DE10339514B4 - Method for exposing a substrate - Google Patents

Abstract

Method for exposing a substrate (17) to a structure (2) of a mask (6), in which the structure (2) on the mask (6) has a regular arrangement of transparent (10, 10 ') and semitransparent (12, 12 '), against each other phase-shifted structural elements having a surface portion and a transmittance each such that the structure (2) is projected at an exposure in an image structure (22) on the substrate (17) whose arrangement is a double spatial frequency of imaged structural elements ( 10 '', 12 ''), wherein the transparent (10 ') or semitransparent (12') structural elements on the mask (6) have an error in the structure width with an average deviation of 5-10 nanometers from a predetermined ideal value,
comprising the steps:
Providing the mask (6) with the structure (2) and the photosensitive resist-coated substrate (17) in a projection apparatus (100),
- Aligning the substrate (17) and setting a value for a focal distance relative to a lens system (51) of the projection apparatus ...

Description

The The invention relates to a method for exposing a substrate with a structure of a mask, in which the structure on the Mask a regular arrangement transparent and / or semitransparent, phase-shifting Structural elements with an area fraction and a transmittance each such that the structure projected on the substrate during exposure in an image structure whose arrangement is a double spatial frequency of mapped Structural elements under consideration the scale the projection has.

The The invention also relates in particular to a method for exposure chromeless, alternating or preferably highly transmissive halftone phase masks.

at the desire to smaller and smaller sizes of Making structures in integrated circuits, you access increasingly based on the concept of phase masks. Under very special, particularly suitable conditions for the structural arrangements The masks are in addition to the contrast-enhancing effect due to the Live features as well still possible, a spatial frequency doubling when mapping the structure from the mask into the respective image plane to achieve. Under frequency doubling is here a doubling the number of resulting in the aerial image in the substrate plane intensity peaks per unit area to understand. Put simply, it will be twice as many Structural elements formed on the substrate as on the mask are.

This effect occurs if and only if the special arrangement of the structural elements within the structure causes exactly the cancellation of the 0th diffraction order. An example of such a frequency-doubling mask structure is from the document EP 1 288 71 A1 known. The periodic arrangement of structural elements specified therein comprises within a cell repeating itself in the mask structure at least two different surface sections or structural elements with different area, transmission and phase property. Optionally, opaque elements are also provided.

The in the document specified condition, with which a frequency doubling is achieved, states that for the transparent or semitransparent elements of the cell the product of area and Transmission must be the same.

For the simple one Case of alternating phase masks yields the trivial condition that the surfaces of the alternating alternating columns, between the opaque lines same. In the case of different transparent structural elements can at high levels of transmission extraordinarily good results in terms of the amplitudes of the intensity peaks be achieved, it is also possible in particular, two-dimensional Structural arrangements, z. B. contact holes to realize. At the alternate Phase masks would be one limited to the one-dimensional line-column patterns.

The Production of phase masks is subject to special requirements to the mask writing instruments, as a rule, two or more levels on the mask independently written, but must be aligned. Of others can just be considered in the case of phase masks critical structure sizes to one nonlinear increase in the error when transferring to a substrate come (mask error enhancement factor).

One Problem arises now that the frequency doubling Feature of phase masks with the special structure arrangements extraordinarily is sensitive to Deviations from the above conditions. At the moment Structural widths in production of, for example, 110 nm are linewidth variations of 5-10 nm based on the wafer scale the aforementioned effects are not uncommon. Although these deviations still within the of the equipment manufacturers or the customer for the acceptance of the integrated circuits specified tolerance limits. However, the frequency-doubling property of the mask structures works already largely lost in these line width errors, or affects at least in a strong asymmetry of the pictured Textured pattern. The term asymmetry here denotes the Inequality of two adjacent intensity peaks in the image or substrate level produced during production frequency doubled Aerial view.

Become only during the projection of frequency doubling structures as well As errors identified as balancing errors are discovered, this is usually the case with an extraordinary high costs associated with stopping production and the corresponding Order the mask from the mask manufacturer again. A solution can therefore seemingly only exist in the settings of the Change lighting system, for example, the numerical aperture or the fill or Coherence factor σ. The expert be satisfying solutions but not achieved.

