DE102013206528B4 - MICROLITHOGRAPHIC PROJECTION EXPOSURE PLANT WITH A VARIABLE TRANSMISSION FILTER - Google Patents

Abstract

Microlithographic projection exposure system (10) with a) a holder (26) for a mask (14), b) a light source (LS) which is set up to generate projection light (PL), c) an illumination system (12) which is used by directs the projection light generated by the light source onto the mask and illuminates an illumination field (16) there, d) a holder (32) for a light-sensitive layer (22), e) a projection lens (20) that places the part of the mask in the illumination field on the images photosensitive layer, characterized by a variable transmission filter (40) comprising: f) a first refractive optical element (42a), the absorption coefficient of which varies according to a first filter function over a surface of the refractive optical element away; g) a second refractive optical element (42b), the absorption coefficient of which varies over a surface of the refractive optical element in accordance with a second filter function, the first filter function being different from the second filter function; h) a displacement device (44) with which the relative position of the two refractive optical elements (42a, 42b) can be changed in a direction (X) perpendicular to an optical axis (OA) of the projection exposure system (10), wherein - In a first relative position, the filter functions of the two refractive optical elements are added to a first overall filter function (GF1N) in which the absorption coefficient is different from zero but independent of location, - in a second relative position the filter functions of the two refractive optical elements to form a second overall filter function (GF2M ), in which the absorption coefficient is different from zero and is location-dependent, and - in a third relative position, which is located between the first and the second relative position, the filter functions of the two refractive optical elements to a third overall filter function (GF1Z) complement each other of the second overall filter function (GF2M) only d differs by a location-independent factor k with 0 <k <1.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to a microlithographic projection exposure apparatus with a variable transmission filter, in particular a variable apodization filter.

2. Description of the Related Art

Integrated electrical circuits and other microstructured devices are typically fabricated by applying a plurality of patterned layers to a suitable substrate, which is typically a silicon wafer. For structuring the layers, they are first covered with a resist that is resistant to light of a certain wavelength range, eg. B. deep ultraviolet (DUV, deep ultraviolet), vacuum ultraviolet (VUV, vacuum ultraviolet), or extreme ultraviolet (EUV, extreme ultraviolet) spectral regions. Subsequently, the thus coated wafer is exposed in a projection exposure apparatus. In this case, a pattern of diffractive structures, which is arranged on a mask, is imaged onto the photoresist with the aid of a projection objective. In general, since the magnification amount is smaller than 1, such projection lenses are sometimes referred to as reduction lenses.

After developing the photoresist, the wafer is subjected to an etching process, whereby the layer is patterned according to the pattern on the mask. The remaining photoresist is then removed from the remaining parts of the layer. This process is repeated until all layers are applied to the wafer.

In projection exposure systems, there is often the need to be able to influence intensity distributions in certain levels in a location-dependent manner. For this purpose, a filter is introduced in the relevant plane, which, when it is passed by the projection light, is referred to as a transmission or gray filter. In addition, filters are known which act in reflection. Such a reflection filter can be realized, for example, by detuning the reflective coating from mirrors in a location-dependent manner.

A particularly important application of filters are so-called apodization filters. This refers to filters which are arranged in a pupil plane of a lens. In general, apodization filters serve the purpose of suppressing unwanted diffraction orders. However, at least in the context of microlithography, the term is often understood in a more general sense. It then designates a filter with which the amplitude term of the optical transfer function can be changed.

Frequently, transmission filters in microlithographic projection exposure systems, and in particular in the case of apodization filters, have a need to be able to change the filter effect quickly and in a location-dependent manner.

From the US 5,444,336 A a projection objective of a microlithographic projection exposure apparatus is known in which different apodization filters can be introduced into a pupil plane of the projection objective. However, the number of different filter distributions that can be realized is logically limited to the number of available apodization filters.

The US 2006/0092396 A1 describes a projection lens in which a transmission filter is constructed from LCD cells that can be individually controlled.

