a reference point of the wafer as an origin of the wafer coordinate system, and the coordinates of all points on the wafer are determined with respect to the origin. In some embodiments, after the wafers W 1 , W 2  are secured in place by the wafer stages  161 ,  162 , light beams, such as the light beams L 1 , L 2  emitted from the optical module OM 1 , irradiate the wafers W 1 , W 2 , respectively. The points aligned with the light beams L 1 , L 2  may be selected to be the origin of the wafer coordinate system of the wafer W 1 , W 2 . In some embodiments, an alignment mark is disposed at each origin of the wafers W 1 , W 2 . The above alignment method is but one suitable process for determining an origin of wafer coordinate system, and any other appropriate alignment method in the field may also be used. 
     At act  630 , a plurality of wafers such as the wafers W 1  and W 2  are inspected simultaneously. In some embodiments, the act  630  includes the act  631  (emitting light from a light source), the act  632  (splitting the light into light beams) and the act  633  (receiving the light beams reflected by the wafers). More detailed description of each of these acts  631 ,  632 ,  633  follows. 
     At act  631 , a light is emitted from a light source. In some embodiments, a light L, such as a laser beam, is emitted by the light source  110 . In some embodiments, the light L emitted by the light source  110  enters an optical amplifier  120 , and is amplified by the optical amplifier  120 . The optical amplifier  120  controls the optical intensity of the light L and increases the optical intensity of the light L to any desired intensity. 
     At act  632 , the light L is split into a plurality of light beams, and the plurality of light beams are directed to the wafers W 1 , W 2  for multi-wafer inspection. In some embodiments, after the light L is amplified by the optical amplifier  120 , the light L is guided to the optical splitting element  140  by the optical directional element  130 . In some embodiments, the light L enters the optical directional element  130 , and is reflected by the reflective mirrors  131 ,  132 ,  133  in sequence, and then directed toward the optical splitting element  140 . 
     Thereafter, the light L is split by the optical splitting element  140  into a first light beam L 1  and a second light beam L 2  along different light paths for inspecting the wafers W 1 , W 2 , respectively. In some embodiments, the light L enters the beam splitter  142  and is split by the beam splitter  142  into a first light beam L 1  and a second light beam L 2 . In some embodiments, a portion (such as, half) of the light L is reflected by the beam splitter  142  as the first light beam L 1  directed toward the wafer W 1 . Another portion (such as, half) of the light L transmits through the beam splitter  142  as the second light beam L 2 . The second light beam L 2  is then reflected by the total reflection mirror  144  and is directed toward the wafer W 2 . In some embodiments, the first light beam L 1  and the second light beam L 2  exiting from the optical splitting element  140  exit in opposite directions along the vertical direction Z. In some embodiments, the first light beam L 1  travels downward to the wafer W 1 , and the second light beam L 2  travels upward to the wafer W 2 . 
     In some embodiments, the first light beam L 1  coming out from the optical splitting element  140  passes through the one-way mirror  151  and shines on the front surface F 1  of the wafer W 1 . The second light beam L 2  coming out from the optical splitting element  140  passes through the one-way mirror  152  and shines on the front surface F 1  of the wafer W 2 . In some embodiments, the light beams L 1 , L 2  are directed to the wafers W 1 , W 2  at a substantially normal angle of incidence. 
     With the first light beam L 1  and the second light beam L 2  shining on the front surfaces F 1  of the wafers W 1 , W 2 , the wafer holder  160  moves in the horizontal directions, such as the directions X, Y. Meanwhile, the optical splitting element  140  is fixed. As such, the wafers W 1  and W 2  carried by the wafer holder  160  simultaneously move along the same horizontal directions as the wafer holder  160  moves, such that the whole front surfaces F 1  of the wafers W 1  and W 2  are scanned by the first light beam L 1  and the second light beam L 2 , respectively. In some embodiments, the first light beam L 1  and the second light beam L 2  shinning on the wafers W 1  and W 2  may have the same intensities, and the intensity of each of the first light beam L 1  and the second light beam L 2  may be half of the intensity of the light L after being amplified by the optical amplifier  120 . 
