Abstract:
A method and system are presented for inspecting a structure containing a pattern in the form of a surface relief fabricated by a pattern-creating tool applied to the structure. Reference data is provided being indicative of photometric intensities of light components of different wavelengths returned from a structure having a pattern similar to the pattern of the structure under inspection. Spectrophotometric measurements are continuously applied to successive locations within the surface relief on the structure so as to form a measurement slice thereon. Measured data in the form of a spectrum indicative of photometric intensities of light components of different wavelengths returned from the successive locations within the slice is detected and analyzed to determine whether it correlates with the reference data in accordance with predetermined criteria results.

Description:
FIELD OF THE INVENTION 
     This invention is generally in the field of optical measurement techniques, and relates to a method and system for optical inspection of a structure formed with a surface relief. 
     BACKGROUND OF THE INVENTION 
     The manufacture of semiconductor devices consists of several procedures applied to a semiconductor wafer to define active and passive elements. The wafer is prepared and one or more layers are deposited thereon. Thereafter, the process of photolithography is performed, in which the surface of a wafer is formed with a pattern conforming to circuit elements. An etching process applied to the uppermost layer follows the photolithography. By desirably repeating these processes, a multi-level semiconductor wafer is produced. Thus, photolithography is one of the main steps in the manufacture of semiconductor devices. It actually consists of the optical image transfer of a pattern from a mask to a semiconductor wafer. The major steps of the photolithography process are as follows: 
     coating a wafer with a photoresist (PR) material followed by a backing procedure; 
     exposing the PR to UV radiation through a mask in order to produce a latent image of the mask on the PR; 
     developing the exposed PR in order to produce a pattern in the form of a plurality of spaced-apart PR-containing regions; 
     etching consisting of removing the wafer layer underneath the PR layer within the spaces between the PR-containing regions; 
     removing the PR layer; and 
     measuring and inspecting the so-obtained patterned structure. 
     Thus, the structure undergoing the inspection is that resulting from the final etching procedure, i.e., with the complete layer pattern. 
     Techniques for monitoring the PR coating procedure have been developed, and is disclosed for example in WO 00/12958. According to this technique, which is based on spectrophotometric measurements, the latter are carried out in-situ during the coating and/or backing procedure. The measurements are applied to separate locations on the uppermost, unpatterned, PR-containing layer during the wafer spinning, and a conventional sampling is performed to determine the layer average result. 
     U.S. Pat. No. 5,872,633 discloses a technique for in-situ measurements carried out during a Chemical Mechanical Plananization (CMP) process, which provides actual thickness of the surface layer of a workpiece. According to this technique, measurements are performed during the rotation of the wafer under polishing. Because of repeating nature of die structure, the wafer surface may be advantageously samples for one full rotation of the wafer, and the measurements taken during that rotation suitable averaged to largely suppress or even cancel out the effect of the non-uniform topology of the dies beneath the uppermost layer (typically SiO 2 ) being polished. In other words, the existing pattern in the underlying layer(s) impedes the thickness measurements on the uppermost layer, and output signals associated with this pattern should therefore be suppressed. 
     SUMMARY OF THE INVENTION 
     There is a need in the art to facilitate inspection of the results of a PR development process, by providing a novel optical inspection method and system. This is associated with the fact that timely detection of defects introduced during the development stage enables to make corrections (reprocessing), if possible, to timely remove defected wafers from the production line, and/or provide feed-forward closed loop control of the coating/development tool. 
     Generally speaking, there is a need for a technique capable of measuring in patterned structures similar to those resulting from the PR development process. Moreover, this technique should be performed automatically, i.e., during the wafer movement through a phototrack (i.e., photolithography tools arrangement). A measurement system has to meet the footprint requirements of the conventional phototrack, and preferably eliminating the need for additional wafer handling means, in addition to those existing in the phototrack (i.e., robot or the like typically transporting the wafer between the photolithography tools). 
