Abstract:
A method and apparatus for providing correction for microloading effects is described. Hybrid proximity correction techniques are used to make the problem computationally more feasible. More specifically, feature edges in a layout can be grouped into those edges, or edge segments, with a large edge separation (group B), e.g. greater than n, and those having less than that separation (group A). The group B features can then be corrected for microloading effects rapidly using rules based correction. Then both groups of edges can be corrected using model based optical proximity correction using the output of the rule based correction as the ideal, or reference, layout.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to the field of semiconductor devices. More particularly, the invention relates to a method and apparatus for correcting for microloading effects.  
           [0003]    2. Description of the Related Art  
           [0004]    One common step in modern approaches to integrated circuit (IC) production is the use of an etching process after exposure of photoresist on the semiconductor to electromagnetic radiation (e.g. light).  
           [0005]    There are number of different etching technologies and methods available including, plasma etching and several types of ion beam etching. In some instances over etching, e.g. etching for an extended period of time compared to the normal etching period, has been used to reduce feature sizes. However, over etching worsens the microloading effect.  
           [0006]    Of particular importance during etching processes is maintaining uniformity. Uniformity refers to evenness of etching for critical dimension, as well as uniformity of etching across a wafer and from wafer to wafer. At the microscopic level, etching rates and profiles depend on features size and feature separation. Microscopic uniformity problems can be grouped into several categories including pattern-dependent etch effects, generally referred to as a microloading. More specifically, microloading refers to the dependence of the etch rate on feature separation for identically sized features and it results from the depletion of reactants when the wafer has a local, higher-density area.  
           [0007]    From a terminology standpoint, critical dimension simply refers to the dimension (e.g. width) of a feature in the relevant direction. For example, a feature corresponding to a transistor can be conceived of as a one dimensional object on the mask since the length will change, but the critical dimension will not. Thus, for example if the transistors are being prepared with a target critical dimension of 1 μm, there can be multiple transistors with different lengths, e.g. some 5 μm, some shorter, some longer, but all might be designed to have critical dimension of 1 μm. (Note, a single mask may include similar features having different critical dimensions.)  
           [0008]    Current optical proximity correction techniques are not well suited to accounting for microloading effects. Further, if existing approaches are used in a straightforward fashion they may be computationally infeasible with present day computer systems and hardware.  
           [0009]    Accordingly, what is needed is a method and apparatus for correcting for microloading effects. Also suitable masks for producing integrated circuits that have been corrected for microloading effects. (As used herein, the term masks includes reticles.)  
         SUMMARY  
         [0010]    A method and apparatus for providing correction for microloading effects is described. Hybrid proximity correction techniques are used to make the problem computationally more feasible. Specifically, if model based optical proximity correction techniques were used alone, the problem would be extremely complicated and further changes made to correct for optical errors would interact with changes made to correct microloading errors.  
           [0011]    The approach groups feature edges in a layout into those edges, edges or edge segments, with a large edge separation (group B), e.g. greater than n, and those having less than that separation (group A). More specifically, the straight line distance from neighboring edges to a given edge can be determined and edges, or edge segments, that are further than the given amount n placed into group B. The value of n is process technology dependent, for an example λ=248 nm wavelength process, n=1.5 μm. Edges having a separation equal to n are placed into either group A or group B, in one embodiment they are placed in group B to be corrected for microloading effects.  
           [0012]    The group B features are then corrected for microloading effects, or etch effects, using rules based correction. Rules based corrections can be applied extremely rapidly since there is minimal computational complexity as the layout is scanned for features, edges, and/or edge segments matching the rule criteria and then the rules are applied. For example, a rule might adjust an edge with a separation of 2.0 μm by 30 nm.  
           [0013]    Next, both groups of edges, e.g. the entire layout portion being corrected, can be corrected using model based optical proximity correction (MOPC). The MOPC is applied using the output of the rule based correction as the ideal, or reference layout. Conceptually this can be viewed as the MOPC is trying to bring the layout so that after optical effects occur the pattern will be such that it is shaped as was computed is better (based on the rules) to account for the later occurring etch process.  