     In some embodiments, depending on the type of defect to be inspected, the wafer may be scanned by a polarized light or an unpolarized light, that is, the first beam L 1  and the second light beam L 2  directed to the wafers W 1  and W 2  may respectively be a polarized (e.g. s-polarized or p-polarized) light or an unpolarized light. In some embodiments in which the polarizer  180  is disposed before the optical splitting element  140 , the light L emitted by the light source  110  is polarized before the light L is split by the optical splitting element  140 , and the light L may be polarized as a p-polarized or s-polarized light. The polarized light L is split by the optical splitting element  140  into polarized light beams L 1  and L 2 . In such embodiments, the first and second light beams L 1  and L 2  have the same polarization. In some other embodiments, the light may be polarized after being split into the first and second light beams. In some embodiments in which the polarizer  180  is omitted, and one or more polarizers are disposed between the optical splitting element  140  and the one-way mirror  151  and/or between the optical splitting element  140  and one-way mirror  152 , the light L is firstly split into the first light beam L 1  and the second light beam L 2 , and thereafter, the first light beam L 1  and/or the second light beam L 2  may be polarized before transmitting through the one-way mirror  151  and/or  152 . In such embodiments, at least one of the first and second light beams L 1  and L 2  shining on the wafers W 1  and W 2  is polarized, and the polarization of the first and second light beams L 1  and L 2  may be the same or different. In some embodiments in which the optical splitting element  140  is a polarizing optical splitting element, the light L is split into two polarized light L 1  and L 2  having different polarizations. In some embodiments, the first light beam L 1  is p-polarized light, and the second light beam L 2  is s-polarized light. In some embodiments, the polarizers are omitted, and unpolarized light L is split by the optical splitting element  140  into two unpolarized light beams L 1  and L 2 . 
     At act  633 , the light beams L 1 , L 2  reflected by the wafers W 1 , W 2  are received by the optical sensors  171 ,  172 , so as to generate the inspection results of the wafers W 1 , W 2 . In some embodiments, the first light beam L 1  and the second light beam L 2  shone on the wafers W 1  and W 2  are reflected by the front surfaces F 1  of the wafers W 1  and W 2 , respectively. The first light beam L 1  reflected by the wafer W 1  is then reflected by the one-way mirror  151  and directed to the optical sensor  171 . The second light beam L 2  reflected by the wafer W 2  is then reflected by the one-way mirror  152  and directed toward the optical sensor  172 . 
     The optical sensor  171  receives the first light beam L 1  reflected from the wafer W 1 , and generates an inspection result of the wafer W 1 . The optical sensor  172  receives the second light beam L 2  reflected from the wafer W 2 , and generates an inspection result of the wafer W 2 . 
     At act  640 , the inspection results of wafers are saved for further analysis. In some embodiments, the inspection results may be transmitted to and stored by a storage medium, such as a computing system. In some embodiments, the inspection results are the images of the front surfaces F 1  of the wafers W 1  and W 2 . In some embodiments, neighbor-die comparison method is performed to determine whether the wafer W 1 , W 2  has a defect or not. Taking the wafer W 1  as an example, the image of the wafer W 1  captured by the optical sensor  171  includes the images of a plurality of dies in die regions of the wafer W 1 . The image of one die (namely, center die) is compared to images of its immediate neighbor dies using an optical testing method. The method then moves to the next die and compares the image of the next die against the images of its neighbor die. The above comparison is repeated until all of the dies in the wafer W 1  have been compared. During the comparison, if any difference (which may be a defect) is found, the difference is noted and the die having the difference is marked. In some embodiments, the coordinate of the difference in the wafer coordinate system is recorded. The coordinate of the difference refers to the position of the difference with respect to the origin of the wafer coordinate system determined at act  620 . Thereafter, a defect review process is performed. In some embodiments, the wafer W 1  is inspected again by a scanning electron microscope (SEM) to get a clearer image of the wafer W 1  to double-check the difference and to determine whether the wafer W 1  has a defect. In some embodiments, another appropriate comparison method may be used. In some embodiments, the images of the wafers W 1  and W 2  are compared with reference images to determine whether the wafers W 1  and W 2  have defects. In some embodiments, the reference images may be images of wafers with or without defects. 
     At act  650 , the wafers W 1  and W 2  are unloaded from the wafer stages  161  and  162 . In some embodiments, the power source of the E-chuck is turned off, and the wafer W 1 , W 2  is unloaded from the wafer stage  161 / 162  by mechanical arm for further processing. If the wafer is determined to have a defect, appropriate processes may be performed to eliminate the defect before performing further semiconductor fabrication process. 
       FIG. 2A  is a schematic view of an inspection apparatus  200  according to various embodiments of the disclosure.  FIG. 2B  is a schematic view of the configuration of wafer stages and optical components of the inspection apparatus  200  according to various embodiments of the disclosure.  FIG. 2B  is a right side view of  FIG. 2A , and some components are omitted in  FIG. 2B  for sake of brevity.  FIG. 2C  is a perspective view of an optical splitting element of the inspection apparatus  200  according to various embodiments of the disclosure. The configuration of the defect inspection apparatus  200  is similar to the defect inspection apparatus  100  shown in  FIG. 1 , except that four wafers can be simultaneously inspected by the inspection apparatus  200 . Like elements are designated with the same or similar reference numbers for ease of understanding and the details thereof are not repeated herein. 