In the textbook by AKK Wong: "Resolution Enhancement Techniques in Optical Lithography", Tutorial Texts in Optical Engineering, Vol. TT47, SPIE Press, 2001, pages 171 to 175, discusses ways to increase resolution. Chapter 8.1 deals with multiple exposures. Thereafter, it is known to increase the depth of field of an optical image by multiple imaging at different focus settings (so-called FLEX method). In addition to the Flex method, alternating phase masks in trim exposure technology are also shown there.

A continuous passage through the focal planes is in the US 2002/0061471 A1 described (see 3 , Section 29). Here, the focus variation serves to reduce the errors in the linewidths on the substrate, thus improving the depth of focus range of imaging a mask onto a substrate.

In the JP 09-138497 A , of the US Pat. No. 6,218,077 B1 and the US 5 255 050 A Lithographic exposure methods are also described.

It is therefore the object of the present invention, the quality and the Throughput of the production of integrated circuits, with the help of frequency doubling mask structures are made to increase. It In particular, an object of the present invention is the influence of asymmetry effects due to balancing errors (minimal linewidth errors) Masks) significantly reduce.

• – Bereitstellen der Maske mit der Struktur und des mit einem photoempfindlichen Resist beschichteten Substrates in einem Projektionsapparat,
• – Ausrichten des Substrates und Einstellen eines Wertes für einen Fokusabstand gegenüber einem Linsensystem des Projektionsapparates,
• – Belichten der Maske zur Projektion der Struktur von der Maske auf das Substrat,
• – Variieren des eingestellten Fokusabstandes während der Belichtung zwischen einem maximalen und einem minimalen Fokusabstand, wobei jedes der auf das Substrat abgebildeten Strukturelemente der regelmäßigen Anordnung mit einer Vielzahl von verschieden eingestellten Werten innerhalb eines Defokusintervalles gebildet wird,
• – so dass der Fehler in der Strukturbreite der Strukturelemente auf der Maske bezüglich der Strukturelemente auf dem Substrat ausgeglichen wird.

The object is achieved by a method for exposing a substrate having a structure of a mask, in which the structure on the mask has a regular arrangement of transparent and semi-transparent, mutually phase-shifted structural elements having a surface portion and a transmittance each such that the structure at an exposure is projected into an image structure on the substrate whose arrangement has a double spatial frequency of imaged structure elements, the transparent or semitransparent structural elements on the mask having an error in the structure width with an average deviation of 5-10 nanometers from a predetermined ideal value, comprising the steps:

• Providing the mask with the structure and the photosensitive resist-coated substrate in a projection apparatus,
• Aligning the substrate and setting a value for a focal distance with respect to a lens system of the projection apparatus,
• Exposing the mask to project the structure from the mask to the substrate,
• Varying the adjusted focus distance during the exposure between a maximum and a minimum focus distance, wherein each of the structure elements of the array arranged on the substrate is formed with a multiplicity of differently adjusted values within a defocus interval,
• - So that the error in the structure width of the structural elements on the mask is balanced with respect to the structural elements on the substrate.

It is during the exposure of the substrate is a change in focus distance performed. The focal distance describes the distance of the surface of the Substrate with the applied resist from a fixed point in the Projection system, such as the objective lens. The focus distance becomes along the optical axis, measured on the Z axis. The method according to the invention According to various values within an interval for the focal distances during the Exposure adjusted. From an ideal, a contrast with maximum sharpness Delivering focal distance deviating Einstellun conditions are in the following also called defocus. This gives the distance or deviation from the optimal focus setting again.

Of the According to the invention, in particular, a low contrast delivering different focus settings during exposure. It has now been found that straight in the inventively used frequency doubling mask structures the asymmetries in the Image structure leading intensity peaks in the presence of a masking error (balancing errors) to higher defocus values first lose weight. Point to a focus error dependent on the mask error adjacent intensity peaks even complete Symmetry on. To even higher Focus deviations out then reverses the sign of asymmetry around, d. H. the previously smaller intensity peaks are now larger than the previously larger intensity peaks.

