Abstract:
The present invention provides a writing technique which can perform high-accuracy overlay writing in electron beam writing equipment performing mark detection by light. Electron beam writing equipment has an electron source; an electron optical system illuminating an electron beam emitted from the electron source onto a sample for scanning to form a desired pattern on the sample; a stage mounting the sample; a mark substrate provided on the stage; means beaming a light beam for position detection which is on the same side as the illumination direction of the electron beam for illuminating the mark substrate; light detection means which is on the same side as the means beaming a light beam for detecting reflected light reflected on the mark substrate; and electron detection means which is on the side opposite the light detection means with respect to the mark substrate for detecting a transmitted electron obtained by illumination of the electron beam onto the mark substrate, wherein relative position information of the light beam and the electron beam is obtained based on the signals of the detected reflected light and transmitted electron.

Description:
CLAIM OF PRIORITY  
       [0001]     The present invention claims priority from Japanese application JP 2003-348019 field on Oct. 7, 2003, the content of which is hereby incorporated by reference on to this application.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a lithography technique. More specifically, the present invention relates to an electron beam writing technique used in a semiconductor process.  
         [0003]     Electron beam measuring equipment which detects a wafer position by light to measure it by electron beam is proposed. In Japanese Patent Application Laid-Open No. 9-22676, mark positions are detected by light and electron beam to identify the distance between both, thereby feeding back a mark detected value by the light to position control of the electron beam.  
         [0004]     In Japanese Patent Application Laid-Open No. 1-214117, a method of detecting a wafer position by light to perform writing by electron beam is proposed.  
         [0005]     In Japanese Patent Application Laid-Open No. 6-275500, a method of measuring the focus and stigma of an electron beam by a transmitted electron which has transmitted through an aperture on a stage is proposed.  
       SUMMARY OF THE INVENTION  
       [0006]     In the above-described prior art (Patent Document 1) method, both light and electron beam use reflected signals from marks are used, resulting in limited accuracy. In the prior art (Patent Document 2), detection is performed by a backscattered electron. In both of the prior art methods, it is hard to say that the accuracy of mark detection is sufficiently considered.  
         [0007]     In the prior art (Patent Document 2), being effective for high-accuracy alignment of an electron beam, no consideration of alignment with a base wafer pattern at writing is given.  
         [0008]     An object of the present invention is to provide an electron beam wiring technique which can perform high-accuracy overlay writing in mark detection using light.  
         [0009]     To achieve the above object, electron beam writing equipment according to the present invention has on a stage a mark substrate illuminated by light beam for position detection and electron beam for writing from the same side and having a support and apertures; an electron detecting surface (here, the term of an electron beam receiving surface used in light is called a detecting surface) provided in the direction opposite the illumination side; and a light detecting surface provided in the same direction as the illumination side, thereby making measurement by the electron beam which is a transmitted beam excellent in efficiency and contrast and the light beam which is reflected light capable of being compatible with wafer mark detection.  
         [0010]     In the above-described prior art (Patent Document 1), since the pattern on a sample by electron beam is measured by a backscattered electron or secondary electron, it is desirable that mark detection by electron beam be performed by a backscattered electron or secondary electron.  
         [0011]     In the writing equipment of the present invention, mark detection by electron beam is performed by a transmitted electron, which cannot adversely affect the accuracy of the equipment. The mark detection by electron beam is performed in positions other than a sample position. A detector can be provided on the stage to use a transmitted electron. The detection method according to the present invention has been found by utilizing the characteristic of the writing equipment.  
         [0012]     In the prior art method of using electron beam reflection, a substrate or its surface must be conductive. The reflection from the background is increased in detection by light to lower the contrast between the signals of reflected lights. Using a mark substrate having apertures can zero the reflectivity in the apertures. An effect for increasing the contrast between the signals of the reflected lights at the same time can be expected.  
         [0013]     To use the present invention more effectively, the devising of a mark structure is important. For instance, the mark substrate is manufactured by coating a metal onto a light element support so as to achieve both high-accuracy aperture formation important for measurement by electron beam and mark reflection important for measurement by light. There is also a method of coating a metal having a principal atom with an atomic number higher than that of a principal atom constituting the support. This can increase the contrast in electron beam measurement while maintaining high-accuracy aperture formation. For instance, it is considered that a principal atom constituting the support is silicon excellent in processability and an element with a higher atomic number is a heavy metal or noble metal. In recent years, the technique for fabricating a stencil substrate having high-accuracy microfabrication pattern apertures withstanding position measurement has been advanced. This utilization is an aim of the present invention.  