           [0014]    In some embodiments, the ordering of the etch and optical effects are switched; however, such embodiments are likely to give less accurate corrections, but may still be useful. Approaches for developing appropriate etch and optical models are described as well. The models can be generated using measurements taken from test exposures. This ensures that the generated models are calibrated for the particular lithography process being used including the stepper, the resist, the etch, etc. In some embodiments, uncalibrated models are used based on assumed data or theoretical computations. This may be appropriate for testing purposes, if suitable test exposures cannot be obtained, and/or if only slight changes to the lithography process for a previously calibrated model are being made, etc.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0015]    [0015]FIG. 1 is a process flow diagram for performing optical proximity correction (OPC) on a layout in a manner that corrects for microloading effects.  
         [0016]    [0016]FIG. 2 is a layout for a portion of an integrated circuit.  
         [0017]    [0017]FIG. 3 depicts rules based OPC correction of a portion of the layout of FIG. 2 for microloading effects.  
         [0018]    [0018]FIG. 4 depicts the model based OPC range for the layout of FIG. 3 for use in performing optical proximity correction for optical and resist effects.  
         [0019]    [0019]FIG. 5 is a process flow diagram for generating OPC rules based on a calibrated etching model.  
         [0020]    [0020]FIG. 6 is a process flow diagram for generating a calibrated model for a given lithography process.  
     
    
     DETAILED DESCRIPTION  
       [0021]    Overview  
         [0022]    As noted, uniformity of critical dimension (CD) of features is an important aspect of etching processes. In order to maintain uniformity of CD, microloading effects should be considered. An approach to correcting for microloading effects to provide uniformity of critical dimension will be discussed first. Next, a more detailed setup of the parameters and models used will be considered. Finally several alternative embodiments will be considered.  
         [0023]    Efficient Microloading Effect Correction  
         [0024]    [0024]FIG. 1 is a process flow diagram for performing optical proximity correction (OPC) on a layout in a manner that corrects for microloading effects. The process of FIG. 1 is best understood in conjunction with the respective processes described on FIGS. 5 and 6 and the example shown in FIGS.  2 - 4 .  
         [0025]    The process starts at step  130  when edge segments of features from a layout  100  are divided into two groups (group A and group B) based on their separation from one another. Group A will be comprised of relatively densely packed edge segments (e.g. separation&lt;n μm), while group B will be comprised of semi-isolated and isolated edge segments (e.g. separation&gt;n μm). (Note: Edge segments with a separation=n μm can be placed in either group A or group B. In one embodiment they are placed in group B to ensure correction for microloading effects.)  
         [0026]    In this example, the separation used is 1.5 μm for a sample λ=248 nm process. More generally, the separation n should be larger than the range of optical proximity effects for the particular process and based on the observed range of microloading/etch effects for the process.  
         [0027]    Different edges, and even portions of a single edge, of a feature in a layout may have different characteristics vis-à-vis their relative isolation from other features. FIG. 2 shows an exemplary layout  200  including a number of features. The grouping of edges for the feature  220  in the layout  200  will be considered. One approach is to measure the line from the corner of other layout objects toward edges of the feature  220  (the measurement line should be perpendicular to the orientation of the edge). Six measurement lines are shown as dashed lines with arrowheads. The edge to edge measure  230  shows the distance between the top corner of a nearby feature and one point on the right edge of feature  220 . Further down the edge  220  another edge to edge measure  232  intersects the right edge of the feature  220 . Still further down, the edge to edge measures  232 ,  234  and  236  intersect the feature  220  along the right edge. Note however, that the region  210  between the edge to edge measure  232  and the edge to edge measure  236  is actually &gt;n μm (here, n=1.5 μm) from other edges. Thus the region  210  of the right edge of the feature  220  will fall into group B. The distance between the left edge of the feature  220  and nearby features is shown by edge to edge measures  238  and  240 . In contrast, the other portions of the side edges of the feature  220  are &lt;1.5 μm from nearby edges and would fall into group A. The grouping can be done in parallel or series for the other layout features being corrected for microloading effects and optical proximity effects.  