     Thus, the main idea of the present invention consists of continuously applying spectrophotometric measurements to a patterned structure progressing on a production line from a pattern-creating tool to a further station, and analyzing measured data to determine whether it satisfies certain criteria results or not. 
     The term “patterned structure” used herein signifies a structure formed with a surface relief in the form of a pattern containing a plurality of spaced-apart projecting regions defining pits within the spaces between these regions. Such a patterned structure may be composed of layers, in which case the pattern is in the form of spaced-apart regions of an uppermost layer spaced by regions of the underneath layer. The uppermost and underneath layers may have different optical properties, such as in semiconductor wafers. 
     The term “pattern-creating tool” signifies a processing tool applied to a structure to produce a surface relief thereon. Such a pattern-creating tool may be a developer tool or the combination of coater and developer tools typically used in the photolithography tools arrangement, etching tool, stamper used in the manufacture of CD or DVD, etc. 
     The term “criteria results” defines the final results of the technique to be obtained, and as used herein signifies certain predetermined characteristics of the pattern to be produced by the pattern-creating tool. These characteristics may be indicative of the absence (entire or partly) of the uppermost layer (e.g., photoresist), the thickness or non-uniformity of thickness of the patterned layer (e.g., the so-called “double coating” of the entire uppermost layer or its separate locations). Generally speaking, the characteristics of the pattern produced by the pattern-creating tool can be indicative of the quality of the pattern-creating process, and can therefore be used for process control, e.g., closed loop control (CLC). 
     There is thus provided according to one broad aspect of the present invention, a method for inspecting a structure containing a pattern in the form of a surface relief fabricated by a pattern-creating tool applied to structure, the method comprising: 
     (a) providing reference data indicative of photometric intensities of light components of different wavelengths returned from a structure having a pattern similar to said pattern of the structure under inspection; 
     (b) continuously applying spectrophotometric measurements to successive locations within the surface relief on the structure so as to inspect a slice thereon; 
     (c) detecting measured data in the form of a spectrum indicative of photometric intensities of light components of different wavelengths returned from the successive locations within the slice; and 
     (d) analyzing the measured data to determine whether it correlates with said reference data in accordance with predetermined criteria results. 
     Preferably, the analysis of the measured data includes analysis of the shape of the detected spectrum, e.g., the number of picks. 
     The reference data may be obtained by prior inspection of the so-called “golden structure”. Alternatively, an optical model could be utilized based on the known features of the pattern to be created by the tool. Such an optical model can be created on the basis of the known Rigoreous Coupled Wave Theory (RCWT). 
     Preferably, the continuous application of the spectrophotometric measurements is carried out during the movement of the structure from the pattern-creating tool towards the further station, with a spectrophotometer stationary mounted with respect to a conveying assembly supporting the structure during this movement. 
     The method can also comprise the step of analyzing the measured data with respect to working parameters of the pattern-creating tool, so as to enable a closed loop feed-forward control of these parameters prior to applying the tool to a further similar structure. 
     Preferably, the pattern-creating tool is a photoresist developer used in the photolithography process applied to the structure, such as a semiconductor wafer. 
     Thus, according to another broad aspect of the present invention, there is provided a method for in-process inspecting of a semiconductor wafer comprising a surface relief of a developed photoresist layer, the method comprising the steps of 
     (i) providing reference data indicative of photometric intensities of light components of different wavelengths returned from the wafer; 
     (ii) continuously applying spectrophotometric measurements to successive locations within the surface relief on the wafer so as to inspect a slice thereon; 
     (iii) detecting measured data in the form of a spectrum indicative of photometric intensities of light components of different wavelengths returned from the successive locations within the slice; and 
     (iv) analyzing the measured data to determine whether it correlates with said reference data in accordance with predetermined criteria results. 
     There are also provided a method for controlling a process consisting of creating a pattern in the form of a surface relief on a structure, and a method for carrying out a photolithography process applied to a semiconductor wafer. 
