Method for transferring a predetermined pattern reducing proximity effects

A method for transferring a predetermined pattern onto a flat support performed by direct writing by means of a particle beam comprises at least: deposition of a photoresist layer on a free surface of the support, application of the beam on exposed areas of the photoresist layer, performing correction by modulation of exposure doses received by each exposed area, developing of the photoresist layer so as to form said pattern. Correction further comprises determination of a substitution pattern (11) comprising at least one subresolution feature and use of the substitution pattern (11) for determining the areas to be exposed when the electron beam is applied. In addition, modulation takes account of the density of the substitution pattern (11) near to each exposed area.

BACKGROUND OF THE INVENTION

The invention relates to a method for transferring a predetermined pattern onto a flat support by direct writing by means of a particle or photon beam, and comprising at least:deposition of a photoresist layer on a free surface of the support,application of the beam on exposed areas of the photoresist layer,performing correction by modulation of exposure doses received by each exposed area,developing of the photoresist layer so as to form said pattern.

STATE OF THE ART

Electron Beam Lithography (EBL) is a lithography technique for direct writing on a flat support, i.e. a writing technique not requiring a mask. The flat support1is more often than not an optical lithography mask (or reticle) or a semi-conducting substrate, for example made of silicon or silicon on insulator (SOI).

Thus, as represented inFIG. 1, transferring a pattern by E-beam lithography onto a flat support1is generally performed by means of a photoresist layer2arranged on a free surface of said support1. An electron beam emitter3then produces an electron beam4(also noted E-beam4) designed to be applied to predetermined areas of the photoresist layer2. Said predetermined areas are also called the exposed areas of the photoresist layer2and they are determined by the pattern to be transferred5. Thus, as represented by the block6inFIG. 1, the electron beam emitter3is controlled such that application of the electron beam4onto the photoresist layer2corresponds either to the pattern to be transferred5or to the complementary part thereof. For example, a pattern to be transferred5is represented inFIG. 3and the hatched parts inFIGS. 4 and 7represent the exposed areas of the photoresist layer2after application of the electron beam4to obtain the pattern to be transferred5in the support1. InFIG. 4, the hatched parts correspond to the elements of the pattern to be transferred5whereas inFIG. 7the hatched parts correspond to the complementary part of said pattern to be transferred5.

Once exposure has been completed, the photoresist layer2is developed so as to free parts of the surface of the support1. If the photoresist is a positive photoresist, the exposed areas of the photoresist layer are then eliminated in the developing step whereas, for a negative photoresist, the eliminated parts correspond to the non-exposed areas of the photoresist. The free parts of the flat support1, i.e. the parts not covered by the photoresist after the developing step, are then etched so as to obtain a pattern corresponding to the pattern to be transferred5in the flat support.

E-beam lithography presents the advantage of being inexpensive and of obtaining a pattern presenting small dimensions, for example less than 10 nm. It is today extensively used in research laboratories, in particular for producing magnetic heads or, in the semi-conductor field, for producing the masks to be used in an optical lithography process.

This technique is however hardly used for producing integrated circuits directly on a semi-conducting wafer. It does in fact present a slow writing speed compared with the writing speed of an optical lithography.

Moreover, in this field of application and in a more general manner, the size of the pattern produced must be perfectly well controlled. Dimensional control of a pattern does however require very precise control of the proximity phenomena liable to occur when the electron beam is applied to the photoresist layer.

The proximity effects in an electron beam lithography process can at present be reduced by modulating the exposure dose received by the photoresist layer according to the writing density, i.e. according to the density of the pattern to be produced, near the area to be exposed.

In the article “Performance Improvement in E-beam Reticle Writer HL-800M”, Hidetoshi Satoh et al. (SPIE Vol. 3096, pages 72-83) propose using proximity effect correction (PEC) hardware to obtain a pattern presenting sufficiently linear edges. Correction is performed directly during application of the electron beam on the photoresist layer. The exposed area of the photoresist layer is thus broken down into a grid of predetermined mesh. The writing density in each mesh is then calculated to then determine the value of the exposure dose to be applied for a given mesh during application of the electron beam.

As indicated by Laurent Pain et al. in the article “65 nm Device Manufacture Using Shaped E-Beam Lithography” (Japanese Journal of Applied Physics, Vol 43. No 6B, 2004), proximity effect correction can also be performed, on the same principle, by a proximity effect correction software such as the PROXECCO® software marketed by PDF solution.

Thus, as illustrated inFIG. 2, control6of the electron beam emitter3is more particularly achieved by previously entering the pattern to be transferred5into a control device and then performing correction by modulating the exposure dose according to the density of the pattern to be transferred5, for example by means of the PROXECCO® software. The electron beam4is then applied to the photoresist layer2so as to expose predetermined areas of the photoresist layer before the latter is developed and the support1is etched.

The U.S. Pat. No. 6,649,452 proposes means for compensating the proximity effects liable to occur when a reticle, designed to transfer an integrated circuit pattern onto a semi-conductor wafer by optical lithography, is produced by E-beam lithography. These means consist in particular in adding additional features (subresolution design features) to the pattern to be transferred, to determine the areas of the photoresist layer to be exposed. These additional features are not resolved on the reticle and they can be of any type of shape and in any type of arrangement. However, the integrated circuit patterns produced on reticles according to the teachings of the U.S. Pat. No. 6,649,452 present dimensions four times larger than those of the final pattern transferred onto the semi-conducting wafer.

To produce a pattern presenting a size smaller than 70 nm on masks and integrated circuits, proximity effect correction does however remain difficult to achieve with these tools. Indeed, it does not enable a good dimensional control of said pattern to be obtained, i.e. within ±10% of the required size.

OBJECT OF THE INVENTION

The object of the invention is to provide a method for transferring a predetermined pattern onto a flat support, by E-beam lithography, remedying the shortcomings of the prior art. More particularly, it is the object of the invention to reduce the proximity effects in simple and efficient manner for any type of pattern to be transferred.

According to the invention, this object is achieved by the appended claims.

DESCRIPTION OF PARTICULAR EMBODIMENTS

Generally speaking, the proximity effects observed are caused by a high backscattering electron ratio at the interface between the support1and the photoresist layer2. It has however been observed that, whatever the type of photoresist used, whether it be negative or positive, the backscattering electron ratio depended essentially on the surface exposed. Thus, the larger the surface of the exposed area, the higher the backscattering electron ratio and therefore the greater the noise, which gives rise to detrimental proximity effects, poor pattern resolution and large dimensional variations.

For example and as illustrated inFIGS. 4 to 6, a pattern to be transferred5such as the one represented inFIG. 3is transferred according to the prior art onto a negative or positive photoresist layer2. As represented inFIG. 4, exposure is performed in such a way that the exposed areas of the layer2correspond to said pattern5. InFIG. 4, four exposed areas7are situated in a dense area, i.e. an area comprising a high density of elements to be transferred, and two exposed areas8are situated in an isolated area, i.e. comprising a low density of elements to be transferred. In a more general manner, it can be considered that the exposed areas7and8correspond respectively to dense portions9and to isolated portions10of the pattern to be transferred5. Each dense portion9comprises a longitudinal axis S1and each isolated portion10comprises a longitudinal axis S2.

The proximity effects can be evaluated by means of two curves α and β. Curve α represents the Gaussian distribution of the incident electron beam (forward electrons) produced by the emitter3and exposing the photoresist layer2, centred on the axis of symmetry of the portion of the pattern to be transferred. Curve β represents the intensity of the ratio of backscattering electrons diffused by the support1. Curve α enables a threshold Cd, called the photoresist threshold, to be determined corresponding to the size required for the portion of the pattern once the latter has been transferred, and the gradient of the curve α, at the level of said threshold, gives indications on the contrast. The width and height of the curve β represent the noise which could impact the resolution of possible transferred neighboring features and therefore cause proximity effects, but also roughness of the photoresist edges after transfer. Thus, the width and height of curve β must be minimal, which can not be obtained in one shot exposure, as present-day proximity effect correction solutions propose to do. In addition, the maximum height D of the curve β and the difference d between the threshold Cdand said maximum can be very small, which indicates that the elements of the transferred pattern are poorly resolved or not resolved due to process window reduction and/or that they are rough, which is caused by the proximity effects.

The curves α and β have thus been represented inFIGS. 5 and 6respectively for a dense portion9and for an isolated portion10of the pattern to be transferred5, for the same threshold Cd, i.e. for transferred portions9and10having the same width L. It can thus be observed inFIGS. 5 and 6that the difference d9is smaller than the difference d10and that the maximum D9is greater than the maximum D10. Thus, in this case, the isolated portions10of the pattern to be transferred are better resolved on the support1than the dense portions9of said pattern and they will be subject to less proximity effects.