At one of the US 2010/0134891 A1 known Apodisierungsfilter the reflective coating of a curved mirror is locally detuned. However, it is difficult to reverse this mood.

From the US 5,614,990 A For example, a projection exposure machine with a variable transmission filter is known which has a plate of photochromic glass and a scanner or an array of light sources to illuminate different areas of the glass with excitation light.

From the WO 2008/092653 A2 For example, a variable transmission filter for a lighting system of a microlithographic projection exposure apparatus is known. In one embodiment, two gray filters may be moved relative to one another along the scan direction.

From the US 3,305,294 A the use of two mutually displaceable phase plates is known in order to influence the phase of the optical light continuously variable. These plates are usually referred to as "Alvarez plates" according to the inventor. In a neutral position, the two plates act as a whole, like a plane plate. If the plates are moved transversely, the overall optical effect corresponds to the effect of a positive or negative rotationally symmetrical lens whose refractive power depends on the relative travel.

From the US 2003/0067591 A1 , of the US 2001/0046039 A1 and the US Pat. No. 6,404,499 B1 In each case, a microlithographic projection exposure apparatus is known, which has a variable transmission filter with two plane-parallel refractive elements.

SUMMARY OF THE INVENTION

The object of the invention is to provide a microlithographic projection exposure apparatus which contains a transmission filter, the filter function, d. H. the two-dimensional distribution of the transmission coefficient across the filter surface, arbitrary and can be changed so rapidly within wide limits that the operation of the projection exposure system does not have to be interrupted for a long time.

A projection exposure apparatus according to the invention which achieves this object has the features of claim 1.

Thus, according to the invention, the principle known from the Alvarez plates is transferred from the range of phase changes to the amplitudes of the optical wavefront. In contrast to the phase changes that add up in two Alvarez plates, the total filter function for amplitude-affecting plates is not the sum but the product of the single-filter functions of the refractive optical elements. Therefore, the formalism known for the Alvarez plates can not be easily transferred to the case of the amplitudes.

It is best if the refractive optical elements are plane plates. In this way, the refractive optical elements are free of refractive power and influence the phase of the light passing through only in a location-independent manner.

To create the absorbing region, the refractive optical elements may carry an absorbent coating. The absorbent coating may be applied in the nature of a continuous gradient gray filter. In addition, a realization of the required absorption profile by the application of individual fully absorbing points into consideration, the density of which varies locally.

Alternatively, it is possible, the material of the refractive optical element, for. As glass or CaF ₂ , by irradiation with high-energy laser radiation high fluence locally damage so that there is a permanent reduction in transmissivity.

In order to reduce light losses due to the possibly numerous refractive optical elements, they can be accommodated in an immersion liquid, which is preferably index-adapted, i. H. has the same refractive index as the refractive optical elements. At the interfaces of the refractive optical elements, there is then neither refraction nor light losses due to partial reflection. In addition, the refractive optical elements can emit heat to the immersion liquid, which is caused by the partial absorption of the projection light.

Structurally, such a transmission filter with an immersion liquid can best be realized if it has a sealed container which is at least partially transparent to the projection light and which is arranged in the beam path of the projection light and in which the refractive optical elements and the immersion liquid are accommodated. Although the traversing device is excluded from the container, it does not have to be provided with seals in order to guide operating rods or the like which connect the traversing device to the individual refractive optical elements out of the container.

In one exemplary embodiment of the invention, the variable transmission filter is assigned a computing unit which is set up to calculate the traversing paths to be set by the traversing device if it is given a two-dimensional nominal filter distribution.

If the refractive optical element is arranged in or near a pupil plane of the projection objective, it can be used as an apodization filter. However, a near-field position of the refractive optical element is also considered, for example to improve the uniformity of the exposure. If field-dependent apodization is desired, the refractive optical element must be located at a position between a pupil plane and a field plane.