     According to yet another aspect of the present invention, there is provided a system comprising a pattern-creating tool to be applied to a structure to form it with a surface relief, and a measurement unit operable for applying optical measurements to the patterned structure and carrying quality control of the surface relief. 
     According to yet another aspect of the invention, there is provided a photolithography tools arrangement for applying to a semiconductor wafer provided with an optical measurement unit and a translation unit operable to carry out a relative displacement of the wafer relative to an optical arrangement of the measurement unit during spectrophotometric measurements within a slice on the wafer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
     FIG. 1 schematically illustrates a part of a production line including a photolithography tool arrangement that utilizing a measurement system for carrying out a method of the invention, and an etching tool; 
     FIG. 2 is a schematic illustration of a patterned structure produced by a pattern-creating tool; 
     FIG. 3 is a block diagram of the main components of the measurement system of FIG. 2; and 
     FIG. 4 graphically illustrates the measurement results. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     More specifically, the present invention is used for controlling a photolithography process used in the manufacture of semiconductor devices, and is therefore exemplified below with respect to this specific application. 
     Referring to FIG. 1, there is illustrated a part of a production line PL showing a semiconductor wafer W progressing on the production line between a photolithography tools arrangement, generally designated  10  (i.e., a phototrack), and an etching tool  12  accommodated downstream of the phototrack. The phototrack is typically composed of such tools as a coater  14 , an exposure tool  16 , and a developer  18 . A robot R (or number of robots) conveys wafers to be sequentially processed by these tools from a load cassette C 1  and returns processed wafers to an unload cassette C 2 . 
     The construction and operation of these tools do not form part of the present invention, and therefore need not be specifically described, except to note the following. A photoresist (PR) layer is deposited onto a metal layer (or insulating substrate) within the coater  14 , then PR layer is exposed to light and developed to create a pattern (surface relief) on the surface of the substrate. In other words, in this specific example, the exposure and developer tools present together a pattern-creating tool. It should be understood that either one of the tools in the arrangement  10 , as well as the entire arrangement, whose working parameters affect the quality of the pattern, could constitute the pattern-creating tool. 
     As shown, a measurement system  20  is installed between the phototrack  10  and the etching tool  12 , so as to be applied to wafers prior to entering the etching tool. The system  20  can be entirely integrated with the phototrack, or partly integrated, such that its optical arrangement to be applied to the wafer is located within the phototrack, while other elements (such as a light source, detector, and processor) are accommodated outside the phototrack in the proximity thereof. In the present example, the same robot R conveys wafers to and away from the measurement system. Moreover, the same robot may progressively support the wafer during measurements with the system  20 . 
     Turning now to FIG. 2, there is illustrated a wafer W as ensuing from the developer tool  14 . The wafer W is a patterned structure, wherein the pattern is in the form of spaced-apart PR regions  22  on the surface  24   a  of a metal layer  24 . It should be understood that, generally, a pattern is in the form of a surface relief of a layer, which may for example be produced by applying a stamper to the layer. Thus, in this specific example, the pattern-creating tool creates PR projections  22 , aimed at etching metal regions within the spaces between these projections, and then removing the PR therefrom. 
     Reference is made to FIG. 3, illustrating the measurement system  20 . The latter is composed of such main constructional parts as an illumination unit  30  of a kind capable of illuminating the wafer with different wavelengths (i.e., the broad band illumination); a light direction/collecting optics  32  typically including an objective lens  33  or a plurality of such lenses; a detection unit  34  including a spectrophotometer; and a control unit  36 . 
     The construction and operation of the illumination and detection units, as well as the light direction/collecting optics, are also known per se and therefore need not be specifically described. 
     The control unit is typically a computer system having a memory for storing reference data, a processor operating by suitable software for analyzing measured data generated by the spectrophotometer, and a monitor for displaying measurement results. The analysis of the measured data may include comparison thereof to the reference data, and/or to theoretical data produced by an optical model. 