     Referring to  FIG. 2A  and  FIG. 2B , in some embodiments, the inspection apparatus  200  includes the optical module OM 2 , the wafer holder  160  for carrying wafers W 1  and W 2 , the wafer holder  260  for carrying wafers W 3  and W 4 , the one-way mirrors  151 , 152 ,  251 , and  252  (one-way mirrors  251  and  252  are not shown in  FIG. 2A ), the optical sensors  171  and  172  and other two optical sensors not shown in the figures. The optical module OM 2  may emit four light beams for inspecting the wafers W 1  to W 4  simultaneously. In some embodiments, the optical module OM 2  includes the light source  110 , the optical amplifier  120 , the optical directional element  130 , and the optical splitting element  240 . 
     In some embodiments, the wafer holder  260  has a similar structure as the wafer holder  160 . In some embodiments, the wafer holder  260  includes a wafer stage  261  and a wafer stage  262  connected to each other by a connecting element  265 . The structural relations of the wafer stage  261 , the wafer stage  262  and the connecting element  265  are similar to those of the wafer support  160 , which are not described again here. It is noted that, the connecting element  161  and  265  of the wafer holders  160  and  260  are not shown in  FIG. 2B  for the sake of brevity. It should be understood that, in  FIG. 2B , the wafer stages  161  and  162  are connected to each other by the connecting element  165 , and the wafer stages  261  and  262  are connected to each other by the connecting element  265 . 
     In some embodiments, similar to the wafer holder  160 , the wafer holder  260  is also rotatable around the rotation axis RA by any reasonable degree, and the wafers carried by the wafer holder  260  may rotate around the rotation axis RA as the wafer holder  260  rotates. In alternative embodiments, the wafer holders  160  and  260  are not rotatable. In some embodiments, the wafer holder  160  is movable in the horizontal directions X and Y, while the wafer holder  260  is movable in the vertical direction Z and the horizontal direction X, such that the wafers carried by the wafer holders  160  and  260  may move along the corresponding directions as the wafer holders move. In some embodiments, the wafer holders  160  and  260  are separate from each other and may separately move along the same or different directions during the wafer inspection. In some embodiments, the inspection position of the wafer holders  160  and  260  are as shown in  FIG. 2A  and  FIG. 2B , the wafer stages  161  and  162  of the wafer holder  160  are configured at opposite vertical sides of the optical splitting element  240 , and the wafer stages  261  and  262  of the wafer holder  260  are configured at opposite lateral sides of the optical splitting element  240 . 
     In some embodiments, the optical splitting element  240  of the optical module OM 2  may split the light from the light source  110  into four light beams L 1 -L 4  for inspecting four wafers W 1 -W 4  carried by the wafer holders  160  and  260 . In some embodiments, as shown in  FIG. 2A  and  FIG. 2C , the optical splitting element  240  includes a first beam splitter  242 , a second beam splitter  244 , a third beam splitter  246  and a total reflection mirror  248 . 
     In some embodiments, besides the optical amplifier  120 , the optical module OM 2  further includes a plurality of optical amplifiers (not shown) before and/or after the light being split by a beam splitter, so as to control the intensities of the light beams shone on the wafers. In some embodiments, as shown in  FIG. 2A  and  FIG. 2B , optical amplifiers  281 - 284  are configured to amplify the light beams L 1 -L 4 , respectively. In detail, a first optical amplifier  281  is disposed between the first beam splitter  242  of the optical splitting element  240  and the one-way mirror  151 ; a second optical amplifier  282  is disposed between the second beam splitter  244  of the optical splitting element  240  and the one-way mirror  152 ; a third optical amplifier  283  is disposed between the third beam splitter  246  of the optical splitting element  240  and the one-way mirror  251 ; and a fourth optical amplifier  284  is disposed between the total reflection mirror  248  of the optical splitting element  240  and the one-way mirror  252 . In some embodiments, the optical amplifiers are arranged between the first beam splitter  242  and the second beam splitter  244 , between the second beam splitter  244  and the third beam splitter  246 , and/or between the third beam splitter  246  and the total reflection mirror  248 . In alternative embodiments, with the optical amplifiers described above, the optical amplifier  120  may be omitted. 
     When the light is split by the beam splitters  242 ,  244 , and  246 , the intensity of the light will drop after being split. With the optical amplifiers arranged on the optical paths, the light beams L 1 -L 4  can be adjusted and controlled within a suitable range for inspecting the wafers W 1  to W 4 . In some embodiments, the light beams L 1  to L 4  may be controlled to have substantially the same intensity to shine on the wafers. 