It is particularly favorable if the overall filter function in the second relative position can be described by a Zernike distribution, in particular a Zn distribution with n greater than or equal to 2, in particular greater than or equal to 4. Such Zn distributions are particularly needed for the purpose of apodization.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the drawings. Show:

1 a highly simplified perspective view of a microlithographic projection exposure apparatus;

2 a simplified meridional section through the in the 1 shown projection exposure apparatus;

3 a meridional section through an inventive variable transmission filter, the part of in the 2 is shown projection lens;

4 the overall filter function of a first pair of refractive optical elements when in a neutral position;

5 the total filter function of the first pair of refractive optical elements when they are maximally relative to each other;

6 the total filter function of the first pair of refractive optical elements when they are in an intermediate position between the in 4 shown neutral position and in the 5 shown position with maximum travel;

7 a graph illustrating the filter distribution of the first refractive optical element of the first pair;

8th a graph illustrating the filter distribution of the second refractive optical element of the first pair;

9 the overall filter function of a second pair of refractive optical elements when in a neutral position;

10 the total filter function of the second pair of refractive optical elements when they are maximally relative to each other;

11 the total filter function of the second pair of refractive optical elements when they are in an intermediate position between the in 9 shown neutral position and in the 10 position shown with maximum travel path.

DESCRIPTION OF PREFERRED EMBODIMENTS

The 1 shows a highly schematic perspective view of a projection exposure system 10 , which is suitable for the lithographic production of microstructured components. The projection exposure machine 10 includes a light source LS, which is used to generate projection light with a Center wavelength of 193 nm is set up, and a lighting system 12 which transmits the projection light generated by the light source LS onto a mask 14 directed and there a narrow, rectangular in the illustrated embodiment illumination field 16 illuminates. Other illumination field shapes, e.g. B. ring segments are also considered.

Inside the lighting field 16 underlying structures 18 on the mask 14 be using a projection lens 20 comprising a plurality of lenses L1 to L4 on a photosensitive layer 22 displayed. The photosensitive layer 22 in which it is z. B. may be a photoresist is on a wafer 24 or another suitable substrate and is located in the image plane of the projection lens 20 , Because the projection lens 20 in general a magnification | β | <1 has, within the illumination field 16 lying structures 18 reduced to a projection field 18 ' displayed.

In the illustrated projection exposure system 10 become the mask 14 and the wafer 24 during the projection proceed along a direction marked Y. The ratio of the travel speeds is equal to the magnification β of the projection lens 20 , If the projection lens 20 the image inverted (ie β <0), the traversing movements of the mask run 14 and the wafer 24 in reverse, as in the 1 is indicated by arrows A1 and A2. In this way, the lighting field 16 in a scanning motion over the mask 14 guided, so that even larger structured areas contiguous to the photosensitive layer 22 can be projected.

The 2 shows the in the 1 shown projection exposure system 10 in a simplified meridional section. In addition, there is a mask traversing table 26 drawn with the mask 14 in an object plane 28 of the projection lens 20 can be moved.

The substrate 24 with the photosensitive layer applied thereon 22 is in the picture plane 30 of the projection lens 20 with the help of a substrate traversing table 32 traversable.

The projection lens 20 contains an intermediate image layer 34 and a first pupil plane 36 and a second pupil plane 38 , In the first pupil level 36 is a variable transmission filter according to the invention 40 arranged in the 2 is shown only schematically. Due to its arrangement in a pupil plane, the transmission filter acts 40 as an apodization filter. The transmission filter comprises a container 43 which is at least partially transparent to the projection light, and a traversing device 44 that with a computing unit 45 connected is. The latter in turn is controlled by a higher-level process control 47 controlled.

The following is the structure of the variable transmission filter 40 closer with respect to the 3 explains which the variable transmission filter 40 in a meridional section shows.

To the variable transmission filter 40 belong in the illustrated embodiment, four refractive optical elements, as a thin plane plates 42a to 42d are formed. The plane plates 42a to 42d consist of glass, to which a filter function was imprinted, what in the 3 indicated by different gray levels. To impress the filter function is the respective plane plate 42a to 42d exposed to high-energy laser radiation high fluence. This leads to permanent material damage locally, which leads to a reduction of the transmission coefficient. The longer the irradiation is carried out, the greater the damage and the more the absorption coefficient increases. In this way, it is possible to produce any filter function in the refractive optical element, without having to apply an absorbent coating for this purpose.