         [0028]    After grouping, the process continues at step  140  with rule based OPC being applied to account for etch effects on group B edges. The OPC etch rules  110  can be used as the rules to modify the group B (separation&gt;n μm) edge segments. The generation of the OPC etch rules  110  will be discussed in greater detail in relation to the process of FIG. 5, below. Turning to the example layout of FIG. 2, the region  210  is in group B and should be corrected for etch effects. Here, FIG. 3 shows a layout  300  that corresponds to the layout  200  after rules based correction of step  140  has been applied for the feature  220 . The rule correction  310  caused the width of the feature in the region  210  to be made smaller (e.g. downward biased, narrowed, reduced in width, etc.) to account for the microloading effect. This correction reduces the width of the feature  320  in that region because semi-isolated and isolated edge segments are likely to etch more slowly and thus be too large in size. (Note: The downward bias is exaggerated for illustrative purposes in FIG. 3. For example, the downward bias might be 30 nm for a 150 nm target critical dimension. The specific bias will have to be determined for each process technology and model.)  
         [0029]    Next, at step  150 , model based OPC is applied to model the resist and optical effects on all edge segments, e.g. both group A and group B. A calibrated optical model  120  can be used to describe those effects and the final OPC layout  160  can be generated. The generation of such an optical model is described in greater detail with reference to FIG. 6.  
         [0030]    In some embodiments, the model based OPC uses the modified shapes generated at step  140  as the ideal (or target) shape. Thus returning to performing this process on the layout  200 , at step  150 , the ideal shape for the feature  220  will be the shape of the feature  320  generated at step  140  during correction for microloading effects.  
         [0031]    [0031]FIG. 4 depicts the model based OPC range for the layout of FIG. 3 for use in performing optical proximity correction for optical and resist effects. Here, the layout  400  includes the feature  320 , the ideal shape that model OPC will attempt to correct the layout to generate. An evaluation point  410  on the ideal layout is shown with an “X”. A dashed line shows the OPC range  420 , which is a circle of radius R from the evaluation point  410 .  
         [0032]    After the model based OPC is applied at step  150 , the OPC layout  160  can be output (not shown). In some circumstances, there may be additional or intermediate steps added to the process of FIG. 1 to permit viewing, simulation, and/or testing of the intermediate and final output layouts.  
         [0033]    In one embodiment, the process of FIG. 1 is added to an OPC software package such as the Photolynx(™) software from Transcription Enterprises, a Numerical Technologies Company, from San Jose, Calif.  
         [0034]    Etch Model and Rule Generation  
         [0035]    [0035]FIG. 5 is a process flow diagram for generating OPC rules based on a calibrated etching model. This process can be used to generate the OPC etch rules  110  for use at step  140  of the process of FIG. 1. The generated rules will provide rule-based OPC correction for certain types of microloading effects.  
         [0036]    The process starts with a test pattern  500 . The test pattern is characterized by a number of line segments of differing widths at different separations. For example, the test pattern might include features with a critical dimension of 1 μm spaced at varying densities, e.g. minimum design pitch up through 10 μm. This could be repeated for each different critical dimension size being used in a particular layout and perhaps at differing orientations, e.g. some placed horizontally and other vertically. At step  510 , a test mask is fabricated according to the test pattern.  
         [0037]    Then, at step  520 , a wafer is exposed using the test mask and the resist on the wafe developed, step  530 . Next, at step  540 , the critical dimension (CD) of features in the resist are measured, and stored as resist CD measurements  545 . The resist CD measurements  545  can be used to calibrate optical models, for more information on that see below.  
