WO2003050619A2

WO2003050619A2 - A method for producing a mask for high resolution lithography, a mask obtained thereby and a multi-layer element for high resolution lithography

Info

Publication number: WO2003050619A2
Application number: PCT/IB2002/005286
Authority: WO
Inventors: Marco Paolo Fontana; Vladimir Troitsky
Original assignee: Infm Istituto Nazionale Per La Fisica Della Materia; BERZINA, Tatiana; BERZINE, Roman; TROITSKY, Anastasia
Priority date: 2001-12-12
Filing date: 2002-12-11
Publication date: 2003-06-19
Also published as: AU2002366691A1; ITPD20010290A1; WO2003050619A3; AU2002366691A8

Abstract

A method is described for producing a mask for high definition lithography, which includes the steps of providing a substrate; depositing on this substrate a first layer of a polymeric material sensitive to exposure to charged particles; depositing on this first layer a second layer of a polymeric material sensitive to electromagnetic radiation; exposing the second layer to electromagnetic radiation according to a predetermined topography so as to define first and second portions of the second layer which have been exposed or not exposed respectively to the electromagnetic. radiation; removing either the first or the second portion of the second layer, and also the step of irradiating the first layer, partially covered by the first or second layer which was not removed, by means of a source of low energy charged particles. A mask is also described for high definition lithography, produced according to the method described above and a multi-layer element for high definition lithography which includes a first layer of a polymeric material sensitive to exposure to charged particles; a second layer of a polymeric material sensitive to electromagnetic radiation, in which the first and the second layer are Langmuir-Blodgett films.

Description

A method for producing a mask for high resolution lithography, a mask obtained thereby and a multi-layer element for high resolution lithography

The present invention relates to a method for producing a mask for high resolution lithography according to the preamble to the main Claim 1. The object of this invention is also to provide a mask according to this method and a semi-finished multi-layer element also obtained by means of the aforesaid method.

It is well known in the field of microelectronics that it is necessary to produce integrated circuits of ever smaller dimensions in order to improve performance and reduce costs. To this end, a lithographic technique is required which makes it possible to achieve a resolution of less than a micron.

In a typical method of manufacturing an integrated circuit, a substrate is coated with a film, the so-called resist, consisting of a material which is sensitive (to electromagnetic radiation, for example) and has protective characteristics whereby it can protect the underlying substrate during the etching process, that is the chemical- physical removal of excess material from the substrate so as to form a predetermined topography.

During one production step, the resist is irradiated with a particular electromagnetic radiation to which it is sensitive. The material constituting the irradiated portion of the resist undergoes a chemical transformation, in particular, this portion is converted into a more soluble form (positive resist) or less soluble form (negative resist) than the portion which was not irradiated. In order to achieve the desired high resolution, it is necessary to develop resists which can be deposited on the substrate in superposed layers which are extremely thin, free of faults, uniform and sensitive, without damaging their protective characteristics.

With this in mind, there has been intensive research into the use of Langmuir-Blodgett film, the thickness of which can be controlled at molecular level, as a resist in deep ultraviolet lithography and in electron beam lithography (A. Barraud, Thin Solid Films, 99 (1983) 305) . Of the compounds that were examined, the most promising for use as resists in electron-beam lithography were the polycyanoacrylates (N.K. atveeva and Yu S. Bokov, Thin Solid Films, 327-329 (1992) 477) .

The use of a Langmuir-Blodgett film, i. particular a polycyanoacrylate, as a resist does have some disadvantages, however, such as the long time it takes to deposit the film. This slowness, meaning that it can take several hours to deposit a film of a thickness of 10-20 run, can lead to the production of a film with faults and impurities, caused by foreign particles, which would be present even in a protective atmosphere, attaching themselves to the substrate. In addition, this slow deposition process is not advantageous from an economic point of view.

Moreover, polycyanoacrylate Langmuir-Blodgett films make excellent resists in electron-beam lithography, a field of application which is very small owing to high costs and to the length of time taken to produce a finished item, but are of no use in deep ultraviolet or in x-ray lithography, techniques which are far more widely used, since they are not at all sensitive to such radiation.

The main object of the present invention is to provide a method for producing a mask for high definition lithography which is designed to overcome the limits complained of with reference to the prior art .

This object, and others which will become clearer from the description which follows, are achieved according to the invention by means of a method and a mask having the characteristics described in the Claims which follow.