There the for the respective mask necessary to establish the symmetry defocus value (Focus deviation) is not immediately known, and its determination a big Would cause effort is according to the method of the invention a plurality of defocus values representing focus settings drove through. This is preferably done continuously. During the opening of a Aperture of the projection optics or a shutter to release the Exposure becomes the substrate holder fixing the substrate (XY-stage) proceed continuously in the Z direction.

But it is also possible to perform focus variations in juxtaposed exposures in the same resist. The term "during" in this document covers the period between the beginning and the end of a single continuous exposure or, in the case of an interrupted exposure - between the beginning of a first partial exposure of the substrate and the end of a last partial exposure of the same substrate.

By the exposure at different focus settings will be through the masking error resulting asymmetry effects over the Defokusintervall averaged. Those at low defocus below the total symmetry Focus value equals the asymmetries due to those exposure settings for the Focus above this optimal focus value. That on the substrate resulting aerial image (aerial image) is therefore formed symmetrical.

In a particularly advantageous embodiment is dependent from the while Exposure currently set focus setting, d. H. the Defocus, set the radiation dose of the exposure source. This can either be the intensity of Exposure source or from the exposure source to the substrate reaching light varies by means of filters or a diaphragm constriction be, or it will - in Trap that one Wafer scanner is used for exposure - the scan speed, d. H. the period of time with which a section of the substrate surface is exposed is varied. This is a particularly advantageous way Weighting of preferred focus settings achieved.

is For example, the assignment of the substrate holder for varying the Focus distance fixed, so can only by means of a variation the radiation dose to the individual mask error account become.

Further advantageous embodiments are the subordinate claims remove.

The Invention will now be described with reference to an embodiment with the aid of a Drawing closer explained become. Show:

1 a section of a frequency-doubling mask structure (left) and the image structure resulting therefrom in a projection on a substrate (right) in the case of ideal balancing (a) or in the presence of a mask error (b);

2 from a periodic mask structure according to 1b resulting image structure for 5 different focus settings beginning at the top left with the in 1b shown image structure at the ideal focus setting;

3 various arrangements for the projection of a frequency-doubling mask pattern from a mask onto a substrate according to the prior art (a), as well as for two embodiments according to the invention (b, c);

4 a flowchart of an embodiment of the invention with discretely set, different focal distances.

1a shows on the left side a section of a frequency-doubling mask structure. In the example shown, the mask structure comprises 2 transparent and semitransparent structural elements 10 . 12 , In the middle of the section shown is a periodically repeating cell 4 the regular arrangement highlighted. The surface of the transparent structural element 10 is about 3 times as small as the area of the semitransparent structural element 12 , The condition to be followed to satisfy the frequency-doubling property relates to the transmittance of the semitransparent structural element 12 , This must be 33% of the incident light according to the area fraction.

In 1a the state of a mask structure with an ideal frequency doubling property is shown. On the right side of the 1a is the result of the projection of the mask structure 2 through a lens system 51 on a substrate 17 resulting image structure 22 represented in a contour plot. The contour lines reflect equal intensities. The rows 7 and columns 8th of structural elements 10 on the mask are immediately in rows 27 and columns 28 in the picture structure 22 displayed. Due to the frequency doubling property arise between the positions of the intensity peaks, in the photosensitive resist immediately the structural elements 10 each represent additional additional intensity peaks in the rows 37 or columns 38 (as well as in the rows 37 ' . 37 '' and the columns 38 ' . 38 '' ).

In 1b the case is shown in which the structural element 10 ' by 5 nm in its structural width from that in 1a shown ideal value (3-σ value of a measured mask error). On the right side of the 1b the result obtained in the substrate plane is shown in a contour plot. It can be seen in the plot shown only with rough resolution that the contrast of the rows 37 and columns 38 additionally strongly decreases in the resulting intensity peaks, ie the ratio of maximum intensity to that at the foot of the intensity peak decreases. This creates an asymmetry between the structural elements which are directly shown in the same position 10 ' and the additional peaks mapped by the frequency doubling property.

2 shows for 5 different focus settings when projecting with the in 1b shown, faulty mask structure in the substrate plane resulting aerial image in each case in a contour line plot. Top left is for the ideal focus ment, which is determined for example by a conventional focusing algorithm, again in 1b obtained state shown. The ideal focus setting represents the focal distance for which maximum contrast in the substrate plane is achieved. Again, the asymmetry is between the intensity peaks of the immediately imaged features 10 ' as well as the additional secondary peaks.