         [0014]     To improve the mark detection accuracy by light, it is desirable to increase the contrast. For instance, a member absorbing light for position detection is provided between the mark substrate and the electron detecting surface to prevent reflection of light from the underside of the mark substrate. The reflectivity of light which has transmitted through the aperture is significantly lowered to increase the contrast between reflected lights. This is a great advantage as compared with the case of using a thick substrate.  
         [0015]     A mark for electron beam is isolated from a mark for light beam, which is significant. An optimum structure for the respective measurement can be provided. In this case, it is important to stabilize the relative distance between the two marks by arranging them to be close to each other. A method of arranging the mark for electron beam in the mark for light beam relatively increased is also advantageous for stabilizing the relative distance between the marks. When isolating the mark for electron beam from the mark for light beam, a light absorber is provided in the direction opposite the illumination side of the mark for light beam. It is effective to prevent light reflection from other than the mark substrate.  
         [0016]     In the case of baseline correction in electron beam writing equipment of a multiple beam system forming a plurality of electron beams which can be subject to blanking independently, a mark detection substrate formed with a mark for detection by light having a pitch being substantially an integral multiple of the pitch of multiple beams can use both marks together. High-accuracy correction can be expected.  
         [0017]     The representative configuration examples of the present invention will be described below.  
         [0018]     (1) Electron beam writing equipment of the present invention has an electron source; an electron optical system illuminating an electron beam emitted from the electron source onto a sample for scanning to form a desired pattern on the sample; a stage mounting the sample; a mark substrate provided on the stage; means beaming a light beam for position detection which is on the same side as the illumination direction of the electron beam for illuminating the mark substrate; light detection means which is on the same side as the means beaming a light beam for detecting reflected light reflected on the mark substrate; and electron detection means which is on the side opposite the light detection means with respect to the mark substrate for detecting a transmitted electron obtained by illumination of the electron beam onto the mark substrate, wherein relative position information of the light beam and the electron beam is obtained based on the signals of the detected reflected light and transmitted electron.  
         [0019]     (2) Electron beam writing equipment of the present invention has an electron source; an electron optical system illuminating an electron beam emitted from the electron source onto a sample for scanning to write a desired pattern on the sample; a stage mounting the sample; a mark substrate provided on the stage; means beaming a light beam for position detection which is on the same side as the illumination direction of the electron beam for illuminating the mark substrate; light detection means which is on the same side as the means beaming a light beam for detecting reflected light reflected on the mark substrate; and electron detection means which is on the side opposite the light detection means with respect to the mark substrate for detecting a transmitted electron obtained by illumination of the electron beam onto the mark substrate, wherein relative position information of the light beam and the electron beam is obtained by using the signals of the detected reflected light and transmitted electron together.  
         [0020]     (3) Electron beam writing equipment of the present invention has an electron optical system independently on/off controlling each of a plurality of electron beams arrayed with a predetermined pitch, deflecting and scanning the on/off controlled electron beams together, and writing a desired pattern on the sample; a stage mounting the sample; a stage mounting the sample; a mark substrate provided on the stage; means beaming a light beam for position detection which is on the same side as the illumination direction of the electron beam for illuminating the mark substrate; light detection means which is on the same side as the means beaming a light beam for detecting reflected light reflected on the mark substrate; and electron detection means which is on the side opposite the light detection means with respect to the mark substrate for detecting a transmitted electron obtained by illumination of the electron beam onto the mark substrate, wherein relative position information of the light beam and the electron beam is obtained based on the detected reflected light and transmitted electron.  
         [0021]     (4) An electron beam writing method of the present invention has the steps of: illuminating and scanning an electron beam emitted from an electron source onto a sample mounted on a stage to form a desired pattern on the sample; illuminating a light beam for position detection onto a mark substrate provided on the stage from the same side as the illumination direction of the electron beam and detecting reflected light reflected on the mark substrate from the same side as the illumination direction of the light beam; illuminating the electron beam onto the mark substrate and detecting a transmitted electron which has transmitted though the mark substrate on the side opposite the side detecting the reflected light with respect to the mark substrate; and obtaining relative position information of the light beam and the electron beam based on the detected reflected light and transmitted electron, wherein a mark position on the sample is detected by the light beam to perform writing by the electron beam according to the obtained relative position information.  