     As shown in FIG. 3, the control unit  36  is connectable to the pattern-creating tool (or its relevant part, e.g., developer), thereby enabling to utilize the analysis results for carrying out a closed loop control of the pattern-creating tool. The analysis of the measured data (spectrum) is indicative of the quality of the pattern, and consequently, indicative of the quality of the pattern-creation process defined by the working parameters of the pattern-creating tool. Hence, the measurement results may be used for the so-called “feed forward” closed loop control of these working parameters prior to applying the tool to a further similar wafer (i.e., the same-lot-wafer). 
     The measurement system operates in the following manner. During the translation of the wafer from the developer tool to the unload cassette, optical measurements are applied to one or more slices on the wafer&#39;s surface. The wafer is supported either by robot R, or by an additional translation stage, for providing reciprocating or rotation of the wafer with respect to the objective lens arrangement within a measurement plane P. It should be understood, that alternatively, the wafer may be kept stationary in the plane P, and a relevant part of the optical system be displaced with respect to the wafer. 
     During the relative displacement of the wafer relative to the objective lens arrangement, measurements are carried out continuously so as to detect light returned from successive locations on the wafer within a slice. Resulting measured data is in the form of a spectrum, i.e., a photometric intensity of the returned light as a function of wavelengths. This spectrum is analyzed by correlating it with the reference data, the certain extent of correlation presenting the criteria results. The minimal spot size should be larger than the dimensions of the largest element of the pattern (feature in the die). 
     The measurement slice may extend across the wafer, along its radius, be spiral-like, etc. The measurement slice is a segment on the wafer for which measured data is obtained. One or more slices may be measured for the inspection purposes. The length of the slice is determined by the spot size (defined by the measurement unit) and by the type of pattern in the structure. In the specific example of wafer inspection, the minimal length L min  of the slice should satisfy the following condition: L min &gt;S die /d, wherein S die  is the surface area occupied by one die, and d is the diameter of the light spot. Maximal time t needed for passing one slice is determined by the saturation time t sat  of the spectrophotometer (specific sensor&#39;s parameter) within the selected range of operating wavelengths, namely, should satisfy the following condition: t&lt;t int . As for the sufficient signal-to-noise ratio of the detected signals, it is determined by an integration time t int  (defined by the system or operator). 
     It should be noted that measurements should preferably be carried out during the uniform motion of the wafer or optics (i.e., without acceleration). To this end, if accelerated motion paths of the wafer under measurements exist (typically when entering and ensuing from the measurement zone), they should be disregarded when analyzing the measured data. 
     FIG. 4 presents the measured data obtained with the above system utilizing the MMS1/VIS-ENL spectrophotometer commercially available from Carl Zeiss. The measured data is in the form of four curves C 1 -C 4 , each presenting a photometric intensity of the detected light as a function of wavelength. Curves C 1  and C 2  correspond to uncoated metal layer (i.e., without PR pattern) obtained for two different, same-lot wafers W 1  and W 2 , respectively. Curves C 3  and C 4  correspond to PR-patterned wafers (i.e., after development) W 3  and W 4 , respectively. It should be understood that the pair of wafers W 3  and W 4  could be the same wafers W 1  and W 2 , but at different manufacturing stages with different parameters (e.g., PR thickness), presenting thereby different patterned structures. Curves C 1  and C 2  are characterized by 0.9953 correlation; curves C 3  and C 4  are characterized by 0.9940 correlation; and curves C 1  (or C 2 ) and C 3  are characterized by 0.9749 correlation. This actually demonstrates that the technique of the present invention provides meaningful results. As clearly seen in the figure, the detected spectra C 3  and C 4  have very similar shapes, i.e., the same number of picks and pitch. 
     Hence, by providing reference data in the form of spectrum (obtained either with a golden wafer at various manufacturing steps or with an optical model), the measured data obtained with a wafer under inspection can be fitted to the “theoretical” data (reference data). This enables to estimate the quality of the patterned layer (e.g., developed PR). 
     Those skilled in the art will readily appreciate that various modifications and changes can be applied to the preferred embodiment of the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims.