WO2008116477A1

WO2008116477A1 - Microlithographic method

Info

Publication number: WO2008116477A1
Application number: PCT/EP2007/002598
Authority: WO
Inventors: Aksel GÖHNERMEIER
Original assignee: Carl Zeiss Smt Ag
Priority date: 2007-03-23
Filing date: 2007-03-23
Publication date: 2008-10-02

Abstract

A microlithographic method, comprises arranging a substrate in a predetermined position relative to a mask, exposing a first layer of a first photoresist on the substrate with radiation transmitted through the mask such that a total first area of the first layer is exposed to a first radiation dose which is at least equal to a first resist exposure threshold dose of the first photoresist, exposing a second layer of a second photoresist on the substrate with radiation transmitted through the mask such that a total second area of the second layer is exposed to a second radiation dose which is at least equal to a second resist exposure threshold dose of the second photoresist, wherein the second layer is different from the first layer, and wherein the total first area has a size which is at least 1.25 times a size of the second total area.

Description

Microlithographic method

Background of the invention

Field of the invention

The present invention relates to a microlithographic method for generating a micropatterned device, and in particular to a microlithographic method allowing generation of micropatterned device having highly resolved pattern features .

Brief Description of Related Art

Lithographic processes are the method of choice in the semiconductor industry for the production of micropatterned devices, such as microchips. In view of the trend to increasing a device density on the individual microchips by decreasing a size and thus critical dimensions of individual devices, such as transistors, improved lithographic methods are required to provide the increased resolution necessary to produce ever smaller and denser device features. A minimum feature size F achievable in a conventional photolithographic process is given by the relationship F = k -λ/NA, wherein k is a process related parameter, λ is an exposure wavelength used in a lithography system and NA is a numerical aperture on an image side of the lithography system. A reduction in F has been achieved in past years by periodically decreasing exposure wavelengths and continually developing lithography systems having higher numerical apertures .

A lithographic process generally involves directing a beam of radiation onto a mask having a given mask pattern of transparent and opaque areas. Radiation passing through the transparent mask areas pass through reduction optics and subsequently expose a photoresist covered substrate. Thus, exposed areas in the photoresist form a reduced-size negative image of the mask pattern. The photoresist is then developed by removing either exposed or unexposed areas of the photoresist in a developer solution thus forming either a negative or a positive image of the mask pattern in the remaining photoresist which typically serves as a mask for further substrate processing. Therefore, a size of resolvable individual features on the mask ultimately limits the feature size on the substrate. The size of the mask features, such as a smallest possible line width and a smallest possible space in between lines, is, in turn, limited by a resolution of a mask manufacturing process.

Therefore, in an attempt to decrease a feature size on the substrate beyond a size limited by a resolution and wavelength of the lithographic process, a process has been developed which involves plural exposure steps and use of two different masks. This process allows generation of a grid of features having a period p with λ/ (4NA) < p < λ/ (2NA) in a lithography system having a numerical aperture NA and utilizing an exposure wavelength λ. The pattern can be realised by using a first mask in a first exposure step and a second mask in a second exposure step. Both the first and the second masks have a grid of features having a period 2p, with the grid of features of the first mask being shifted a distance p relative to the grid of features of the second mask. A period p of a grid of equidistant lines of the same width generally defines a distance from one center of a line to a center of the next line, or in other words a sum of a line width and a width of an area in between adjacent lines. It is also commonly referred to as a pitch.

This known lithographic process is described in the following in more detail with reference to Figure 1. Figure 1 shows a cross- section of a first mask 100, and a substrate 10 coated with a first masking layer 11 and a first photoresist 21. The first mask 100 comprises a pattern of opaque mask features 1 and transparent mask features 2. The opaque mask features 1 form a grid having a period 2p.

In a first exposure step, a beam of radiation directed onto the first mask 100 is transmitted through the transparent mask features 2. Radiation transmitted through the mask 100 generates a first pattern of exposed areas 21e in a first photoresist 21. For ease of illustration, the exposed areas 21e are depicted as having a same size as a corresponding transparent mask feature 2 in Figure 1. Thus, the first pattern of exposed areas 21e substantially corresponds to the pattern of transparent mask features 2, whereas unexposed areas 21u of the first photoresist 21 form a pattern that substantially corresponds to the pattern of opaque mask features 1 (Figure Ia) .

In a second step, the first photoresist 21 is developed. The photoresist being a positive photoresist, the first pattern of exposed areas of first photoresist 21e is removed upon development of the first photoresist 21, such that a first photoresist pattern of unexposed areas of first photoresist 21u is left behind, as shown in Figure Ib. The substrate 10 is then removed from the lithography system. In a subsequent process step, the first photoresist pattern of unexposed areas 2Iu of first photoresist 21 is used as an etch mask for etching the underlying first masking layer 11. Etching thus transfers the first pattern of exposed areas 21e of first photoresist 21 to the first masking layer 11 to form a first pattern of trenches 11a, lib in the first masking layer 11, as shown in Figure Ic. The first pattern of trenches 11a, lib in the first masking layer 11 has a period 2p . Thereafter, remains of first photoresist 21u are removed to release the first masking layer 1 having the first pattern of trenches 11a, lib (Figure Id) .

In order to enable a second exposure, the trenches 11a, lib in the first masking layer 11 are filled with a second masking material 31 such that a surface of the second masking material 31 is about level with a surface of the first masking layer 11. A second photoresist 41 is then coated onto the first masking layer 11 and the second masking material 31, as shown in Figure Ie. Thereafter, the substrate 10 is reintroduced into the lithography system for a second exposure step.

In the second exposure step, a second mask 200 is used which has the same pattern of features of period 2p as the first mask 100, which pattern is shifted by p relative to the pattern of features in the first mask 100. Arranging the substrate 10 in a same position relative to the second mask 200 results in radiation transmitted through transparent mask features 2 in the second mask 200 being incident at other locations of the substrate 10 as compared to the first exposure step. Thus, although a same total area of photoresist is exposed to radiation during the second exposure step due to the same pattern of features being used in the exposure, the areas on the substrate 10 that are exposed in the second exposure step are shifted relative to the areas exposed during the first exposure step with no overlap between areas exposed in the first exposure with areas exposed in the second exposure. Thus, previously unexposed areas can be exposed in the second exposure step.

In the second exposure step, radiation transmitted through the second mask 200 generates a second pattern of exposed areas 41e in the second photoresist 41. The second pattern of exposed areas 41e in the second photoresist 41 is shifted relative to the previous first pattern of exposed areas 21e in the first photoresist 21, as illustrated in Figure If. The second photoresist 41 is also a positive photoresist such that exposed areas 41e of the photoresist 41 are removed in a subsequent development step to form a second photoresist pattern of unexposed areas 41u of second photoresist 41 (Figure Ig) . The second photoresist pattern of unexposed areas 41u of the second photoresist 41 is used as an etch mask in a subsequent etching process of the first layer 11, resulting in a second pattern of trenches lie, Hd being formed in the first masking layer 11, the second pattern of trenches lie, Hd having period 2p.

The trenches Ha, b formed after the first exposure step are interdigitated with the trenches Hc, d formed after the second exposure step. Thus, a denser pattern of features can be generated with the final pattern of trenches encompassing the first and second pattern of trenches Ha, Hb, Hc, Hd having a period p. This pattern may be used for further processing of the substrate . A main disadvantage associated with this process lies in a significant increase in production costs since manufacturing two different masks is requisite.

Therefore, it is an object of the present invention to provide a lithographic process which allows generation of a dense pattern of features on a substrate whilst requiring only one mask.

Summary of the Invention

The present invention provides a microlithographic method, comprising: arranging a substrate in a predetermined position relative to a mask, exposing a first layer of a first photoresist on the substrate with radiation transmitted through the mask such that a total first area of the first layer is exposed to a first radiation dose which is at least equal to a first resist exposure threshold dose of the first photoresist, exposing a second layer of a second photoresist on the substrate with radiation transmitted through the mask such that a total second area of the second layer is exposed to a second radiation dose which is at least equal to a second resist exposure threshold dose of the second photoresist, wherein the second layer is different from the first layer, and wherein the total first area has a size which is at least 1.25 times a size of the second total area.

In exemplary embodiments, the size of the total first area is at least 1.5 or 1.75 times the size of the second total area. The size of the total first area may further be twice, 3 times, 4 times, 5 times, 7.5 times, 10 times, 20 times, 30 times or 40 times the size of the second total area, to name a few further examples .

The term microlithographic process shall refer to a lithographic process for generating a micropatterned device, i.e. a device having a pattern with features having dimensions in the micrometer or even nanometer range . It is to be understood that an area having received a certain minimum radiation dose does not need to have received the same radiation dose across the entire area, but may have received different radiation doses in different portions of the area, as long as every portion of the area has received the minimum radiation dose.

Herein, the first resist exposure threshold dose is defined as a minimum exposure dose which is sufficient to chemically alter an area of the first layer exposed to at least the first resist exposure threshold dose for it to be selectively and totally separable from an area of the first layer exposed to less than the first resist exposure threshold dose. Likewise, the second resist exposure threshold dose is defined as a minimum exposure dose which is sufficient to chemically alter an area of the second layer exposed to at least the second resist exposure threshold dose for it to be selectively and totally separable from an area of the second layer exposed to less than the second resist exposure threshold dose. This corresponds to an assumption of infinite contrast for the photoresists. A radiation dose, which is used synonymously with the term exposure dose herein, defines an amount of radiation per unit area.

In a simplified use of terminology relating to photoresist exposure, an area of a layer having received a radiation dose above the resist exposure threshold dose would simply be called an exposed area whereas an area of the layer having received a radiation dose below the resist exposure threshold dose would typically be called unexposed. However, even radiation transmitted through a mask having mask features that are either completely opaque or transparent to radiation does not result in an intensity distribution with only two discrete intensity values of the radiation incident on the photoresist. This is illustrated with reference to Figure 2. Transmission of radiation by transparent mask features is high, as indicated by reference sign HT whereas there is zero transmission through opaque mask features, as indicated by reference sign LT. Thus, an intensity of the radiation across the mask has one of two discrete values. Due to diffraction and other phenomena, an intensity distribution of radiation in an image plane where a photoresist layer is disposed differs from the initial intensity distribution generated by the mask. It has, in particular, a more gradual transition between high and low intensity values, for instance an about sine-type distribution.

Thus, rather than differentiating between two discrete states of unexposed and exposed areas of photoresist, it is herein distinguished between photoresist having received more or less than its resist exposure threshold dose. For simplicity's sake, in the following description, exposing with an exposure dose less than the resist exposure threshold dose may also be referred to as sub-threshold exposing, whereas exposing with an exposure dose of at least the resist exposure threshold dose may also be referred to as threshold exposing.

In the case of a conventional positive photoresist, for instance, those areas of the photoresist which have received at least the resist exposure threshold dose will be rendered soluble and therefore removed in a subsequent development process whereas those areas which have received less than the resist exposure threshold dose will remain insoluble.

In a conventional negative photoresist, those areas having received less than the resist exposure threshold dose will be soluble in a development process whereas those areas having received more than the resist exposure threshold dose are rendered insoluble.

In those conventional photoresists, selective separation is typically achieved on the basis of a difference in solubility in a developer solution. A further increase in a difference in chemical and/or physical characteristics of the respective sub- threshold exposed and threshold-exposed areas may be achieved by one or more of a number of known methods, such as a so-called post-bake or a pre-bake. Processes involving multiple subthreshold exposures of the same layer with intermediate baking steps may also be feasible. The first and second photoresists may be any suitable type of photoresist known in the art. Examples for suitable photoresists are chemically amplified resists or Novolac type resists.

The microlithographic process according to the present invention only requires one mask. Of course, the process according to the present may be used without realising all or any advantages thereof. For instance, it would be conceivable to use two separate masks having the same mask features. Likewise, it would be feasible to use two identical mask sections of the same or two different masks. In principle, a same mask pattern, whether it belongs to the same or different masks, may advantageously be used in a same position relative to the substrate.

The microlithographic method according to the present invention thus preferably involves exposing a first layer of a first photoresist on the substrate with radiation transmitted through the mask, with the substrate being arranged in the predetermined position relative to the mask, and exposing a second layer of a second photoresist on the substrate with radiation transmitted through the mask, with the substrate being arranged in the predetermined position relative to the mask, i.e. using the same mask in a same position relative to the substrate.

The microlithographic exposure process enables exposure to radiation transmitted through the same mask to be used to define a first pattern in the first layer that may be transferred to a first masking layer to form a first pattern in the first masking layer, and to define a second pattern in the second layer that may be transferred to the first masking layer to form a second pattern in the first masking layer, which second pattern has a different position from the first pattern. In analogy to the process described above with reference to Figure 1, a final pattern in the first masking layer may be comprised of the first and the second patterns of trenches .

In advantageous embodiments, the microlithographic method further comprises developing the first and second layers. Areas of the first layer which are exposed to less than the first radiation dose define a first pattern, and areas of the second layer exposed to the second radiation dose define a second pattern which may serve for pattern transfers.

Using the same mask in the same position relative to the substrate will result in patterns of threshold-exposed areas in both the first and second layers, which generally differ from each other not so much in terms of their position but in terms of their sizes. Taking a simple cross as a simple exemplary mask feature, the above described process can be used to generate a cross of threshold-exposed areas in the first layer with this cross being an enlarged version of a cross of threshold-exposed areas generated in the second layer. The cross would have practically the same position in both layers, such that the cross in the second layer would, if both layers were stacked, be completely overlapped by and centred with respect to the cross in the first layer.

As outlined above, the microlithographic method according to the present invention may further comprise: transferring the first pattern to a first masking layer on the substrate such that a first pattern of trenches is formed in the first masking layer, and transferring the second pattern to the first masking layer on the substrate such that a second pattern of trenches is formed in the first masking layer.

The present invention provides a microlithographic method, comprising: arranging a substrate in a predetermined position relative to a mask, exposing a first layer of a first photoresist on the substrate with radiation transmitted through the mask, with the substrate being arranged in the predetermined position relative to the mask, such that a first pattern of areas exposed to a radiation dose below a first resist exposure threshold dose is generated in the first layer, exposing a second layer of a second photoresist on the substrate with radiation transmitted through the mask, with the substrate being arranged in the predetermined position relative to the mask, such that a second pattern of areas exposed to a (second) radiation dose equal to or above a second resist exposure threshold dose is generated in the second layer, the first layer being different from the second layer, and transferring the first pattern to a first masking layer on the substrate to form a first pattern of trenches in the first masking layer and transferring the second pattern to the first masking layer on the substrate to form a second pattern of trenches in the first masking layer.

In this embodiment, an inverse pattern of the first pattern is exposed to a (first) radiation dose which is equal to or higher than the first resist exposure threshold dose.

Exposure of the first and second layers may be achieved in a number of ways. Generally, the microlithographic method of the present invention may involve the first and second layers being exposed in a single exposure step or the first layer being exposed in a separate exposure step from the second layer.

Both embodiments have in common that a ratio of the first radiation dose absorbed by the first layer versus the first resist exposure threshold dose is substantially different from a ratio of the second radiation dose absorbed by the second layer versus the second resist exposure threshold dose.

To achieve this, for instance, a total radiation dose of an exposure step may be adjusted, a sensitivity (resist exposure threshold dose) of the first photoresist may be different from that of the second photoresist, a first layer thickness may be different from a second layer thickness, to name but a few.

The first and second layers are different at least in that they are individual, physically separate layers.

In an exemplary embodiment, using a first layer of a same thickness and composed of a same photoresist as the second layer, a radiation dose received and absorbed by the first layer may be set to be higher than a radiation dose received by the second layer. For instance, the microlithographic method may be carried out such that a ratio of the first radiation dose to the second radiation dose is at least 1.25. In exemplary embodiments wherein the first and second layers have a same resist exposure threshold dose, the ratio of the first radiation dose to the second radiation dose may be at least 5, it may be at least 8, 10, 15, 20 or 50, to name but a few examples.

In exemplary embodiments, a ratio of first to second radiation doses may be adjusted in dependence of a pattern of the mask, in particular in dependence of a desired line to space ratio to be realised in the pattern (grid) that is to be finally formed on the substrate. By way of example, the ratio of first to second radiation doses may be chosen to increase with a decrease in the desired line to space ratio. A line to space ratio is generally used to characterize regular patterns such as grids and defines a width of a line in respect to a width of an adjacent space, i.e. gap in between lines, and is commonly also referred to as a duty cycle.

The process according to the present invention allows taking advantage of the particular radiation intensity distribution occurring within a layer of photoresist during photolithographic exposure thereof. Changing an intensity of radiation transmitted through the same mask not only leads to a change in maximum and minimum intensities but also to a difference in an area which is irradiated with a certain intensity. This is schematically illustrated with reference to Figure 3. Radiation transmitted through a transparent mask area 2 is incident on a layer of positive photoresist 21. Figure 3 shows dose contours in the layer 21 of photoresist at increasing exposure doses Dl through D4. Exposure to dose Dl would not be sufficient to develop the photoresist layer across an entire thickness of the layer 21, i.e. the layer 21 has received less than its resist exposure threshold dose. Increasing exposure doses from D2 to D4 is accompanied by an increasing volume and also width of the photoresist layer 21 having received a dose above the resist exposure threshold value. Development of the photoresist layer after exposure to exposure dose D3 would lead to about a width w3 of photoresist being removed, exposure to dose D4 would lead to a larger width w4 of photoresist being removed. Figure 3 also demonstrates that it is possible for w4 to be larger than a width wlOO of a corresponding imaged transparent area, whereas w2 , w3 < wlOO, (assuming a hypothetical reduction factor of 1) . In a lithography system comprising a reduction system for reducing an image size of the mask, this would correspond to w4> wlOO * β > w2 , with β representing the reduction factor of the lithography system. Exposure to different exposure doses Dl to D4 results in exposures with different ratios of radiation or exposure dose vs. resist exposure threshold dose of the photoresist .

In a further exemplary embodiment, exposing the first layer comprises exposing the first layer such that radiation transmitted through a transparent mask feature having an area of a first size Al exposes an area of a second size A2 in the first layer with a radiation dose equal to or above the first resist exposure threshold dose, wherein exposing the second layer comprises exposing the second layer such that radiation transmitted through the transparent mask feature having the area of the first size Al exposes an area of a third size A3 in the second layer with a (second) resist exposure radiation dose equal to or above the second threshold dose, wherein a ratio A2/A3 of the second size A2 to the third size A3 is at least 1.25, may be at least 5 , 8 , 10, 15, 20, 30, 40 or more. In further exemplary embodiments, A2 may be larger than Al * β, with β being a reduction factor of the lithographic system used for exposure. In further exemplary embodiments, A3 may be smaller than Al * β .

An embodiment of the present invention which is illustrative of a concept underlying the present invention is explained in the following with reference to Figure 4. Radiation transmitted through mask 100 having transparent and opaque mask areas 1, 2 exposes areas 21e of the first layer of first photoresist 21 with at least the first resist exposure threshold dose, and exposes areas 21u with less than the first resist exposure threshold dose. Sub-threshold-exposed areas 21u define a first pattern which will subsequently be transferred to form a corresponding first pattern of trenches 11a, b in the first masking layer 11. Radiation transmitted through the same mask 100 arranged in the same position relative to the substrate 10 exposes areas 4Ie of the second layer 41 of second photoresist with at least the second resist exposure threshold dose, and exposes areas 4Iu with less than the second resist exposure threshold dose. A ratio of a radiation dose absorbed by the photoresist of the first layer vs. its resist exposure threshold dose is larger than a ratio of a radiation dose absorbed by the photoresist of the second layer vs. its resist exposure threshold dose, resulting in a larger total area of the first layer of photoresist being threshold-exposed. Threshold-exposed areas 4Ie of the second layer 41 define a second pattern which will be subsequently transferred to the first masking layer 11 to form a corresponding second pattern of trenches lie, d therein. Threshold-exposed areas 41e of the second layer 41 do not overlap with sub-threshold exposed areas 2Iu of the first layer 21, i.e. the first and second patterns do not overlap, but are rather offset from one another. Using a mask 100 having a feature pattern with period 2p thus allows generation of a pattern of trenches in the first masking layer which has period p .

In exemplary first embodiments, the first and second photoresists are positive photoresists. In those exemplary first embodiments, the method further comprises developing the first layer to form a first photoresist pattern, and developing the second layer to form a second photoresist pattern, such that the first pattern of trenches in the first masking layer is a transfer of the first photoresist pattern and the second pattern of trenches in the first masking layer is a transfer of a negative image of the second photoresist pattern.

In those first exemplary embodiments, a total area covered by the first photoresist pattern may be smaller than a total area covered by the second photoresist pattern, wherein a ratio of a size of the total area covered by the second photoresist pattern to a size of the total area covered by the first photoresist pattern may be at least 1.25, at least 5, 8, 10, 15, 20 or more.

As mentioned above, the process according to the present invention may comprise simultaneously or sequentially exposing the first and second layers of first/second photoresist.

Evidently, simultaneous exposure requires the first and second layers to be simultaneously present on the substrate whereas sequential exposure only requires one of the first and second layers per exposure.

In the following, an exemplary embodiment of the microlithographic process will be described which involves two separate exposure steps. In this exemplary single-mask-double exposure embodiment, exposing and developing the first layer precedes exposing and developing the second layer. The first and second photoresists are positive photoresists such that upon developing the first layer a first photoresist pattern is formed, and upon developing the second layer a second photoresist pattern is formed. Thus, the first pattern is formed by the first photoresist pattern and the second pattern is formed by a negative image of the second photoresist pattern.

Thus, the microlithographic process comprises in this embodiment :

(a) arranging a substrate in a predetermined position relative to a mask,

(b) exposing a first layer of a first photoresist on the substrate with radiation transmitted through the mask such that a total first area of the first layer is exposed to a first radiation dose which is at least equal to a first resist exposure threshold dose of the first photoresist,

(c) developing the first layer of the first photoresist to form a first photoresist pattern, (d) exposing a second layer of a second photoresist on the substrate with radiation transmitted through the mask such that a total second area of the second layer is exposed to a second radiation dose which is at least equal to a second resist exposure threshold dose,

(e) developing the second layer of the second photoresist to form a second photoresist pattern.

The exemplary single-mask-double exposure embodiment may further comprise one or more of the following:

depositing the first masking layer onto areas on the substrate which are not covered by the first photoresist pattern after development of the first layer, and removing the first photoresist pattern to form the first pattern of trenches in the first masking layer, thus transferring the first pattern to the first masking layer on the substrate;

filling the first pattern of trenches in the first masking layer with a second masking material, and coating the first masking layer and second masking material with the second layer of the second positive photoresist before exposing and developing the second layer to form the second photoresist pattern;

transferring the second pattern to the first masking layer on the substrate by using the second photoresist pattern as an etch mask to etch the first masking layer thus forming the second pattern of trenches in the first masking material; and

removing the second masking material and the second photoresist pattern.

Thus, in this embodiment, the first masking material is only deposited on the substrate after the first layer of the first photoresist was exposed and developed. In particular, the first masking material is patterned by the first photoresist pattern which serves as a mould for the first masking material, such that the first masking material has an inverse pattern as compared to the first photoresist pattern. The additional set of trenches to be formed in the first masking material may involve filling the set of trenches left behind after removal of the first photoresist pattern, depositing the second layer of the second photoresist on the filled first masking material, exposure and development to form the second photoresist pattern, and subsequent etching.

Exposing the first layer with radiation transmitted through the mask may comprise the transmitted radiation having a first average intensity, and exposing the second layer with radiation transmitted through the mask may comprise the transmitted radiation having a second average intensity, with a ratio of the first to the second average intensities being at least 1.25, for instance .

In an alternative example of the single-mask-double-exposure embodiment, at least one of the first and second photoresists is a negative photoresist. In order to achieve the same pattern of trenches in the first masking layer as in the embodiment described above, additional process steps would be required. For instance, if the first photoresist was a negative photoresist, the following changes to the above described exemplary embodiment would be necessary:

Upon developing the first layer, a first photoresist pattern would be formed, and the first pattern would be formed/defined by a negative image of the first photoresist pattern. A further process step would comprise filling trenches in the first photoresist pattern with a filling material which is thus given the pattern of a negative image of the first photoresist pattern. The first photoresist pattern would then be removed leaving behind the patterned filling material. A pattern of the filling material would thus correspond and take a function of the first photoresist pattern in the above described embodiment. Subsequently, the first masking layer could be deposited. Similar considerations apply in cases where the second photoresist is a negative photoresist. In the following, an exemplary embodiment of a lithographic process will be described which involves only a single exposure step. In this exemplary single-mask-single exposure embodiment, exposing the first and second layers comprises exposing the first and second layers simultaneously with radiation transmitted through the mask. In this embodiment, the second layer is sandwiched between the first layer and the first masking material and the first masking layer is sandwiched between the second layer and the substrate. Due to this sandwiching, the second layer receives less radiation than the first layer. The first and second photoresists are positive photoresists, such that upon developing the first layer, a first photoresist pattern is formed. Likewise, upon developing the second layer, a second photoresist pattern is formed. As a result, the first pattern is formed/defined by the first photoresist pattern and the second pattern is formed/defined by a negative image of the second photoresist pattern. In other words, the first pattern of trenches in the first masking layer is formed by transfer of the first photoresist pattern to the masking layer and the second pattern of trenches in the first masking layer is generated by transfer of a negative image of the second photoresist pattern to the first masking layer.

This embodiment has an additional advantage in that only one exposure step is required, which allows higher throughput thus saving time and expense. Since there is no requirement to rearrange the substrate in a same position relative to the mask, problems arising from misalignment and alignment tolerances that may lead to undesired deterioration of a quality of a resulting feature pattern can be avoided.

The exemplary single-mask-single exposure embodiment may further comprise one or more of the following:

developing the first layer and developing the second layer such that the first photoresist pattern is disposed on the second photoresist pattern; transferring the second pattern to the first masking layer on the substrate by using the second photoresist pattern as an etch mask to etch the first masking layer thus forming the second pattern of trenches in the first masking material;

transferring the first photoresist pattern into the second photoresist pattern to form a secondary second photoresist pattern;

transferring the first photoresist pattern into the second photoresist pattern in a same process step as etching the first masking layer using the second photoresist pattern as the etch mask;

depositing a second masking layer in the second pattern of trenches in the first masking material and in areas where second photoresist was removed, removing the secondary second photoresist pattern to form a second masking layer pattern which is a negative image of the first photoresist pattern, and etching the first masking layer using the second masking layer pattern as an etch mask thus forming the first pattern of trenches in the first masking material; and

removing remaining second masking layer.

In a further exemplary embodiment of a single-mask-single- exposure process, the substrate is coated with a stack of layers which comprise, in an order of increasing distance from the substrate: the first masking material, a second interim layer, the second layer of second positive photoresist, a first interim layer and the first layer of first positive photoresist. Exposing the first and second layers comprises exposing the first and second layers simultaneously with radiation transmitted through the mask. Like in the previous described example, upon developing the first layer, a first photoresist pattern is formed, and upon developing the second layer, a second photoresist pattern is formed. Thus, the first pattern is formed by the first photoresist pattern and the second pattern is formed by a negative image of the second photoresist pattern. In this alternative example, the process may further comprise one or more of the following in the following or any other suitable sequence:

developing the first layer including removing areas of the first interim layer to form a first interim layer pattern which is substantially identical to the first photoresist pattern and separates the first layer from the second layer;

developing the second layer including removing areas of the second interim layer to form a second interim layer pattern which is substantially identical to the second photoresist pattern and separates the second layer from the first masking material ;

transferring the negative image of the second photoresist pattern to the first masking layer on the substrate by using the second photoresist pattern as an etch mask to etch the first masking layer thus forming the second pattern of trenches in the first masking material;

transferring the first photoresist pattern into the second photoresist pattern to form a secondary second photoresist pattern, wherein the first and second interim layer patterns serve as etch stops;

depositing a second masking layer in the second pattern of trenches in the first masking layer and in areas where second photoresist, first and second interim layers were removed;

removing remaining first interim layer, the secondary second photoresist pattern and the second interim layer where the secondary photoresist pattern was removed to form a second masking layer pattern which is a negative image of the first photoresist pattern, and etching the first masking layer using the second masking layer pattern as an etch mask thus forming the first pattern of trenches in the first masking material; and removing remaining second masking material and remaining second interim layer.

Thus, the present invention further provides a microlithographic method, comprising: arranging a substrate coated with a first layer of a first positive photoresist in a predetermined position relative to a mask, exposing the first layer with radiation transmitted through the mask and developing the first layer to form a first photoresist pattern, depositing a first masking layer to cover areas on the substrate which are not covered by the first photoresist pattern to form a first pattern of trenches in the first masking layer, removing the first photoresist pattern, filling the first pattern of trenches in the first masking layer with a second masking material, coating the first masking layer and second masking material with a second layer of a second positive photoresist, arranging the substrate in the predetermined position relative to the mask, exposing the second layer with radiation transmitted through the mask and developing the second layer to form a second photoresist pattern, etching the first masking layer using the second photoresist layer as an etch mask to form a second pattern of trenches in the first masking material, the second pattern of trenches being a negative image of the second photoresist pattern, and removing the second photoresist pattern and the second masking material .

The present invention further provides a microlithographic method, comprising: arranging a substrate in a predetermined position relative to a mask, the substrate being coated with a stack of layers, the stack comprising in an order of increasing distance from a substrate surface: a first masking layer, a second interim layer, a second layer of a second positive photoresist, a first interim layer and a first layer of a first positive photoresist, exposing the first and second layers with radiation transmitted through the mask, developing the first layer to form a first photoresist pattern and removing areas of the first interim layer to form a first interim layer pattern which is substantially identical to the first photoresist pattern and separates the first from the second layer, developing the second layer to form a second photoresist pattern and removing areas of the second interim layer to form a second interim layer pattern which is substantially identical to the second photoresist pattern and separates the second layer from the first masking layer, etching the first masking layer using the second photoresist pattern as an etch mask to transfer a negative image of the second photoresist pattern to the first masking layer on the substrate thus forming a second pattern of trenches in the first masking material, transferring the first photoresist pattern into the second photoresist pattern to form a secondary second photoresist pattern, filling the second pattern of trenches in the first masking layer and areas where second photoresist, first and second interim layers were removed with a second masking material, removing remaining first interim layer, secondary second photoresist pattern and second interim layer where the secondary photoresist pattern was removed to form a second masking layer pattern which is a negative image of the first photoresist pattern, etching the first masking layer using the second masking layer pattern as an etch mask thus forming the first pattern of trenches in the first masking material , and removing remaining second masking material and remaining second interim layer.

Use of the first interim layer allows controlling transmission of radiation into the second layer of second photoresist. In those embodiments wherein photoresists are used which release one or more certain chemicals upon exposure, diffusion of those one or more chemicals can also be controlled by the first interim layer. In conventional photoresists, those one or more chemicals typically comprise an acid.

The first and second interim layers may be made of a same or different materials. For instance, at least one the first and second interim layers may be comprised of bottom anti-reflective coating (BARC) .

The first masking material may be an inorganic hardmask material, such as SiO₂, silicon nitride (Si₃N₄, Si_xN_y) , silicon oxy nitride (SiO_xN_y) . The second masking material may be an organic hardmask material, for instance.

The first layer of first photoresist may have a substantially same thickness as the second layer of second photoresist. In addition or alternatively, the first photoresist may have a same or a different resist exposure threshold dose as the second photoresist. The first photoresist may be made from the same material as the second photoresist.

One or more resolution enhancements techniques known in the art may be applied in the process according to the present invention, such as use of off-axis illumination, OPC, immersion lithography, polarised illumination as well as use of attenuating/attenuated phase shift masks.

Suitable masks for use in the present invention comprise binary structure masks, attenuating phase shift masks and high transmission attenuating phase shift masks.

Examples of wavelengths of radiation which are suitable for use in the present invention are wavelengths below 250 nm, in particular the wavelengths of 248 nm or 193 nm typically used in lithographic methods. The present invention is also suited for use with wavelengths in the EUV-region, in particular 13 nm.

The present invention may also make use of an immersion system, i.e. a lithography system wherein a liquid medium is interposed between the optics of the lithography system and the substrate. The liquid medium has a refractive index greater than one. In a lithography system using the 193 nm wavelength produced by ArF excimer lasers, the liquid medium typically used is ultra-pure water.

Brief Description of the Drawings

The foregoing as well as other advantageous features of the present invention should become more apparent from the following detailed description of two exemplary embodiments of the invention with reference to the accompanying schematic drawings. It is noted that not all possible embodiments of the present invention necessarily exhibit each and every, or any, of the advantages identified herein.

Figure 1 illustrates a lithographic process known in the art;

Figure 2 illustrates a spatial intensity distribution of radiation generated by a mask, in a plane of the mask and in an image plane;

Figure 3 illustrates dose contours in a layer of photoresist;

Figure 4 illustrates a concept underlying embodiments of the present invention;

Figure 5 illustrates process steps in a single-mask-double- exposure embodiment;

Figure 6 illustrates an arrangement of layers on a substrate for use in single-mask-single-exposure embodiment;

Figure 7 illustrates process steps in a single-mask-single- exposure embodiment ;

Figure 8 illustrates a dependency of a line to space ratio on a ratio of first to second radiation doses; and

Figure 9 illustrates a dependency of a line to space ratio on a ratio of an area width exposed in the first layer to an area width exposed in the second layer.

Detailed Description of Depicted Embodiments

In the exemplary embodiments described below, components that are alike in function and structure are designated as far as possible by alike reference numerals. Therefore, to understand the features of the individual components of a specific embodiment , the descriptions of other embodiments and of the summary of the invention should be referred to.

An first exemplary embodiment of a single-mask-double-exposure process in which the first layer is exposed and developed before the second layer is exposed and developed is explained with reference to Figure 5. For ease of illustration, exposure processes are illustrated as non-reducing imaging processes, i.e. without reduction optics.

A silicon wafer is provided as a substrate 10. A first layer 21 of a first positive photoresist is coated on the substrate 10. In this exemplary embodiment, a thickness of the first layer is about 100 ran. The first layer 21 of positive photoresist has a first resist exposure threshold dose, i.e. an area exposed to the first resist exposure threshold dose can be selectively removed from areas exposed to less than the resist exposure threshold dose in a subsequent development process. For a first and a second exposure in the process, a same mask 100 is used which comprises a pattern of opaque pattern features 2 and transparent pattern features 1 which form a grid of opaque and transparent lines having a pitch 2p. The pitch indicates a distance of a center of one opaque line 1 to a center of an adjacent opaque line 1. The mask 100 employed in this embodiment is an attenuating phase shift mask having a pitch of about 260 nm. A wavelength of radiation used in the first and second exposures is 193nm in this embodiment. The projection optics of the lithography system used for exposure has a numerical aperture of 0.93.

For a first exposure of the substrate 10 coated with the first layer 21 of first positive photoresist, the substrate 10 is arranged in a predetermined position relative to the mask 10 in a lithography system. Then, the first layer 21 on the substrate 10 is exposed to radiation transmitted through the mask 100. As a result, areas 21e of the first layer 21 have been exposed to a

(first) radiation dose above the first resist exposure threshold dose. In addition, a first pattern of areas 21u is generated which areas 21u have been exposed to less than the first resist exposure threshold dose, as shown in Figure 5a. The exposure conditions and photoresist properties are chosen such that radiation transmitted through a transparent line 2 having an area Ai exposes an area A2 of the first layer 21 with a radiation dose of at least the first resist exposure threshold dose. Figure 5a further shows that an area A2 is larger than the area Al of transparent line 2 (assuming no reduction) . This means that a ratio of total areas 21u to total areas 21e is different from a ratio of total areas of opaque lines 1 to total areas of transparent lines 2 in the mask. This is attributed to exposure conditions having been set to achieve an overexposure of the first layer 21. The first layer 21 now comprises a first pattern of areas 21u which have received a radiation dose below the first resist exposure threshold dose.

The exposed substrate 10 is then removed from the lithography system for further processing. In a second process step, areas 21e of the first layer 21 having been exposed to at least the first resist exposure threshold dose are removed in a photoresist development step. With those areas 21e of the first layer 21 removed, a first photoresist pattern 21u is formed, as illustrated in Figure 5b.

In a third process step, a first masking material is deposited on the substrate 10 such that void areas of the first photoresist pattern 21u are filled, thus forming a first masking layer 11. An amount of first masking material deposited is adjusted such that a surface of the first masking layer is about level with a surface of the first photoresist pattern 21u. The first masking material used is a hardmask material.

In a fourth process step, the first photoresist pattern 21u is removed by a stripping process thus uncovering the first masking layer 11 which has a first pattern of trenches 11a, lib formed therein (Figure 5d) . The first pattern of trenches 11a, lib corresponds to the first pattern of areas 21u of the first layer 21 which have been exposed to less than the first resist exposure threshold dose and the first photoresist pattern 2Iu. It is possible that dimensions of the first pattern of trenches 11a, lib differ somewhat from dimensions of the first pattern of sub-threshold-exposed areas 21u of the first layer 21 as a result of the processing of the photoresist.

In a fifth process step, the trenches 11a, lib in the first masking layer are filled with a second masking material 31 such that a surface of the second masking material 31 is about level with the surface of the first masking layer 11, as shown in Figure 5e. The second masking material is an organic hardmask material. In addition, a second layer 41 of a second positive photoresist is coated onto the first masking layer 11 and the second masking material 31 by a spin-coating process. The second photoresist used is made of the same material as the first photoresist, and a thickness of second layer 41 is about the same as the thickness of the first layer 21. Under those conditions, the second resist exposure threshold dose is about equal or at least very similar to the first resist exposure threshold dose.

The substrate 10 is then reintroduced into the lithography system and rearranged in the predetermined position relative to the mask 100. In a sixth process step, the substrate 10 is exposed for the second time. The exposure conditions for the second exposure are set such that the second layer 41 on the substrate 10 is exposed to radiation transmitted through the mask 100 such that areas 41u of the first layer 41 are exposed with a radiation dose less than the second resist exposure threshold dose. In addition, a second pattern of areas 41e is generated which has been exposed with at least the second resist exposure threshold dose, as shown in Figure 5f. In the second exposure, the transparent line 2 of area Al in the mask 100 exposes an area A3 of the second layer 41 with a radiation dose of at least the second resist exposure threshold dose. Figure 5f further shows that the area A3 is smaller than the area Al of transparent line 2 (assuming no reduction) . Area A3 is also smaller than the corresponding area A2 of the previously exposed first layer 21. Thus, a smaller percentage of the second layer 41 has been exposed with at least the second resist exposure threshold dose as compared to a percentage of the first layer 21 exposed to at least the first resist exposure threshold dose. The second layer 41 now comprises a second pattern of areas 24e which have received a radiation dose equal to or above the second resist exposure threshold dose.

The substrate is again removed from the lithography system and the second layer 41 developed in a same manner as described above in connection with the first layer 21. Upon developing, a second photoresist pattern 41u is formed, which is a negative image of the second pattern. An area of the second photoresist pattern 4Iu is relatively larger than an area of the first photoresist pattern 21u. In a further process step, the second photoresist pattern 41u is used as an etch mask in an etch step for transferring a negative image of the second photoresist pattern 41u into the first masking layer 11, as illustrated in Figure 5g. Removing the second photoresist pattern 4Iu and the second masking material 31 uncovers a first masking layer 11 having an additional second pattern of trenches lie, Hd. Thus, a grid of trenches lla-d having pitch p is formed in the first masking layer 11.

Figure 6 illustrates an arrangement of layers on a substrate 110 for use in an second embodiment, which involves a single-mask- single-exposure process. The arrangement 110 comprises a stack of layers on a substrate 10, comprised in increasing distance from the substrate 10 by: a first masking layer 11, a second interim layer 50, a second photoresist layer 21, a first interim layer 50 made of the same material as the first interim layer 50, and a first layer 41 of first photoresist. The first photoresist and the second photoresist, the first masking layer 11 and the substrate 10 comprise the same materials as in the first exemplary embodiment of the "single-mask-double-exposure process described above with reference to Figure 5.

Figure 7 illustrates process steps of a single-mask- single- exposure process according to a second embodiment .

The substrate 10 is introduced into a lithography system for a single exposure using the same mask 100 and exposure wavelength as in the first embodiment. The substrate 10 is arranged in a predetermined position relative to the mask 100. Radiation transmitted through the mask 10 in the exposure step exposes both the first and second layers 21, 41 in one exposure step. Since the second layer 41 receives a smaller total radiation dose than the first layer 21 due to the first layer 31 and the first interim layer 60 absorbing and/or scattering and/or reflecting radiation, a total size of areas 41e in the second layer 41 which have received an exposure dose of at least the second resist exposure threshold dose is smaller than a total size of areas 21e in the first layer 21 which have received at least the first resist exposure threshold dose, as illustrated in Figure 7a. In correspondence to the above described first single-mask-double-exposure embodiment, a first pattern is defined by areas 21u of the first layer 21 having received less than the first resist exposure threshold dose, and a second pattern is defined by areas 4Ie in the second layer 41 which have received a radiation dose at least equal to the second resist exposure threshold dose.

The substrate is then removed from the lithography system and the first and second layers 21, 41 are developed. Developing also involves removal of portions of first and second interim layer 50, 60. The development step results in formation of a first photoresist pattern 21u, a first interim layer pattern which is substantially the same as the first photoresist pattern, a second photoresist pattern, and a second interim layer pattern which is substantially the same as the second photoresist pattern, as shown in Figure 7b.

In a subsequent process step, the first masking layer 11 is etched using the second photoresist pattern 41u as an etch mask. Thus, a negative image of the second photoresist pattern 4Iu is transferred into the first masking layer 11 to form a second pattern of trenches lie, Hd therein. The etching process also removes the first photoresist pattern 21u. The first and second interim layer patterns 50, 60 serve as etch stops in the etching process. As a result, the first interim layer pattern 60, which corresponds to the first photoresist pattern 21u, is transferred into the second photoresist pattern 41u. Thus, a secondary photoresist pattern 41u is formed, which corresponds to the removed first photoresist pattern 21u, as illustrated in Figure 7c.

In a subsequent process step, areas left void by the first masking layer 11, the first and second interim layers 50, 60 and the secondary photoresist pattern 41u are filled with a second masking material to form a second masking layer 51 (Figure 7d) . The second masking material is the same material as used in the previous described embodiment.

In a further process step, remains of the first interim layer 60, the secondary photoresist pattern 41u and those areas of the second interim layer 50 which are not covered by the second masking layer 51 are removed. A pattern thus formed in the second masking layer 51 corresponds to the first pattern and the removed first photoresist pattern 21u (Figure 7e) .

In a further process step, the first masking layer 11 is etched using the second masking material 51, inclusive of the second interim layer 50, as an etch mask. Thus, the first pattern is transferred into the first masking layer 11 to form a corresponding first pattern of trenches 11a, lib (Figure 7f ) .

In an additional process step, the second masking layer 51 and remaining second interim layer 50 are removed, thus uncovering the first masking layer 11 having a first and a second pattern of trenches 11a - d (Figure 7g) .

In Figure 8, a ratio of the first radiation dose to the second radiation dose (y-axis) that is desirable for achieving a predetermined line to space ratio (x-axis) for first and second layers of the same thickness and material is shown in the depicted graph. As evident from the graph, for achieving a line to space ratio of 1, as it would be realised in a regular pattern of equidistant lines having the same widths as the spaces separating the adjacent lines, for instance, a ratio of first versus second radiation dose should preferably be about 6 in the embodiment forming the basis for the depicted graph. For realisation of a much smaller line to space ratio, a higher value of dose ratio may be desirable, for instance a ratio of about 100 for a line to space ratio of below 0.2. These figures are merely illustrative and may be different for other combinations of layers, materials, exposure conditions etc.

In Figure 9, a ratio of the first total area to the second total area is illustrated by indicating an area width of a feature that is threshold-exposed in the first layer and the corresponding area width of the same feature that is threshold - exposed in the second layer, in dependence of a desired line to space ratio. As apparent from the depicted graph, for realising a desired line to space ratio of 1, a ratio of area widths of about 2.5 would be suitable in the embodiment forming the basis for the depicted graph. Again, the figures in the graph are merely illustrative and may be different for other combinations of layers, materials, exposure conditions etc.

While the invention has been described also with respect to certain specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention set forth herein are intended to be illustrative and not limiting in any way. Various changes may be made without departing from the spirit and scope of the present invention as defined in the following claims.

Claims

What is claimed is:

1. A microlithographic method, comprising:

arranging a substrate in a predetermined position relative to a mask,

exposing a first layer of a first photoresist on the substrate with radiation transmitted through the mask such that a total first area of the first layer is exposed to a first radiation dose which is at least equal to a first resist exposure threshold dose of the first photoresist,

exposing a second layer of a second photoresist on the substrate with radiation transmitted through the mask such that a total second area of the second layer is exposed to a second radiation dose which is at least equal to a second resist exposure threshold dose of the second photoresist,

wherein the second layer is different from the first layer, and

wherein the total first area has a size which is at least 1.25 times a size of the second total area.

2. The microlithographic method according to claim 1, wherein a ratio of the first radiation dose to the second radiation dose is at least 1.1.

3. The microlithographic method according to claim 1 or 2, further comprising developing the first and second layers.

4. The microlithographic method according to one of the preceding claims, wherein areas of the first layer exposed to less than the first radiation dose define a first pattern, and areas of the second layer exposed to the second radiation dose define a second pattern.

5. The microlithographic method according to claim 4, further comprising:

transferring the first pattern to a first masking layer on the substrate such that a first pattern of trenches is formed in the first masking layer and transferring the second pattern to the first masking layer on the substrate such that a second pattern of trenches is formed in the first masking layer.

6. A microlithographic exposure lithographic method, comprising:

arranging a substrate in a predetermined position relative to a mask,

exposing a first layer of a first photoresist on the substrate with radiation transmitted through the mask, with the substrate being arranged in the predetermined position relative to the mask, such that a first pattern of areas exposed to a radiation dose below a first resist exposure threshold dose is generated in the first layer,

exposing a second layer of a second photoresist on the substrate with radiation transmitted through the mask, with the substrate being arranged in the predetermined position relative to the mask, such that a second pattern of areas exposed to a radiation dose equal to or above a second resist exposure threshold dose is generated in the second layer,

the first layer being different from the second layer,

and transferring the first pattern to a first masking layer on the substrate to form a first pattern of trenches in the first masking layer and transferring the second pattern to the first masking layer on the substrate to form a second pattern of trenches in the first masking layer.

7. The microlithographic method according to claim 6, wherein

exposing the first layer comprises exposing the first layer such that radiation transmitted through a transparent mask feature having an area of a first size Al exposes an area of a second size A2 in the first layer with a radiation dose equal to or above the first resist exposure threshold dose,

wherein exposing the second layer comprises exposing the second layer such that radiation transmitted through the transparent mask feature having the area of the first size

Al exposes an area of a third size A3 in the second layer with a radiation dose equal to or above the second resist exposure threshold dose,

and wherein a ratio A2/A3 of the second size A2 to the third size A3 is at least 1.25.

8. The microlithographic method according to one of claims 4 to 7, wherein the first photoresist is a positive photoresist, wherein the second photoresist is a positive photoresist, and wherein the method further comprises: developing the first layer to form a first photoresist pattern and developing the second layer to form a second photoresist pattern, wherein the first pattern is defined by the first photoresist pattern and wherein the second pattern is defined by a negative image of the second photoresist pattern.

9. The microlithographic method according to one of claims 4 to 8, wherein exposing and developing the first layer of the first photoresist precedes exposing and developing the second layer of the second photoresist.

10. The microlithographic method according to claim 9, wherein transferring the first pattern to the first masking layer on the substrate comprises depositing the first masking layer onto areas on the substrate which are not covered by the first photoresist pattern after development of the first layer, and removing the first photoresist pattern to form the first pattern of trenches in the first masking layer.

11. The microlithographic method according to claim 10, further comprising filling the first pattern of trenches in the first masking layer after removal of the first photoresist pattern with a second masking material, and coating the first masking layer and second masking material with the second layer of the second positive photoresist before exposing and developing the second layer thus forming the second photoresist pattern.

12. The microlithographic method according to claim 11, wherein transferring the second pattern to the first masking layer on the substrate comprises using the second photoresist pattern as an etch mask to etch the deposited first masking layer to form the second pattern of trenches in the first masking material .

13. The microlithographic method according to claim 12, further comprising removing the second masking material and the second photoresist pattern.

14. The microlithographic method according to one of the preceding claims, wherein exposing the first layer with radiation transmitted through the mask comprises the transmitted radiation having a first average intensity, wherein exposing the second layer with radiation transmitted through the mask comprises the transmitted radiation having a second average intensity, a ratio of the first to the second average intensities being at least 6.

15. The microlithographic method according to one of claims 4 to 13, wherein exposing the first and second layers comprises exposing the first and second layers in a single exposure with radiation transmitted through the mask, wherein the second layer is sandwiched between the first layer and the first masking material and the first masking layer is sandwiched between the second layer and the substrate.

16. The microlithographic method according to claim 15, further comprising developing the first layer to form a first photoresist pattern and developing the second layer to form a second photoresist pattern such that a first photoresist pattern is disposed on a second photoresist pattern.

17. The microlithographic method according to claim 16, wherein transferring the second pattern to the first masking layer on the substrate comprises using the second photoresist pattern as an etch mask to etch the first masking layer to form the second pattern of trenches in the first masking material .

18. The microlithographic method according to claim 17, further comprising transferring the first photoresist pattern into the second photoresist pattern to form a secondary second photoresist pattern.

19. The microlithographic method according to claim 18, wherein transferring the first photoresist pattern into the second photoresist pattern is accomplished in a same process step as etching the first masking layer using the second photoresist pattern as the etch mask.

20. The microlithographic method according to one of claims 18 and 19, further comprising depositing a second masking layer in the second pattern of trenches in the first masking material and in areas where second photoresist was removed, removing the secondary second photoresist pattern to form a second masking layer pattern which is a negative image of the first photoresist pattern, and etching the first masking layer using the second masking layer pattern as an etch mask to form the first pattern of trenches in the first masking material .

21. The microlithographic method according to claim 20, further comprising removing remaining second masking layer.

22. The microlithographic method according to claim 8, wherein exposing the first and second layers comprises exposing the first and second layers in a single exposure with radiation transmitted through the mask, wherein the substrate is coated with a stack of layers which comprise in an order of increasing distance from the substrate: the first masking material, a second interim layer, the second layer, a first interim layer and the first layer.

23. The microlithographic method according to claim 22, wherein developing the first layer also comprises removing areas of the first interim layer to form a first interim layer pattern which is substantially identical to the first photoresist pattern and separates the first from the second layer, developing the first layer being followed by developing the second layer which also includes removing areas of the second interim layer to form a second interim layer pattern which is substantially identical to the second photoresist pattern and separates the second layer from the first masking material.

24. The microlithographic method according to claim 23, wherein transferring the negative image of the second photoresist pattern to the first masking layer on the substrate comprises using the second photoresist pattern as an etch mask to etch the first masking layer to thereby form the second pattern of trenches in the first masking material.

25. The microlithographic method according to claim 24, further comprising transferring the first photoresist pattern into the second photoresist pattern to form a secondary second photoresist pattern, wherein the first and second interim layer patterns serve as etch stops .

26. The microlithographic method according to claim 25, further comprising depositing a second masking layer in the second pattern of trenches in the first masking layer and in areas where second photoresist, first and second interim layers were removed, followed by removing remaining first interim layer, the secondary second photoresist pattern and the second interim layer where the secondary photoresist pattern was removed to form a second masking layer pattern which is a negative image of the first photoresist pattern, and etching the first masking layer using the second masking layer pattern as an etch mask thus forming the first pattern of trenches in the first masking material.

27. The microlithographic method according to claim 24, further comprising removing remaining second masking material and remaining second interim layer.