         [0022]     According to the present invention, high-accuracy overlay writing of the electron beam writing equipment can be performed to form a high-resolution device pattern. In particular, the present invention is considered to be effective for electron beam writing equipment of a multiple beam system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a schematic diagram of assistance in explaining the configuration of equipment according to Embodiments 1 and 2 of the present invention;  
         [0024]      FIG. 2  is a diagram of assistance in explaining a mark peripheral portion according to Embodiment 1;  
         [0025]      FIG. 3  is a diagram of assistance in explaining a process for performing overlay writing according to the present invention;  
         [0026]      FIG. 4  is a diagram of assistance in explaining a mark peripheral portion according to Embodiment 2;  
         [0027]      FIG. 5  is a schematic diagram of assistance in explaining the configuration of equipment according to Embodiment 3 of the present invention;  
         [0028]      FIG. 6  is a diagram of assistance in explaining the cross-sectional structure of a mark substrate according to Embodiment 3; and  
         [0029]      FIG. 7  is a top view of assistance in explaining the shapes of marks for electron beam and marks for light beam of the mark substrate according to Embodiment 3.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Embodiment 1  
       [0030]      FIG. 1  shows the configuration of equipment according to this embodiment. In this embodiment, electron beam writing equipment which can use together a variable shaping method and a cell projection method together is targeted.  
         [0031]     An electron beam accelerated to 50 kV by an electron source  101  illuminates a first mask  102  formed with a rectangular aperture  117 . The image of the rectangular aperture is formed on a second mask  111  by two projection lenses  107  and  108 . A rectangular aperture  118  for variable shaping and a plurality of cell apertures  119  for cell projection are formed on the second mask  111 . The position of the first mask image on the second mask is controlled by a shaping deflector  106  and a beam shape control circuit  133  between the two masks. The transmitted electron beam formed by the two masks is demagnified by two demagnification lenses  112  and  113  to be finally projected onto a sample  124  placed on a stage  125  by objective lenses  114  and  115 . These lenses are driven by a lens control circuit  135 . At the same time, the electron beam is axis aligned by an aligner control circuit  134 . The position of the electron beam on the sample  124  is controlled by an objective deflector  116 .  
         [0032]     A mark substrate  126  for position detection is provided on the stage  125 . A laser interferometer, not shown, measuring the positions of the mark substrate  126  and the stage  125 , a transmitted electron detector  127 , a signal processing circuit  137  and a stage control circuit  138  are used to measure the position of the electron beam. The equipment has a reflected light detector  123  and a light source  128  in addition to the mark substrate  126  for position detection provided on the stage  125  and can measure the position of light. The entire control of these is performed by a data control circuit  131 . The controlled result and the measured result are displayed by a display  132 .  
         [0033]      FIG. 2  shows an enlarged view around the mark substrate. An electron beam  201  used here is an electron beam for writing. A light beam  202  is light for wafer mark detection. Actually, the light and the electron are incident in different positions. For convenience of the description, they are shown in the same position. To simply show the drawing, the light source and the optical fiber are not shown in  FIG. 2 . The transmitted electron detector  127  and an electron detecting surface  211  are provided on the side opposite the plane in which the electron beam  201  is illuminated onto the mark substrate  206 , enabling high sensitivity and high contrast. The reflected light detector  123  and a light detecting surface  210  are provided on the same side as the plane in which the light beam  202  is illuminated onto the mark substrate  126 . This can detect reflected light from the wafer mark.  
         [0034]     Using the transmitted electron and the reflected light together is found to be important for baseline correction of the equipment detecting a wafer mark by light to perform writing by electron beam. In particular, the amount of current per beam is smaller in a multiple beam system. Detection of a highly sensitive transmitted electron is essential.  
         [0035]     In this embodiment, as shown in  FIG. 2 , a silicon stencil  204  is selected as a support. Its thickness is 2 μm. The reason why a light element is selected as the support is that it is excellent in processability of an aperture to make the accuracy of the aperture shape higher. As candidates of other materials, silicon carbide and diamond are considered. To facilitate the process, the silicon stencil  204  is a thin film having a thickness of 2 μm. This is ½ or below of the range of an electron beam used for writing. The silicon having a thickness of 2 μm cannot shut off the electron beam of 50 kV. It is 10 times or more larger than the mean free path and can scatter the electron beam. When an aperture for scattered electron  207  is provided such that no scattered electrons are incident upon the transmitted electron detector  127 , high contrast can be obtained. The electron beam  201  which has transmitted through the aperture is directly incident upon the transmitted electron detector  127 . High sensitivity can be obtained.  