For the other contour plots, the focus distance was increased by 0.1 μm. A comparison of the plots for a defocus of 0 μm and 0.1 μm shows that the larger the defocus, the greater the contrast of the secondary peaks in the rows 37 and columns 38 is. For a focus adjustment with a defocus of 0.2 μm, an identity in the intensity peaks is achieved. However, this is at the expense of the overall achievable contrast for the aerial photo.

To even greater values of the defocus, the asymmetry between directly imaged structural elements increases 10 ' and additionally mapped secondary peaks in the reverse direction, ie the minor peaks of the rows 37 and columns 38 are larger than those of the immediately depicted intensity peaks of the rows 27 and columns 28 ,

3a shows an arrangement for exposing a substrate 17 , ie a semiconductor wafer, according to the prior art. The exposure takes place in a wafer scanner. A radiation source 15 emits a light beam through a mask slot 18 towards a mask (reticle) 6 , From the mask 6 is the one in the 1b shown mask structure 2 with the structural elements 12 ' . 10 ' made with non-ideal, frequency doubling feature. About a lens system 51 , which is shown here only schematically, that is through the mask 6 structured light on a substrate 17 on which a photosensitive resist is applied. The structural elements 12 ' . 10 ' become image structure elements 10 '' . 12 '' displayed.

This is a four-fold reducing image, with the mask in the wafer scanner 6 with a scanning motion 45 and the substrate 17 with a four times slower scan motion 46 opposite the mask slot 18 is moved. A usually also existing wafer slot immediacy bar above the position of the substrate 17 is not shown in the schematic diagram for the sake of simplicity.

With the in 3a shown projection apparatus 100 For exposure, the substrate is first 17 aligned with the projection optics (lens system 51 ) as well as the mask 6 , Typically, a focusing algorithm is carried out for a first wafer of a lot, in which an ideal focus distance is determined with respect to the projection optics of the projection apparatus. In the step of aligning, a so-called level / tilt algorithm is performed, which ensures that each surface section is removed from the surface of the substrate 17 at the ideal focus distance from the lens system 51 stands. With the projection apparatus 100 Subsequently, an exposure is performed, which corresponds to the in 1b results shown, which disadvantageous asymmetry effects between the directly from the structural elements 10 ' having imaged intensity peaks and the additional minor peaks.

3b shows an embodiment of the present invention. In the projection apparatus 100 becomes the substrate 17 with an inclination α to that through the lens system 51 defined ideal focal plane 90 arranged. When scanning 46 becomes the substrate 17 parallel or antiparallel to the gradient direction of the inclination α of the substrate 17 moves (within the drawing plane of the 3b ). At the same time, by means of a movement of the substrate holder 57 (not shown here) in the z-direction a vertical movement 49 performed, due to which the focus distance is varied for each of the inclined surface sections.

Because of the slot 18 the exposed cutout on the substrate 17 in 3b wanders right, the focus distance to the lens system 51 that is, the vertical movement leads 49 with regard to the currently exposed section to compensate for this effect. One of the cutout of the slot 18 detected element on the surface of the substrate 17 has during exposure so initially a small focus distance, which is due to the movement 49 increases until the surface element due to the scanning movement 46 the light section of the slot 18 leaves again. The scanning movement 46 , the vertical movement 49 and the inclination α are in a fixed relationship to each other, so that each surface element passes through the same focus interval.

In the exemplary embodiment, for imaging the image structure elements 10 '' . 12 '' Defocus values from 0 μm to 0.4 μm, as in 2 shown, go through. The exposure process is not interrupted in this embodiment, so that this Defokusintervall is continuously traversed. In the 2 contour plots shown are thus averaged, so that the different asymmetry effects cancel each other substantially. The result is essentially the same as in 1a illustrated contour plots for a mask structure without line width error, ie without balancing errors. There is only a slightly reduced contrast to be accepted. The changed contrast can be accommodated by changing the overall radiation dose compared to the ideal setting. The total radiation dose depends in particular

3c shows another embodiment of the present invention. In the projection apparatus 100 is the substrate 17 with the substrate holder 57 similar to the prior art within the ideal focal plane 90 arranged. The substrate holder 57 (XY-stage) is with a moving unit 55 ie, a motor connected, which moves in the vertical Z-direction with the substrate holders 57 can perform. The trajectory 55 is with a central control unit 59 connected to the projection apparatus.