The four plane plates 42a to 42d are in the fluid-tight container 43 taken, which is can-shaped in the illustrated embodiment. On opposite sides is the container 43 with windows 46 . 48 through which the projection light PL passes during the operation of the projection exposure apparatus.

The container 43 is with an immersion liquid 50 filled the entire of the container 43 filled in enclosed volume. The immersion liquid 50 has at least approximately the same refractive index as the plane plates 42a to 42d , so that the projection light PL the plane plates 42a to 42d can pass through, without it comes at the interfaces to refraction or reflection to a significant extent.

Every plan plate 42a to 42d is about a linkage 52a to 52d with the shuttle 44 connected. The plane plates 42a to 42d In this way, individually with the help of the moving device along the X-direction, ie perpendicular to the optical axis OA of the projection lens 20 to be traded, as by double arrows 54a to 54d in the 3 is indicated.

The two plane plates 42a and 42b form a first pair and the other two plane plates 42c and 42d a second pair. The filter functions of the plane plates of each pair are matched in a manner which will be described below with reference to FIGS 4 to 11 is explained in more detail.

In the 4 the total filter function of the first pair is shown when the two plane plates 42a . 42b in a first relative position to each other along the X direction, which is referred to below as the neutral position. It can be seen that in the total filter function GF1N of the first pair in the neutral position, the absorption coefficient is different from zero but location-independent.

The 5 shows the total filter function of the first pair in a second relative position where the absorbing effect is maximal. The total filter function GF1M of the first pair in this maximum relative position is characterized by the fact that the absorption coefficient in the middle of the area penetrated by the projection light PL is greatest and drops continuously towards the edge. This total filter function GF1M corresponds to a Zernike distribution Z4.

The 6 shows the overall filter function of the first pair in a third relative position that is between the first neutral position and the second maximum relative position. Accordingly, the overall filter function GF1Z is also located between the total filter functions GF1N and GF1M, which are included in the 4 and 5 are shown. This overall filter function GF1Z is also a Z4 distribution, except that the overall filter function GF1Z is globally reduced by a location-independent factor compared to the overall filter function GF1M.

The 7 and 8th illustrate the filter functions of the individual plane plates 42a respectively. 42b with which in the 4 to 6 shown overall filter functions were achieved by the relative position along the X direction between the plane plates 42a . 42b with the help of the moving device 44 was changed. To produce in the 5 and 6 The overall filter functions GF1M and GF1Z shown are the two plane plates 42a . 42b proceed by equal amounts in opposite directions along the X-axis.

The following explains how the in the 7 and 8th shown filter functions of the plane plates 42a . 42b can be calculated.

The total filter function T _zn (x, y) results as a product of the individual filter functions T ₁ (x), T ₂ (x) of the _{plane plates} 42a . 42b according to the equation (1): T _Zn (x, y) = T ₁ (x + Δx, y) T ₂ (x, y), (1)

It is again assumed below that the desired overall filter function T _Zn (x, y) is a 24 distribution which is given by Equation (2): T _Z4 (x, y) = β - αΔx (2x ² + 2y ² - 1), (2)

Therein, β denotes a necessary transmission offset, where β = 0.95, and α = (1-β). The values for x and y can vary between -1 and +1. If equation (1) is developed into a Taylor series, equation (3) is obtained:

By comparing the coefficients of equations (2) and (3), it can be seen that T ₁ (x, y) T ₂ (x, y) = β (4)

Besides that is

Using equation (4), one obtains

The first term on the left hand of equation (6) is the derivative with respect to x of the logarithm of T ₁ (x). For the filter function T ₁ (x) we obtain the following exponential function:

The integration constant c can be determined by the transmission value of T ₁ (x) at the position x = y = 0 T ₁ (0, 0) = √β. (8th)

Consequently, it is c = - β / αlnββ (9)

The 9 to 11 show total filter distributions GF2N, GF2M or GF2Z for the second pair of plane plates 42c and 42d , Again, there is again a neutral position in which the absorption coefficient is location independent, cf. 9 , At maximum relative position of the two plane plates 42c . 42d along the X-direction you get in the 10 shown Gesamtfilterfunkion GF2M, which corresponds to a Z2 distribution. In intermediate positions one also contains Z2 distributions, as described in the 11 are shown by way of example and which differs from the filter function GF2M only by a location-independent factor.