         [0036]     The aperture for scattered electron  207  is a light absorber. Specifically, it is a blackbody of carbon. This absorbs the light which has transmitted through the silicon so that no irregular reflected light around the detection part is incident upon the reflected light detector  123 . There is also a method of manufacturing the aperture for scattered electron  207  using a material having good processability, such as aluminum, to coat its surface with the carbon. A light absorber  212  is arranged on the electron detecting surface, which is effective from the same reason.  
         [0037]     In  FIG. 2 , gold (Au)  203  as a noble metal is deposited on the silicon. This can increase the reflectivity of light from the support. The substrate of a mark for electron beam and a mark for light beam can be shared. This is a great merit in ensuring the accuracy of baseline correction. Deposition of the gold having an atom with an atomic number higher than that of a principal atom constituting the support is effective for improving the scattering power to the electron beam. The double merit can be obtained. In  FIG. 2 , deposition of the gold is performed only from the upper surface. Since the function of reflecting light is important, the deposition may be performed from both surfaces or only from the lower surface. In other words, the gold may be coated at least on the surface on the side of means reflecting the light beam or on the opposite side. The thickness of the gold of this embodiment is 40 nm. This is twice or more the depth of penetration of the light used for detection. This can obtain a sufficient reflectivity. When the film thickness is larger than the size of the aperture pattern, it adversely affects the aperture pattern shape. Examples of other candidates of noble metal or heavy metal coated can include platinum, palladium, tungsten and tantalum. An alloy of these may be used. To form a two-layer film, there are a method of depositing a noble metal film after fabricating an aperture in a silicon substrate and a method of forming an aperture after fabricating a heavy metal film in a silicon substrate by the CVD method. The aperture shape has a 1-μm line with a 2-μm pitch.  
         [0038]     The wavelength of the light for mark detection is 590 nm. It is incident almost vertically upon the mark substrate  126  through an optical fiber  129  shown in  FIG. 1 . The reflectivity of the light in a certain portion of the support is 90% or above, which is significantly contrasted with 0% of the aperture. The detection probability of the electron beam scattered in a certain portion of the support is 1% or below. It is found that signals in almost ideal contrast in both beams can be obtained.  
         [0039]     The above equipment is used to perform overlay writing to a silicon wafer according to the process of  FIG. 3 . The mark positions shown in  FIG. 2  are detected by light and electron beam (steps  301  and  302 ) to obtain relative position information of both (step  303 ). In this case, the light source  128 , the reflected light detector  123 , the signal processing circuit  137  and the stage control circuit  138  are utilized. Mark detection of the wafer is performed by light (step  304 ) to detect the pattern position of the base layer. The obtained relative position information is processed by the data control circuit  131  to be finally fed back to the objective deflector  116  for performing writing by electron beam (step  305 ). As a result, an alignment accuracy of 30 nm can be realized by 3σ.  
         [0040]     As is apparent from the above description, the transmitted electron is larger in the aperture and the reflected light is larger other than the aperture. Using the reverse phase can perform high-speed baseline correction. Both signals of the transmitted electron and the reflected light are synthesized with each other. The relative distance between the light and the electron in which the contrast is lowest is obtained, making it possible to perform correction. This method is one application method of the present invention.  
       Embodiment 2  
       [0041]     In this embodiment, equipment equivalent to that of  FIG. 1  is used and the mark detection part shown in  FIG. 4  is used. In  FIG. 4 , a mark for electron beam  407  is isolated from a mark for light beam  408 . This is done to increase the accuracy by using a mark pattern suitable for the respective marks. Since the relative distance between the marks must be stable to heat and stress, both must be positioned as close as possible to each other. In this embodiment, as a mark substrate, aluminum (Al)  403  as a metal having a thickness of 100 nm is deposited on a silicon stencil  404  having a thickness of 5 μm. The electron beam scattering power of the aluminum is not very different from that of the silicon. An effect for increasing the reflectivity of the light can be expected.  
         [0042]     In general, an electron beam  401  is hard to be deflected largely so that a small mark is preferable. A light beam  402  for mark detection has poor resolution so that a large mark is preferable. The mark for light beam  408  is larger than the mark for electron beam  407 . In  FIG. 4 , the mark for light beam  408  having a 4-μm line with a 20-μm pitch is formed following the aperture shape having 1-μm line with a 2-μm pitch of the mark for electron beam  407 . A light absorber  409  is provided under the mark for light beam  408  to prevent irregular reflection of light. The light absorber  409  is desirably a conductor to prevent electrification due to the electron beam transmitting through therenear. Carbon is used as the light absorber.  