Characteristics and advantages of the invention will become more apparent from the detailed description of a preferred embodiment thereof, illustrated, purely by way of non- limitative example, with reference to the appended drawings, in which:

Figure la is a sectioned, schematic view of a step in the method of the invention for producing a mask for high definition lithography;

Figure lb is an enlarged detail of the step of the method of Figure la;

Figures 2a-2g are schematic and sectioned views of a plurality of steps of the method of Figure la;

Figure 3 is a schematic view of a further step in the method of Figure 2.

With reference first to Figures 2a-2g, a substrate is indicated 1, with a multi-layer element 2 deposited on it for manufacturing a high-resolution lithographic mask 3 produced according to the method of the invention. A film of a metal material 12, in the preferred example chromium, or another material which can to be removed later in an etching process so as to form a desired topography, is deposited on a substrate surface 1, a silicon wafer, for example.

The multi-layer element 2 includes a first layer of resist 4, formed in particular of a material with protective characteristics and sensitive to irradiation by charged particles, in particular electrons, deposited on the film 12, using the Langmuir-Schaefer method with any appropriate adjustments .

In the Langmuir-Schaefer (LS) Technique, illustrated in Figure 1, molecules of an amphiphilic substance, in aqueous solution with a volatile solvent, one part hydrophobic and one part hydrophilic, are distributed on a water surface 15. Once the solvent has evaporated, a monomolecular layer 5, known as a Langmuir layer, of the substance being used ₍ remains at the air-water interface . The molecules of the compound are compressed by a barrier (not shown) so as to increase surface pressure up to a desired value in which the molecules at the air-water interface are orientated with the hydrophobic group in the gaseous phase.

The next step consists in depositing the monomolecular layer 5 thus obtained onto a solid support, in particular onto the film 12 of metal material. A mechanical system including a plurality of movable barriers 6 is provided for separating the water surface 15 into various independent portions after the monomolecular layer 5 has been compressed, thereby preventing molecules of the compound from seeping from an adjacent portion into a portion with no molecules, which has already been deposited on the film 12. In order to transfer the monomolecular layer 5 from the water surface to the film 12, the substrate 1, which is orientated parallel to the water surface 15, is first moved towards this surface until the film 12 touches the layer 5 and is then moved away again from that surface. In this way, the layer 5 remains bonded to the film 12.

In order to deposit a new monomolecular layer 5 onto the first layer already deposited, the process of moving the substrate 1 towards and then away from the water surface 15 whereby the molecules of the compound are distributed, is repeated for a number of times equal to the number of monomolecular layers 5 intended to make up the first layer 4 of resist.

An additional monomolecular layer of a conductive material 8, operable to remove any electrical charges which may have accumulated, is deposited on the free surface of the final monomolecular layer 5 of compound.

The method also provides, should this be necessary, for a monomolecular layer of adhesive 16 to be deposited on the film 12 of metal material before the first layer 4 is deposited.

Thanks to the deposition method described above, it is possible to determine the thickness of the first layer 4 extremely accurately, to a degree of accuracy of 0.1-0.5 run. In addition, thicknesses of dozens of monomolecular layers are achieved in only a few minutes, with each layer being between 0.6 and 1 nm thick, and almost totally free of faults (there is no time for foreign bodies to be deposited) , on a substrate with a longitudinal dimension which is preferably of between 100 and 150 mm.

A second layer 9 of a polymeric material which is both photosensitive and sensitive to electromagnetic rays, referred to henceforth as a photoresist, is deposited on the free surface of the first layer 4, that is that which is not in contact with the film 12. In order to produce miniaturized topographies, the most interesting electromagnetic rays are the deep ultraviolet (UV) rays and X-rays, the wavelengths of either being sufficiently short to produce the desired resolution: a polymeric material sensitive to one of these types of radiation is selected. The protective characteristics of such a photoresist are not relevant, as will be described in greater detail hereinafter, thanks to a particular characteristic of the invention, meaning that the choice of this material is particularly simple and advantageous .

The second layer 9 is deposited by means of the Langmuir- Schaefer method described above.

The thickness of the multi-layer element 2, shown in Figure 2a and including the first layer 4 and the second layer 9, is determined according to the method described in detail later and is generally between 10 and 50 nm, in dependence on the application for which it is intended.

In a subsequent step of the method of the invention, shown in Figure 2b, the second layer 9 of the multilayer element 2 is exposed to electromagnetic light of an appropriate wavelength (represented by the unbroken, parallel arrows indicated UV in Figure 2b) , that is, the wave length to which the compound constituting the second layer 9 is sensitive, and according to a predetermined topography.