     In some embodiments, the optical module OM 2  of the inspection apparatus  200  is free of any polarizer. In alternative embodiments, the optical module OM 2  may include one or more polarizer for polarizing one or more of the light beams L 1 -L 4 , and the one or more polarizer may be disposed between the optical splitting element  240  and the one-way mirrors  151 / 152 / 251 / 252 , or between the optical directional element  130  and the optical splitting element  240 . 
     The inspection method using the inspection apparatus  200  is described below with reference to  FIG. 2A  to  FIG. 2C  and  FIG. 6 . 
     At act  610  and act  620 , the wafers W 1  and W 2  are loaded to the wafer stages  161  and  162  of the wafer holder  160 , the wafers W 3  and W 4  are loaded to wafer stages  261  and  262  of the wafer holder  260 , respectively, and an alignment process is performed to determine origins of the wafer coordinate systems of the wafers. The loading method and alignment method are similar to those described with reference to  FIG. 1 . In some embodiments in which the wafer holders  160  and  260  are rotatable, each wafer may be loaded to the corresponding wafer stage at the loading position with the receiving surface of the wafer stage facing up. In some embodiments, before loading a wafer, one of the wafer stages  161 , 162 , 261 , 262  rotates to the loading position for loading one of the wafers W 1 -W 4 , and thereafter, the wafer stage carrying the wafer rotates to another position, and another one of the wafer stages rotates to the loading position for loading another one of the wafers. Herein, the loading position refers to the position where the receiving surface of the wafer stage faces up, such as the position where the wafer stage  161  is located shown in  FIG. 2A . Thereafter, the rotation and wafer loading are repeated until the four wafers W 1 -W 4  are loaded to the four wafer stages  161 ,  162 ,  261 ,  261 , respectively. As described above, the wafers W 1  to W 4  may be secured by the wafer stages through electrostatic force and/or mechanical force. 
     In some embodiments, after the wafers W 1 -W 4  have been loaded to the wafer stages  161 , 162 ,  261 ,  262  in place, the wafer holders  160  and  260  rotate to the inspection position for wafer inspection. In some embodiments, the wafer stages of the wafer holder  160  rotate to overlap the optical splitting element  240  in the vertical direction Z, such that the wafers W 1  and W 2  carried by the wafer stages  161  and  162  are located on opposite vertical sides of the optical splitting element  240 . The wafer stages of the wafer holder  260  rotate to overlap the optical splitting element  240  in the horizontal direction Y, such that the wafers W 3  and W 4  carried by the wafer stages  261  and  262  are located on opposite lateral sides of the optical splitting element  240 . In some embodiments, the vertical distance between the wafer W 1  and the optical splitting element  240  is substantially the same as or different from the vertical distance between the wafer W 2  and the optical splitting element  240 , and the lateral distance between the wafer W 3  and the optical splitting element  240  is substantially the same as or different from the lateral distance between the wafer W 4  and the optical splitting element  240 . Thereafter, the wafer holders  160  and  260  may stop rotating and keep at the inspection position for wafer inspection. 
     In some embodiments in which the wafer holders  160  and  260  are not rotatable, the wafer holders  160  and  260  are configured at the inspection position, and the wafers W 1  to W 4  are respectively loaded to the wafer stages  161 ,  162 ,  261 ,  262  at the position shown in  FIG. 2A  and  FIG. 2B . 
     Thereafter, at act  630 , multi-wafer inspection is performed. In some embodiments, at act  631 , the light L is emitted by the light source  110 . The light L may be optionally amplified by the amplifier  120 . Thereafter, the light L is directed to the optical splitting element  240  through the optical direction element  130 . 
     At act  632 , the light is split into a plurality of light beams, and the light beams are directed to the wafers carried by the wafer holders, respectively. In some embodiments, as shown in  FIGS. 2A to 2C , the light L is split by the optical splitting element  240  into a first light beam L 1 , a second light beam L 2 , a third light beam L 3  and a fourth light beam L 4  for inspecting the wafers W 1 -W 4 , respectively. 
     In some embodiments, as shown in  FIGS. 2A and 2C , the light L first enters the first beam splitter  242 , and is split by the first beam splitter  242  into two lights L 1  and L 2 ′. A first part of the light L is reflected by the first beam splitter  242  and directed toward the wafer W 1  as a first light beam L 1 . A second part of the light L (i.e. the light L 2 ′) goes through the first beam splitter  242  and enters the second beam splitter  244 . 
     The light L 2 ′ is then split by the second beam splitter  244  into two lights L 2  and L 3 ′. A first part of the light L 2 ′ is reflected by the second beam splitter  244  and directed toward the wafer W 2  as a second light beam L 2 . A second part of the light L 2 ′, that is, the light L 3 ′ goes through the second beam splitter  244  and enters the third beam splitter  246 . 