In the embodiment, the moving part is 55 prepared to do an in 3c schematically illustrated sawtooth curve 50 of the substrate holder 57 traversing in the Z direction, so that the focus distance also rises or falls linearly on the time axis and falls back at regular intervals back to a basic value steep / jerky. The periodicity of the temporal curve 50 correlates with the width of the light cutout through the slot 18 as well as the scan speed 46 , Every surface element of the substrate 17 thus passes exactly once - or integer multiples - the desired focus interval.

It goes without saying for the expert, instead of the one in 3c shown sawtooth curve 50 Other periodic curves that use sinusoids, symmetrical sawtooth profiles, etc.

With the central control unit 59 is also the source of illumination 15 connected. Depending on the current focus distance is by means of the control unit 59 the intensity of the radiation source 15 varied. This allows a weighting of example in 2 achieved intensity results are achieved. For example, if it is known that the symmetry of the intensity peaks is achieved at a defocus of 0.1 μm instead of 0.2 μm, the focus interval through the curve 50 however, given a fixed value between 0.0 μm and 0.4 μm, the intensity of the radiation can be reduced to the size of the focus values to a vanishingly small value. In this way it is possible to convert masks with arbitrary balancing error via the variation of focus and radiation dose without asymmetries in image structures on a semiconductor wafer.

4 shows in a flowchart an embodiment of the invention, in which not as in the previous example continuously within a continuous exposure, the focus distance is varied, but gradually and alternie end the exposure and then each a focus variation is undertaken.

After deploying 1001 from mask 6 and substrate 17 if a defocus interval is selected, step 1002 , These are indications of an interval of deviations from an ideal or optimum focal distance, ie relative numerical values. The interval preferably becomes dependent on the mask provided 6 selected, because the line width or phase error is assumed to be known here, it should be corrected by applying the embodiment just.

Meanwhile, the substrate becomes 17 and the mask 6 to each other and to the lens system 51 aligned, step 1003 , Subsequently, a focusing algorithm is started, which provides an ideal focus adjustment or focus distance, step 1004 , For a fraction of the actual required exposure time is now an exposure 1005 with the set focal distance - here the ideal focus distance - performed. The length of this partial exposure corresponds to a weighting of the focal distance. Through a variation 1010 the exposure time of a partial exposure, a variation of the radiation dose is achieved. If all focus distances within the assigned defocus interval are to be weighted equally, the total exposure time is divided by the number of focal distances to be set. Alternatively or additionally, the step 1010 also the change of the intensity of the radiation source 15 include.

In the following step 1006 the focus distance is varied according to the default of the selected defocus interval. In a loop are now exposures successively 1005 performed under the respective different focal distances. Leaving the defocus interval will be in an abort condition 1007 checked, their entry finally to completion 1008 the exposure leads.

The steps 1005 and 1006 can also be executed parallel in time.

2

mask structure with structural elements

4

Cell, repeating periodically

6

mask or reticle

7

string of structural elements on mask

8th

columns of structural elements on mask

10

transparent Structural element, ideal

10 '

transparent Structure element, with line width error

10 ''

Picture structural element in substrate level

12

semi-transparent Structural element, ideal

12 '

semi-transparent Structure element, with line width error

12 ''

Picture structural element in substrate level

15

radiation source

17

substratum in substrate level

18

mask slot

22

image structure

27 27 ', 27' '

string imaged structural elements in substrate plane

28 28 ', 28' '

columns imaged structural elements in substrate plane

37

string the additionally imaged by frequency doubling structural elements in the substrate plane

38

columns the additionally imaged by frequency doubling structural elements in the substrate plane