With the variable transmission filter 40 Thus, both Z2- and Z4-type filter distributions can be generated as well as overlays thereof in any proportions.

The following explains how the variable transmission filter 40 for example, can be controlled:
First, during an exposure break, an optical sensor 120 into the picture plane 30 of the projection lens 20 proceed as in the 2 through an arrow 122 is indicated. The image sensor 120 misses the optical wavefronts in the image plane and determines in this way the imaging quality of the projection lens 20 , The central process control 47 calculated on the basis of the sensor 120 provided measurement results a desired apodization, by the transmission filter 40 should be set. This apodization is in the form of a desired filter function to the arithmetic unit 45 to hand over. In a numerical optimization procedure, this determines the travel paths of the individual plane plates 42a to 42d in the transmission filter 40 in such a way that the multiplicative superimposition of the total filter functions generated by the pairs approximates as closely as possible the desired desired filter function. The corresponding setting commands are then from the arithmetic unit 45 to the shuttle 44 passing the plane plates with the aid of the servomotors contained therein 42a to 42d so inside the container 43 shifts that results in the desired filter function for the passing through projection light PL. The resulting apodization then leads to an improvement of the imaging properties of the projection lens 20 ,

Claims (8)

Microlithographic projection exposure apparatus ( 10 ) with a) a holder ( 26 ) for a mask ( 14 ), b) a light source (LS), which is set up to generate projection light (PL), c) a lighting system ( 12 ), which directs the projection light generated by the light source on the mask and there a lighting field ( 16 ), d) a holder ( 32 ) for a photosensitive layer ( 22 ), e) a projection lens ( 20 ) which images the part of the mask lying in the illumination field onto the photosensitive layer, characterized by a variable transmission filter ( 40 ), comprising: f) a first refractive optical element ( 42a ) whose absorption coefficient varies over a surface of the refractive optical element according to a first filter function; g) a second refractive optical element ( 42b ) whose absorption coefficient varies according to a second filter function across an area of the refractive optical element, the first filter function being different from the second filter function; h) a shuttle ( 44 ), with which, in a direction (X) perpendicular to an optical axis (OA) of the projection exposure apparatus ( 10 ), the relative position of the two refractive optical elements ( 42a . 42b ), wherein - in a first relative position, the filter functions of the two refractive optical elements complement each other to a first total filter function (GF1N) in which the absorption coefficient is different from zero, but location-independent, - in a second relative position, the filter functions of the two refractive optical Complement elements to a second overall filter function (GF2M) in which the absorption coefficient is non-zero and location-dependent, and - in a third relative position located between the first and second relative positions, the filter functions of the two refractive optical elements to a third total filter function (GF1Z), which differs from the second total filter function (GF2M) only by a location-independent factor k with 0 <k <1. Microlithographic projection exposure apparatus according to claim 1, characterized in that the refractive optical elements ( 42a . 42b ) Planplatten are. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the refractive optical elements ( 42 . 42b ) carry an at least partially absorbent coating. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the refractive optical elements in an immersion liquid ( 50 ) are included. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the variable transmission filter has an arithmetic unit ( 45 ), which is adapted to be used by the shuttle ( 44 ) to calculate traversing paths to be set if it is given a two-dimensional target filter distribution. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the variable transmission filter in a pupil plane ( 36 ) of the projection lens ( 20 ) is arranged. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the total filter function in the second relative position can be described by a Zernike distribution. Microlithographic projection exposure apparatus according to claim 7, characterized in that the Zernike distribution is a Zn distribution with n greater than or equal to 2.

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