         [0038]    According to one embodiment, only resist CD measurements for separations less than a predetermined distance n, e.g. &lt;1.5 μm, are stored in the resist CD measurements while only the resist CD measurements for separations greater than (or equal to) the predetermined distance are used for the remaining steps of the process of FIG. 5. For the remainder of the discussion of FIG. 5 and FIG. 6 it will be assumed that this “grouping” of resist CD measurements has occurred and that the resist CD measurement  545  contains only the measurements for separations less than the predetermined amount and that the processing at step  550 - 580  uses only measurements for separations greater than (or equal to) the predetermined amount. Here, the predetermined separation is 1.5 μm.  
         [0039]    The process continues at step  550 , with the etching of the wafer, e.g. plasma etch. At step,  560  the critical dimensions of features after etch are measured. These measurements are used to develop an etch model  570 . The etch model predicts the variability of critical dimension for &gt;1.5 μm separations.  
         [0040]    Finally, from the etch model, at step  580  OPC rules are generated and stored, e.g. as OPC etch rules  110 . The etch rules can take the form of specific (or ranged) data for target critical dimension and edge separation, e.g. for critical dimension target of 150 nm and a feature separation of (1.5 μm, 2 μm], the feature should be downward biased by 30 nm.  
         [0041]    In some embodiments, the ModelGen and RuleGen software products from Numerical Technologies, Inc., of San Jose, Calif., can be used at step  570  and step  580 , respectively, of the process of FIG. 5.  
         [0042]    Process Model  
         [0043]    [0043]FIG. 6 is a process flow diagram for generating a calibrated model for a given lithography process. This process can be used to generate the calibrated optical model  120  for use at step  150  of the process of FIG. 1.  
         [0044]    The process starts with input of stepper, and other process, settings  600 . This information is used to generate an optical model at step  610 . For example, the ModelGen(™) software from Numerical Technologies, Inc., San Jose, Calif., could be used to generate the model. At step  620 , the model can be calibrated based on results from a sample exposure, e.g. the resist CD measurements  545 . As noted above, the resist CD measurements  545  may in some instances only contain measurements for feature separations less than a predetermined amount. In some embodiments, the ModelCal(™) software from Numerical Technologies, Inc., is used to generate the calibrated model. The calibrated model can be stored as the calibrated optical model  120 .  
         [0045]    Alternative Embodiments  
         [0046]    Embodiments of the invention can be used with deep ultraviolet (DUV), extreme ultraviolet (EUV), x-ray, and/or other lithography techniques. The particular mask substrate and protective areas should be adapted for the specific lithographic process. Additionally, the rules (or simulation) will be based on the model of the specific systems being used. The examples herein and the n given for separation were for an exemplary λ=248 nm process.  
         [0047]    Note also that at either stage of the process of FIG. 1, rule based correction could be substituted for model based correction and vice versa. The particular arrangement was selected for speed and overall accuracy. Similarly, the ordering of the correction for etch effects and optical effects can be swapped in some embodiments of the invention. This is somewhat less desirable however since by performing the optical correction second it is possible to use the ideal target shape computed earlier for the etch effects, which in terms of wafer processing come later.  
         [0048]    Note also that the value of n used for grouping at step  130  may to some extent be empirically determined. Specifically factors such as the wavelength of the light used (λ), the numerical aperture (N.A.), the resist used, and more.  
         [0049]    The data structures and code described in this detailed description can be stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated).  
         [0050]    For example, the transmission medium may include a communications network, such as the Internet. In one embodiment, the computer readable storage medium includes one or more computer programs for performing rules based optical proximity correction for microloading effects, model based optical proximity correction for optical and resist effects, and grouping edges of features according to separation from surrounding features. In one embodiment, the electromagnetic waveform comprises computer programs accessed over a network, the computer programs for rules based optical proximity correction for microloading effects, model based optical proximity correction for optical and resist effects, and grouping edges of features according to separation from surrounding features.  
         [0051]    In one variation of this embodiment, the computer data for layouts is formatted as one or more GDS-II data files. In other embodiments, the electromagnetic waveform includes a computer program accessed across the network for modifying the layout to correct for microloading effects and/or for optical and resist effects.  
         [0052]    The foregoing description of embodiments of the invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations will be apparent. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.