         [0043]     In addition to simply lowering the reflectivity of the material, there is a method of lowering the reflectivity of light by a convex and concave structure. The mark for light beam is detected by light beam and the mark for electron beam is detected by electron beam. The relative relation between the electron beam and the light beam is obtained by taking into account the distance between both patterns. Mark detection of the wafer is performed by the light to detect the pattern position of the base layer for performing writing by the electron beam according to the obtained relative position information. As a result, an alignment accuracy of 25 nm can be realized by 3σ.  
         [0044]     In the drawing, the numeral  405  denotes a light detector;  406 , a mark substrate;  410 , an aperture for scattered electron;  411 , an electron detector;  420 , a light detecting surface; and  421 , an electron detecting surface.  
       Embodiment 3  
       [0045]      FIG. 5  shows the configuration of equipment according to this embodiment. In this embodiment, electron beam writing equipment of a multiple beam system is targeted. An electron beam  511  accelerated to 50 kV by an electron source  510  is a parallel beam through a condenser lens  512  to be isolated into a plurality of point beams by an aperture array  513  having a plurality of apertures. The point beams are imaged onto an intermediate image  516  of the point beams by a lens array  514  at the later stage. In order to on/off control the plurality of point beams individually, a blanker array  515  and a blanking aperture  519  are provided. The thus-produced multiple point beams are demagnified by a doublet lens  522  having a first projection lens  518  and a second projection lens  521  to be imaged onto a sample  524 . There is a deflector  520  between the two lenses of the doublet lens, which defines the writing position on the sample  524 .  
         [0046]     There is a mark substrate  526  for position detection on a stage  525  mounting the sample  524 . A laser interferometer, not shown, measuring the position of the stage  525  and a transmitted electron detector  527  are used to measure the position of the electron beam. The equipment has a reflected light detector  523  and a light source  528  in addition to the mark substrate  526  for position detection provided on the stage  525  and can measure the position of the light.  
         [0047]     Aligners  517  having two stages are provided above the first projection lens  518  as the first lens of the doublet lens  522 . They are engaged with each other to align the incident angle and the incident position of the electron beam onto the lens. The aligners  517  are driven by an aligner control circuit  504 . The doublet lens  522  is driven by a lens control circuit  505 . In this example, specifically, electric currents are supplied. The set values of the electric currents are determined by information provided from a data control circuit  501 . A focus control circuit  502  and a pattern generator circuit  503  operate the corresponding optical elements by supplying voltages. The set values of these are determined by information provided from the data control circuit  501 . The data control circuit  501  has a display  509  and performs calculation for determining the amount of operation of the lenses and aligners using information obtained from a signal processing circuit  507  and a stage control circuit  508 .  
         [0048]     In this embodiment, as another example in which the mark for electron beam is isolated from the mark for light beam, a substrate having a cross-sectional structure as shown in  FIG. 6  is employed. The shape seen from the top is shown in  FIG. 7 . In  FIG. 6 , the light detecting surface, the light detector, the electron detecting surface and the electron detector are not shown. The basic structure is the same as those shown in  FIGS. 2 and 4 .  
         [0049]     In this embodiment, a mark for electron beam  605  is positioned in a mark for light beam  604 . Detection of light  602  may use reflected light from a large two-dimensional area for processing a reflected image by a CCD. An electron beam  601  is hard to use a backscattered electron from the large two-dimensional area since a usable electric current is significantly limited by the Coulomb effect and aberration. It is suitable to use a mark smaller than the mark for light beam. As in this embodiment, for the mark shape, the mark for electron beam  605  is included in the mark for light beam  604 . While securing the respective detection accuracies, the stability of the relative distance between both marks can be ensured. In this case, when the signal of the reflected light is processed by avoiding a central mark area for electron beam  700  (area which has not been used for light measurement), interference of both can be prevented.  
         [0050]     This equipment is used to detect the pattern position of the base layer to perform writing by the electron beam according to the obtained relative position information. As a result, an alignment accuracy of 23 nm can be realized by 3σ.  
         [0051]     In the above embodiments, light is used as an energy beam for position detection and an electron beam in probe form is used as an energy beam for pattern forming. It is apparent that it can be applied to lithography equipment using other energy beams (e.g., projection type electron beam and ion beam, and short-wavelength light).