It can be shown that the reverse phenomenon occurs if the pattern represented inFIG. 3is transferred according to the prior art onto a positive photoresist layer2, performing an exposure as illustrated inFIG. 7, i.e. exposing areas corresponding to the complementary part of the pattern5. Proximity effects and poor resolution of the patterns will occur for the isolated portions10of said pattern5whereas the dense portions9will be protected. It can in fact be shown that, in this case, for the isolated portions10, the energy level β is in the same order of magnitude as the threshold Cd, corresponding to the energy necessary to obtain the required size L of said portion. The noise energy level is such that the isolated portions10are totally exposed, which prevents their resolution.

The backscattering electron ratio and therefore the proximity effects thereby depend strongly on the width of the exposed area and on the density of features to be transferred. Thus, the backscattering electron ratio is high if the width of the exposed surface is large and vice-versa.

The method for transferring according to the invention proposes to reduce the proximity effects in simple and efficient manner, whatever the shape of the pattern to be transferred5. Thus, as represented inFIG. 8, control6of the electron beam used to perform transfer of a predetermined pattern5onto the flat support1is achieved, according to the invention, by replacing entry of the pattern to be transferred5by determination and entry of a substitution pattern11comprising at least one subresolution feature (also called fine element).

In this way, instead of determining the areas to be exposed when the electron beam4is applied according to the pattern to be transferred5, as in the prior art, the method for transferring according to the invention uses a predetermined substitution pattern11comprising at least one subresolution feature.

Correction by modulating the exposure doses received by each exposed area thus takes account of the density of the substitution pattern11, in proximity to each exposed area. Correction by modulating exposure doses can for example be performed using the PROXECCO® software.

The electron beam4is then applied on the photoresist layer2so as to expose predetermined areas of said layer in accordance with the substitution pattern11. The photoresist layer2then undergoes a revelation step to free parts of the free surface of the support1.

Determination of the substitution pattern11and determination of the subresolution feature or features depends in particular on the outline of the pattern to be transferred and on the exposure mode and/or the type of photoresist used.

According to a first embodiment, at least one portion of the pattern to be transferred5is replaced, in the substitution pattern11, by a set of at least two subresolution features parallel to a longitudinal axis of said portion. Moreover, the exposed areas of the photoresist layer are formed by the substitution pattern11. This first embodiment is more particularly suited to the case where the density of the pattern to be transferred5is high at the level of the portion to be replaced by a set of at least two subresolution features.

As an example illustrated inFIG. 9, the substitution pattern11comprises four features12. The features12replace the dense portions9in the pattern to be transferred5represented inFIG. 3. Each feature12thus has a longitudinal axis that coincides with the longitudinal axis S1of a corresponding dense portion9. It is formed by three distinct, parallel and equidistant subresolution features13. In addition, inFIG. 9, one of the subresolution features13is superposed on the corresponding axis S1. The substitution pattern11comprises elements corresponding to the rest of the pattern to be transferred5, and in particular the exposed areas8corresponding to the isolated portions10. The subresolution features13of each set of features12present an identical length equal to the length of the associated dense portion9. Furthermore, the width of each set of features12is equal to that of said the associated portion, after transfer. In some cases, the width of each set of features12can also be equal to that of said the associated portion of the pattern to be transferred.

The number of subresolution features13forming a set12is not limited to three. It is determined according to the width of the portion to be replaced. Moreover, determination of the width of each subresolution feature13can be performed empirically. For example, the number of subresolution features13constituting a set12is determined beforehand, then, for a constant pitch p between different dense portions9of fixed width L′, the width L actually obtained on the support1is measured for different width values of the subresolution features13and distance values between two subresolution features13. The width of the subresolution features and the distance between two subresolution features are then determined as soon as the value of the width L coincides substantially with that of the fixed width L′.

For example, dense portions9having a width L′ of 60 nm and a pitch p between two dense portions9of 60 nm can each be replaced by a set12of three subresolution features13each having a width of 15 nm with a 7.5 nm spacing between two adjacent subresolution features13of said set12.

Electron beam exposure is performed following a grid the pitch of which grid corresponds to a minimum unit proper to each application device or emitter used. Thus, if the emitter used presents a grid pitch having a fineness of 1 nm, such as for example the exposure device marketed under the name of LEICA, SB 350 DW, the 7.5 nm spacing can be obtained by performing two successive exposure steps, with spacings of respectively 7 nm and 8 nm between the adjacent subresolution features13of the sets12, so as to obtain a mean spacing of 7.5 nm.

Dense portions9having a width L′ of 60 nm and a pitch p between two dense portions9of 120 nm can each be replaced by a set12of three subresolution features13each having a width of 16 nm, with a 6 nm spacing between two subresolution features13.

The first embodiment is particularly well suited for portions of patterns located in a dense environment and having a high backscattering electron ratio. The subresolution features are in fact of smaller sizes than the dense portion9and they present a low backscattering electron ratio compared with a dense portion9. Moreover the set formed by the subresolution features13provides the energy necessary for formation of the pattern, at the required size.

InFIG. 10, the curves α and β represented correspond to superposition of the respective curves α1, α2, α3, and β1, β2and β3of each subresolution feature13of a set12as represented inFIG. 9and designed to replace a dense portion9. However, the curve a of the set12is finer than the curve a of a dense portion9, as represented inFIG. 5. In addition, the gradient of the curve α of the set12, at the level of the threshold Cd, is steeper than that of the dense portion9, which indicates an improvement of the contrast of the transferred pattern. Furthermore, compared to the height and width of the curve α of a dense portion9(FIG. 5), the height and width of the curve β of the set12are very small, which indicates that the impact of the backscattering electrons has considerably decreased on account of the substitution pattern11. Thus, the sum of the curves α of each subresolution feature13of a set12reproduces the replaced dense portion9, while improving the lithographical performances. Correction by exposure dose modulation, performed in accordance with the substitution pattern11as represented inFIG. 9, is moreover facilitated due to the fact that the ruggedness range of the control device is increased.

According to a second embodiment, the substitution pattern11can be formed by the pattern to be transferred during a first electron beam application period, then by at least one subresolution feature during a second electron beam application period. The subresolution feature is parallel to a longitudinal axis of a portion of the pattern to be transferred and it presents a length equal to the length of said portion. The density of the pattern to be transferred is preferably high at the level of said portion and the exposed areas of the photoresist layer2are formed by the substitution pattern11.

More particularly and as represented inFIGS. 11 and 12, the substitution pattern11is formed by the pattern to be transferred5during the first application period of the electron beam4and by a plurality of sets12respectively formed by two subresolution features13during the second application period. Each set12corresponds to a dense portion9of the pattern to be transferred5. The two subresolution features13of each set12are then symmetrical with respect to the longitudinal axis S1of said corresponding portion9and the width of each set12is equal to that of said associated portion9, after transfer. More particularly, the width of each set12can also be equal to that of said associated portion9of the pattern to be transferred. As represented inFIGS. 13 and 14, the exposure dose applied during the first application period is preferably lower than the exposure dose applied during the second application period. In this case, particular care must be taken over control6of the electron beam. Indeed, the areas of the photoresist layer2, exposed during the first application period, have to be exposed again during the second application period, according to the set12associated with said portion9. Thus, the two exposures must be perfectly superposed at the level of the dense portions9, in order not to affect the final resolution of the pattern. It can also be observed inFIG. 15that the gradient of the curve α is much steeper than that of the curve α according toFIG. 5, which indicates an improvement of the contrast. Furthermore, the width and height of the curve β are smaller than those of the curve β inFIG. 5. The second embodiment of the substitution pattern therefore provides an improvement of the contrast and a reduction of the noise and therefore of the proximity effects compared with a transfer method not comprising proximity effect correction.

Determination of the width of the subresolution features is preferably obtained empirically, for example by successively varying the exposure dose, the size of the subresolution features and the size of the space between said subresolution features. For example, for a dense portion9with a width of 45 nm separated from another dense portion9by a pitch of 90 nm, the set12can be formed by two subresolution features13with a width of 15 nm separated from one another by 30 nm.

In an alternative embodiment represented byFIG. 16, the substitution pattern11, during the second application period, can be formed by a plurality of sets12each comprising a single subresolution feature13superposed on the longitudinal axis S1of said portion9. For example, for a dense portion9with a width of 45 nm separated from another dense portion9by a pitch of 90 nm, a subresolution feature13can be used with a width of 9 nm the longitudinal axis whereof is superposed on that of the corresponding portion9. For a portion9with a width of 60 nm separated from another portion9by a pitch of 120 nm, a subresolution feature13can be used with a width of 32 nm whereas for the same portion separated from another portion7by a pitch of 60 nm, the width of the subresolution feature can be 48 nm.

As forFIGS. 13 to 15,FIGS. 17 to 19enable it to be observed that the exposure dose applied during the first application period is lower than that applied during the second application period. The gradient of the curve α inFIG. 19is much steeper than that of the curve α inFIG. 5. Moreover, the width and height of the curve β inFIG. 19are smaller than those of the curve β inFIG. 5. Thus, the alternative to the second embodiment of the substitution pattern also provides an improvement of the contrast and a reduction of noise and therefore of the proximity effects.

In a third embodiment, the substitution pattern11is formed by the pattern to be transferred5, completed by subresolution features13arranged outside at least one portion of the pattern to be transferred. More particularly and as represented inFIG. 20, the exposed areas of the photoresist layer are formed by the areas complementary to the substitution pattern11. The portion completed by subresolution features13is an isolated portion10, i.e. the density of the pattern to be transferred is low at the level of said portion. More particularly, the isolated portions10are surrounded by large exposure surfaces14. Thus, the subresolution features13are arranged outside at least one isolated portion10, in parallel manner to a longitudinal axis S2of said portion10, in said exposure surfaces14. They are situated near to said portion10, for example at a distance less than 10 times the width of said portion.

The subresolution features13present a length smaller than or equal to the length of said portion. In addition, the number and/or width of each subresolution feature13and/or the distance between a subresolution feature and the corresponding portion and/or the width of said portion are determined according to the width of a complementary area of the pattern to be transferred adjacent to said portion. More particularly, these parameters are determined empirically. For a fixed value L′ of a portion10of the pattern to be transferred5, the result of the width L actually obtained on the support1as a function of the value W of the exposed surface can for example be noted. This makes it possible to determine the critical dimension from which the proximity effects begin for each value L′, i.e. the size W from which a decrease of the actual value L is observed with respect to the fixed value L′. The size of the subresolution features and the distance between each subresolution feature and the portion10is then determined empirically. For each portion impacted by proximity effects, subresolution features are added having a width varying from 5 nm to 20 nm and/or a distance between each subresolution feature and the portion10that also varies. The required width and distance are obtained when the actual width L is equal to or close to the required value L.

For example, to obtain a portion10with a width of 40 nm achieved in a positive photoresist, two subresolution features with a width of 40 nm can be used arranged on each side of a longitudinal portion with a width of 80 nm, respectively at a distance of 40 nm from said longitudinal portion. The portion10obtained then presents a width of 40 nm and the subresolution features are not resolved when the photoresist layer is developed.

Unlike the embodiments described in the U.S. Pat. No. 6,649,452, the subresolution features of the third embodiment of the substitution pattern11are arranged close to isolated portions10in a large exposure surface and they have a length smaller than or equal to that of the portion10. The arrangement of the subresolution features13enables the background noise to be locally reduced, whatever the size of the isolated portion, thus reducing the proximity effects and writing time while improving the contrast and resolution, in particular compared with the embodiment as represented inFIG. 7. In the U.S. Pat. No. 6,649,452, an isolated portion of the pattern to be transferred is arranged in a lattice of equidistant parallel lines. Certain lines are interrupted by the isolated portion so as to free a space around the whole periphery of the isolated portion. The presence of said lines enables the backscattering electron energy level to be homogenized.

According to the invention, it is possible to obtain a portion presenting a minimum size that can be less than 70 nm, whereas with the embodiments described in the U.S. Pat. No. 6,649,452 the portion obtained would have a minimum size of 130 to 140 nm.

The invention is not limited to the embodiments described above.

Thus, in the first and second embodiments, when the substitution pattern11comprises several subresolution features13, the latter can have different widths. The set formed by the set of subresolution features13replacing a predetermined portion of the pattern to be transferred (1stembodiment) or the set formed by said portion during a first application period and by at least one subresolution feature formed during a second application period (2ndembodiment) can for example have a larger width, preferably larger by a maximum of 10%, than the width of the predetermined portion of the pattern to be transferred. Determination of this width is more particularly a function of the size of the pattern to be transferred and not of the grid of the emitter device. The chosen width is considered to be satisfactory when the transferred pattern has the required size, with a minimum line roughness of 3% with respect to said required size.

Furthermore, the lithography process can be replaced by any type of direct writing method and the electron beam can be replaced by a particle beam such as an ion beam or by a photon beam.