     The light L 3 ′ is then split by the third beam splitter  246  into two lights L 3  and L 4 . A first part of the light L 3 ′ is reflected by the third beam splitter  246  and directed toward the wafer W 3  as a third light beam L 3 . A second part of the light L 3 ′, that is, the light L 4 , goes through the third beam splitter  246 , and the light L 4  is then reflected by the total reflection mirror  248  and directed toward the fourth wafer W 4  as a fourth light beam L 4 . 
     In some embodiments, as shown in  FIG. 2B , after the light L is split into the light beams L 1 -L 4 , the light beams L 1 -L 4  may be amplified by the optical amplifiers  281 - 284 , respectively, so as to have suitable intensities for inspecting the wafers. In some embodiments, after being amplified, the light beams L 1  to L 4  have substantially the same intensity. Thereafter, the light beams L 1 -L 4  pass through the one-way mirrors  151 ,  152 ,  251 ,  252  and shine on the front surfaces F 1  of the wafers W 1 -W 4 , respectively. Depending on the defect to be detected, the light beams L 1  to L 4  may be polarized or unpolarized, respectively. When a polarized light beam is needed, the light beam may be polarized by a polarizer before passing through the one-way mirror. 
     Referring to  FIG. 2A  and  FIG. 2B , during the inspection, with the light beams L 1 -L 4  shining on the front surfaces F 1  of the wafers W 1 -W 4 , the wafer holder  160  moves along the horizontal directions X and Y, such that the wafer stages  161  and  162  carrying the wafers W 1  and W 2  move along the horizontal directions X and Y simultaneously; the wafer holder  260  moves along the horizontal and vertical directions X and Z, such that the wafer stages  261  and  262  carrying the wafers W 3  and W 4  moves along the horizontal and vertical directions X and Z simultaneously, such that the whole front surfaces F 1  of the wafers W 1 -W 4  are scanned. In some embodiments, the wafer holders  160  and  260  move simultaneously along the same direction or different directions, such that the wafers W 1 -W 4  are inspected simultaneously. 
     At act  634 , the light beams reflected from the wafers are received by optical sensors and inspection results of the wafers are generated. In some embodiments, the first light beam L 1  is reflected by the wafer W 1 , and the first light beam L 1  reflected by the wafer W 1  is then reflected by the one-way mirror  151  and received by the optical sensor  171 . The second light beam L 2  is reflected by the wafer W 2 , and the second light beam L 2  reflected by the wafer W 2  is then reflected by the one-way mirror  152  and received by the optical sensor  172 . The third light beam L 3  is reflected by the wafer W 3 , and the third light beam L 3  reflected by the third wafer W 3  is then reflected by the one-way mirror  251  and received by an optical sensor (not shown). The fourth light beam L 4  is reflected by the wafer W 4 , and the fourth light beam L 4  reflected by the wafer W 4  is then reflected by the one-way mirror  252  and received by an optical sensor (not shown). It is noted that, the optical sensors for receiving the light beams L 3  and L 4 , and the reflections of the light beams L 3  and L 4  from the wafer W 3  and W 4  to the one-way mirrors  251 ,  252  and the reflections of the light beams L 3  and L 4  from the one-way mirrors  251 ,  252  to the optical sensors are similar to those described with respect to the light beams L 1  and L 2  and the optical sensors  171 / 171 , and are not specifically shown in the figures. 
     After receiving the light beams L 1 -L 4  reflected by the wafers W 1 -W 4 , the optical sensors generate the inspection results of the wafers W 1 -W 4 , respectively. At act  640 , the inspection results (such as images) of the wafers are saved for further analysis. The defect determination method of the wafers are similar to those described with reference to  FIG. 1 , which are not described again here. Thereafter, at act  650 , the wafers W 1 -W 4  are unloaded from the wafer stages. 
       FIG. 3  is a schematic view of an inspection apparatus  300  according to some embodiments of the disclosure. The configuration of the defect inspection apparatus  300  is similar to the defect inspection apparatus  100  shown in  FIG. 1 , except that multiple layers of wafer holders are included in the inspection apparatus  300 . Like elements are designated with the same or similar reference numbers for ease of understanding and the details thereof are not repeated herein. 
     Referring to  FIG. 3 , in some embodiments, the inspection apparatus  300  includes an optical module OM 3 , a wafer holder set including a plurality of wafer holders (e.g. wafer holder  160  and  360 ) for carrying a plurality of wafers, one-way mirrors  151 ,  152 ,  351 ,  352 , and optical sensors  171 ,  172 ,  371 ,  372 . In some embodiments, the inspection apparatus  300  includes at least two wafer holders. The plurality of wafer holders may be arranged in the vertical direction Z and overlap each other in the vertical direction Z. In some embodiments, the inspection apparatus  300  includes a wafer holder  160  and a wafer holder  360  below the wafer holder  160 . The wafer holder  360  includes substantially the same structure as the wafer holder  160 . In some embodiments, the wafer holder  360  includes a wafer stage  361  for carrying a wafer W 3  and a wafer stage  362  for carrying a wafer W 4  connected to each other by a connecting element  365 . Similar to the wafer holder  160 , the wafer holder  360  may also be rotatable or not rotatable, and is movable along the horizontal directions, such as the directions X and Y. Although two wafer holders  160  and  360  are shown in  FIG. 3 , the disclosure is not limited thereto. More than two wafer holders may be included in the inspection apparatus  300 , as represented by the ellipsis. 
     The optical module OM 3  may include the light source  110 , an optical amplifier  120 , an optical directional splitting unit  330 , and a plurality of optical splitting elements  140 ,  340 . 
     The optical directional splitting unit  330  is configured to split the light L from the light source  110  into a plurality of lights and direct the plurality of lights toward the plurality of optical splitting elements  140 ,  340 . In some embodiments, the optical direction splitting unit  330  includes reflective mirrors  131 ,  132 , one or more beam splitters, such as the beam splitter  333 , and an optical component  334 . The optical component  334  may be a total reflection mirror when the wafer holder  360  is the last wafer holder in a wafer hold set, which is most distal the light source along the path of the optical directional splitting unit  330  (e.g. when the wafer holder  360  is a bottommost wafer holder). In some embodiments in which more wafer holders are disposed under the wafer holder  360 , the optical component  334  is a beam splitter, and a total reflection mirror is disposed corresponding to the bottommost wafer holder. 
     The optical splitting elements  140 ,  340  are disposed between the wafer stages of the corresponding wafer holder  160 ,  260  and configured to split the light from the optical directional splitting unit  330  into a plurality of light beams for inspecting the corresponding wafers. The configuration of each optical splitting element and corresponding wafer holder is similar to that of the optical splitting element  140  and the wafer holder  160  as described with reference to  FIG. 1 . 
     The inspection method using the inspection apparatus  300  is similar to that of the inspection apparatus  100 . At act  610 , the wafers W 1 -W 4  are loaded to the wafer stages  161 ,  162 ,  361 ,  362  of the wafer holders  160 ,  360 . In some embodiments, the wafers W 1 -W 4  are carried by the wafer stages  161 ,  162 ,  361 ,  362  by electrostatic force and/or mechanical force, respectively. At act  620 , an alignment is executed to determine origins of the wafer coordinate system of the wafers W 1 -W 4 . Thereafter, at act, multi-wafer inspection is performed. 
     In some embodiments, at act  631 , the light L is emitted by the light source  110 . The light L may be amplified by the optical amplifier  120  before entering the optical directional splitting unit  330 . At act  632 , the light L is split into a plurality of light beams L 1  to L 4  directed toward wafers W 1  to W 4 . In some embodiments, the light L enters the optical directional splitting unit  330 , the light L is reflected by the reflectors  131  and  132  in sequence and then split by the beam splitter  333  into two portions L′ and L″. The first portion L′ of the light L is reflected by the beam splitter  333  and directed toward the optical splitting element  140 . The second portion L″ of the light L transmits through the beam splitter  333  and may be partially or completely reflected by the optical component  334  and directed toward the optical splitting element  340 . In some embodiments in which the wafer holder  360  is the bottommost wafer holder and the optical component  334  is a total reflection mirror, the second portion L″ of the light L is substantially completely reflected by the total reflection mirror  334  and directed toward the optical splitting element  340 . In some embodiments in which other wafer holders are disposed below the wafer holder  360  and the component  334  is a beam splitter, the second portion L″ of the light L is further split by the beam splitter  334  into two part. In some embodiments, half of the second portion L″ of the light L is reflected by the beam splitter  334  and directed toward the optical beam splitting element  340 , and the other half of the second portion L″ of the light L transmits through the beam splitter  334  for the wafers carried by the wafer holders in next layers. In some embodiments, after the light L is split into two portions L′ and L″ and before the first portion L′ and the second portion L″ of the light L enter the optical splitting elements  140  and  340 , the first portion L′ and second portion L″ of the light L are amplified by optical amplifiers (not shown) which may be disposed between the optical directional splitting element  330  and the optical splitting elements  140 ,  340 , so as to adjust the intensities of the light beams L 1 -L 4  directed to the wafers. 
     The first portion L′ of the light L is then split by the optical splitting element  140  into a first light beam L 1  and a second light beam L 2  for inspecting the wafers W 1  and W 2  carried by the wafer holder  160 . The second portion L″ of the light L is then split by the optical splitting element  340  into a third light beam L 3  and a fourth light beam L 4  for inspecting the wafers W 3  and W 4  carried by the wafer holder  360 . In some embodiments, the adjusting of the intensities of the light beams L 1 -L 4  may be performed after the first and second portions L′ and L″ being split by the optical splitting elements  140 ,  340 . In some embodiments, the light beams L 1 -L 4  may be amplified to desired intensities before going through the one-way mirrors by the amplifiers (not shown) disposed between the optical splitting element  140 ,  340  and the corresponding one-way mirrors  151 ,  152 ,  351 ,  352 . The light beams L 1 -L 4  may be unpolarized or polarized, respectively. In some embodiments, at least one of the light beams L 1 -L 4  is polarized by the polarizer(s) (not shown) disposed between the optical splitting element and the corresponding one-way mirror. 
     Thereafter, the light beams L 1 -L 4  respectively transmit through a corresponding one-way mirror  151 ,  152 ,  351 ,  352  and shine on the wafers W 1 -W 4 , respectively. 
     During the wafer inspection, with the light beams L 1 -L 4  shining on the wafers W 1 -W 4 , the wafer holders  160  and  360  may move along the horizontal direction X/Y simultaneously, such that the wafers W 1 -W 4  carried by the wafer stages of the wafer holders  160  and  360  move along the horizontal directions X/Y simultaneously, and the whole front surfaces F 1  of the wafers W 1 -W 4  may be scanned by the light beams L 1 -L 4 , respectively. 
     At act  634 , the light beams L 1 -L 4  are reflected by the wafers W 1 -W 4 , and the light beams reflected by the wafers W 1 -W 4  are then reflected by the one-way mirrors  151 ,  152 ,  351 ,  352  and received by the optical sensors  171 ,  172 ,  371 ,  372 , respectively. Thereafter, upon receiving the light beams L 1 -L 4  reflected by the wafers W 1 -W 4 , the optical sensors  171 ,  172 ,  371 ,  372  generate the inspection result of the wafers W 1 -W 4 , such as the images of the wafers W 1 -W 4 . At act, the inspection result is saved. At act  650 , the wafers W 1 -W 4  are unloaded from the wafer holders  160  and  360 . 
       FIG. 4  is a schematic view of an inspection apparatus  400  according to a fourth embodiment. The configuration of the inspection apparatus  400  is similar to the inspection apparatus  100  shown in  FIG. 1 , except that the optical directional element is an optical fiber  480  instead of a reflective mirror unit. The optical fiber  480  is configured to guide the light L to the optical splitting element  140 . In alternative embodiments, the optical directional element may include a combination of reflective mirrors and optical fiber. 
     In some embodiments, after being amplified by the optical amplifier  120 , the light L emitted by the optical source  110  is guided to the optical splitting element  140  by the optical fiber  480 . The optical fiber  480  is more flexible in transmitting the light L. Also, by using the optical fiber  480 , the size of the optical directional element may be reduced. The other structures of the inspection apparatus  400  and the inspection method using the inspection apparatus  400  are similar to those described with reference to  FIG. 1 , which are not described again here. 
     In the foregoing embodiments, the light source  110  is disposed over the wafer holder(s), and the optical directional element is needed to adjust the light path of the light emitted by the light source  110 . However, the disclosure is not limited thereto. 
       FIG. 5  is a schematic view of an inspection apparatus  500  according to at least one embodiment of the disclosure. The configuration of the inspection apparatus is similar to the defect inspection apparatus shown in  FIG. 1 , except that the light source  110  is disposed aside the wafer holder  160 , and the optical splitting element  140  is replaced by an integrated optical element  520 . The integrated optical element  520  is configured to split the light from the light source  110  into a plurality of light beams for inspecting multiple wafers. 
     In some embodiments, the light source  110  is disposed laterally aside the integrated optical element  520 , and the light L emit from the light source  110  is directed toward the integrated optical element  520  directly. As such, the directional element for guiding the light L may be omitted. When the light L enters the integrated optical element  520 , the light L is split by the integrated optical element  520  into a plurality of light beams for inspecting the wafers carried by the wafer holder  160 , respectively. 
     In some embodiments, the integrated optical element  520  is or includes an optical coupler, arrayed waveguide grating (AWG), combinations thereof or the like. In some embodiments, the integrated optical element  520  includes a plurality of optical components which are combined to fulfill some complex functions. Such optical components, for example, may be optical filters, modulators, amplifiers, splitters or the like. These optical components, for example, can be fabricated on the surface of some crystalline material (such as silicon, silica, or LiNbO 3 ) and connected with waveguides. 
     In some embodiments, the integrated optical element  520  includes an arrayed waveguide grating AG. A structure of arrayed waveguide grating AG in accordance with at least one embodiment is illustrated in  FIG. 8 . In some embodiments, the arrayed waveguide grating AG includes an input optical fiber F, free space propagation regions S 1  and S 2 , a waveguide array including a plurality of channel waveguides W 1 , W 2 , W 3  . . . W n , and a plurality of output optical fibers F 1 , F 2  . . . F n . The input optical fiber F is configured to input an incident light. The free space propagation region S 1  is coupled to an end of the optical fiber F and may include an input cavity, coupler part or slab waveguide. The channel waveguides W 1 , W 2 , W 3  . . . W n  are connected to an end of the free space propagation region S 1 . The channel waveguides have different lengths and are arranged side by side. The free space propagation region S 2  is connected to ends of the channel waveguides W 1 , W 2 , W 3  . . . W n  and may include an output cavity, coupler part, or slab waveguide. The output optical fibers F 1 , F 2  . . . F n  are connected to an end of the free space propagation region S 2  and configured to output a plurality of lights split from the incident light. In some embodiments, an incident light such as a light L is fed using the optical fiber F into the free space propagation region S 1 . The light L may be a wavelength multiplexed light having various wavelengths. The light L passes through the free propagation region S 1  and enters the channel waveguides W 1 , W 2 , W 3  . . . W n . The phase delay proportional to wavelength is introduced to light signals passed from different channel waveguides of different lengths. These phase delayed signals are made to pass from the free propagation region S 2 . Light signals after passing through different lengths of channel waveguides interfere with one another and are refocused at the output optical fibers F 1 , F 2  . . . F n . As a result, each of the output optical fiber F 1 , F 2  . . . F n  is fed with one unique wavelength of light having maximum amplitude. In some embodiments, a plurality of lights L 1 , L 2  . . . L n  is output from the optical fibers F 1 , F 2  . . . F n . In other words, the incident light having at least two wavelengths is split by the arrayed waveguide grating AG into at least two lights, and each of the at least two lights has a single wavelength. It is noted that, the number of the channel waveguides, the number of the output optical fibers and the number of lights output from the optical fibers are not limited in the disclosure. 
     Referring back to  FIG. 5 , the integrated optical element  520  may split the light L from the light source  110  into at least two light beams, such as the light beams L 1  and L 2 , for inspecting the wafers W 1  and W 2 . The integrated optical element  520  may further amplify the light L, so as to control the intensities of the light beams directed to the wafers carried by the wafer holder  160 . The other structures of the inspection apparatus  500  and the inspection method using the inspection apparatus  500  are similar to those described with reference to  FIG. 1 , which are not described again here. 
     In the embodiments of the disclosure, the inspection apparatus includes one or more wafer holders including a plurality of wafer stages for carrying a plurality of wafers, and the optical module of the inspection apparatus is configured to emit a plurality of light beams for inspecting the plurality of wafers simultaneously, so as to implement multi-wafer inspection. As such, the wafer inspection efficiency is improved, and the yield is thus improved. 
     In accordance with some embodiments of the disclosure, a wafer inspection apparatus includes a light source, a first optical splitting element, a first wafer holder, a first optical sensor and a second optical sensor. The light source is configured to emit a light. The first optical splitting element is configured to split the light from the light source into a first light beam and a second light beam. The first wafer holder includes a first wafer stage for carrying a first wafer and a second wafer stage for carrying a second wafer. The first wafer is configured to reflect the first light beam, and the second wafer is configured to reflect the second light beam. The first optical sensor is configured to receive the first light beam reflected by the first wafer carried by the first wafer stage. The second optical sensor is configured to receive the second light beam reflected by the second wafer carried by the second wafer stage. 
     In accordance with some embodiments of the disclosure, a wafer inspection apparatus includes an optical module, at least one wafer holder for carrying a plurality of wafers and a plurality of optical sensors. The optical module is configured to emit a plurality of light beams for simultaneously scanning the plurality of wafers carried by the at least one wafer holder. The plurality of optical sensors is configured to receive the light beams reflected by the plurality of wafers. 
     In accordance with some embodiments of the disclosure, a wafer inspection method includes: loading a plurality of wafers to at least one wafer holder; and inspecting the plurality of wafers simultaneously, comprising: emitting a plurality of light beams from an optical module, and the plurality of light beams are directed toward the plurality of wafers; and receiving the plurality of light beams reflected by the wafers through a plurality of optical sensors. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.