45

scanning movement the mask

46

scanning movement of the substrate

49

vertical Movement to focus variation

50

time Traverse curve for substrate holder

51

lens system

55

traversing for substrate holder

57

substrate holder

59

central control unit

90

ideal focal plane

100

projector

α

Tilt Focal plane substrate level

1001

Provide of mask and substrate

1002

Select one Defokusintervalles

1003

Align (Alignment) of the substrate

1004

Focus with focusing algorithm

1005

Expose with focus distance set

1006

variation the adjusted focus distance

1007

Termination condition: "Focus distance within the defocus interval? "

1008

The End the exposure

1010

variation the radiation dose (exposure time and / or intensity)

Claims (10)

Method for exposing a substrate ( 17 ) with a structure ( 2 ) of a mask ( 6 ), in which the structure ( 2 ) on the mask ( 6 ) a regular arrangement of transparent ( 10 . 10 ' ) and semi-transparent ( 12 . 12 ' ), against each other phase-shifted structural elements having a surface portion and a transmittance each having such a structure that 2 ) when exposed in an image structure ( 22 ) on the substrate ( 17 ) whose arrangement is a double spatial frequency of imaged structural elements ( 10 '' . 12 '' ), the transparent ( 10 ' ) or semitransparent ( 12 ' ) Structural elements on the mask ( 6 ) have an error in feature width with a mean deviation of 5-10 nanometers from a given ideal value, comprising the steps of: - providing the mask ( 6 ) with the structure ( 2 ) and the photosensitive resist-coated substrate ( 17 ) in a projection apparatus ( 100 ), - alignment of the substrate ( 17 ) and setting a value for a focal distance with respect to a lens system ( 51 ) of the projection apparatus ( 100 ), - exposing the mask ( 6 ) for the projection of the structure ( 2 ) from the mask ( 6 ) on the substrate ( 17 ), - varying the set focus distance during the exposure between a maximum and a minimum focus distance, each of which is focused on the substrate ( 17 ) illustrated structural elements ( 10 '' . 12 '' ) of the regular array is formed with a multiplicity of differently set values within a defocus interval, so that the error in the structural width of the structural elements ( 10 ' . 12 ' ) on the mask ( 6 ) with regard to the structural elements ( 10 '' . 12 '' ) on the substrate ( 17 ) is compensated. Method according to claim 1, characterized in that that the minimum or maximum focus distance is the same as the Step of aligning set focus distance. Method according to one of claims 1 or 2, characterized in that the focus distance set in the alignment step is an ideal focus distance determined in a focusing algorithm, with which a structural element ( 10 ' . 12 ' ) with maximum contrast on the substrate ( 17 ) is displayed. Method according to one of claims 2 or 3, characterized that this Defocus interval between the maximum and minimum focus distance less than 0.4 microns. Method according to one of claims 1 to 4, characterized in that the projection apparatus ( 100 ) is a wafer scanner, in which during the exposure both the mask ( 6 ) as well as the substrate ( 17 ) perpendicular to an optical axis of the lens system ( 51 ) and by means of a slot ( 18 ) are irradiated in sections. Method according to claim 5, characterized in that a through the substrate ( 17 ) substrate plane opposite one through the lens system ( 51 ) defined focal plane ( 90 ) is inclined, so that the focus distance due to the Movement of the substrate ( 17 ) changes continuously perpendicular to the optical axis in the substrate plane. Method according to claim 6, characterized, that - each Focus distance under which part of the exposure is performed a value for associated with the radiation dose to be used at this focal distance becomes, - during the Exposure dependent from the respective focal distance, the radiation dose is varied. Method according to claim 5, characterized in that the substrate ( 17 ) during the exposure with another, along the optical axis of the lens system ( 51 ) extending direction is reciprocated periodically, so that the of the slot ( 18 ) of the wafer scanner irradiated section of the substrate ( 17 ) in its focal distances with respect to the lens system ( 51 ) is varied. Method according to claim 8, characterized in that that the periodic to-and-fro motion starting from a first focus distance continuously increases linearly up to a second focal distance and subsequently immediately falls back to the first focus distance. Method according to one of claims 1 to 9, characterized in that the transparent ( 10 . 10 ' ) or semitransparent ( 12 . 12 ' ) Structural elements on the mask ( 6 ) have a phase error with a deviation of their mutually phase-shifting property from 180 degrees greater than +/- 5 degrees.