The desired topography is formed in the second layer 9 by irradiating the multilayer element 2 through an additional mask 10 (Figure 2b) . A first, irradiated portion 17 of the second layer undergoes a chemical transformation whereby the photoresist is converted into a more soluble form than a second portion 18 which is protected by the additional mask 10 and thus not irradiated.

The first layer 4 is composed of a material which is not sensitive to the type of electromagnetic radiation used, whereby these rays can penetrate it without causing any alteration in the material of which it is composed.

Using an appropriate chemical solvent, the first portion 17 is then rendered soluble and removed (Figure 2c) . The topography of the remaining second portion 18 of the second layer 9 is therefore the same as that defined initially by the additional mask 10. The surface of the first layer 4 is thereby divided into a first exposed area 19 not covered by the second portion 18 of the second layer 9 and a second area 20 covered by it.

According to a further characteristic of the method of the invention, the multi-layer element is then irradiated with charged particles, in this case electrons, diffused with low energy, preferably energy of between 0.5 and 2 keV (see Figure 2d; the electrons are indicated schematically by unbroken arrows indicated e ' ) • A source of randomly diffused electrons is used for this purpose. Since the resist constituting the first layer is sensitive to the electronic irradiation, a first portion 25 of the first layer 4, corresponding to the first area 19 exposed to the electrons undergoes a chemical transformation (cross-linking, for example) and can be removed by means of a chemical agent (Figure 2e) . The topography present in the second layer 9 is therefore transferred to the first layer 4, of which only a second portion 24, complementary to the first portion 25, remains after treatment with the chemical agent .

In the same way, the second portion 18 of the second layer 9, which is not required after the described electron radiation step, can be removed by an appropriate chemical agent.

The thicknesses of the first layer 4 and the second layer 9 are determined in such a way that the electrons of the selected energy cannot cross the thickness of the second portion 18 of the second layer 9 covering the second area 20 of the first layer 4 and also in such a way that the first portion 25 of the first layer 4 is fully exposed to the electrons, throughout its depth.

In any case, in order to achieve the highest resolution offered by this method, the first and second layers must be as thin as possible; it is therefore first necessary to determine the minimum thickness of the first layer in dependence on this giving sufficient protection to the film 12 of metal material during the subsequent etching step, that is the removal of superfluous zones not covered by the first layer 4. The energy of the electrons required in order fully to cross link all the exposed portion of the first layer is determined on the basis of this minimum thickness. In particular, the depth to which the electrons penetrate can be determined in tests and is equal, for example, to a value A. The first layer 4 is therefore selected to be of a thickness less than A (full cross linking) , with the minimum thickness of the second layer 9, which will in any event be greater than A, being determined as that which will protect the first layer 4 from being penetrated by electrons of the selected energy. Since low energy electrons are never destructive, the resist constituting the second layer 9 need have no protective properties.

This prevention of any electrons passing through the second layer 9, while limiting the thickness of the latter, is achieved thanks to accurate adjustment of the thicknesses of the first and second layers, made possible by the LS deposition method. In this way, the overall thickness of the multilayer element 2 can be kept down to a fraction of that of prior art multilayers, without compromising any protective characteristics during subsequent etching.

Irradiation with electrons is very fast, taking between 0.5 and 5 minutes in dependence on the sensitivity of the first layer 4, as opposed to how things normally proceed in electron beam lithography, since it is possible to expose a plurality of wafers 11 evenly using a very simple source of randomly diffused electrons.

An example of such exposure is illustrated in Figure 3, where an electron gun 22 is used and the electrons are diffused by a screen 23.

An etching step (see Figure 2f) follows the removal of the first portion 25 of the first layer 4, which had been exposed to the electrons, using any method known per se whereby those portions of the film 12 of metal material which are not covered by the first layer 4 are removed, thereby reproducing the desired topography in the film 12. The remaining second portion 24 of the first layer, corresponding to the second area 20, serves to protect the underlying film 12 during this step and is removed on its completion (Figure 2g) .

The resolution which can be obtained according to the method of the invention, on completion of the etching step on the film 12, carried out in particular on a typical metal material (chromium, for example) used in microelectronics, through a mask 3 thus produced, is of around 0.1 μm.

Although Figures 2a-2g show first and second layers of a positive resist, the method of the invention can be carried out using either two negative resists or one positive resist and one negative.

In addition, the method of the invention can be used to produce topographies in ultra-thin molecular layers for applications in nanoelectronics and also in the manufacture of materials which include molecular nanostructures . In such cases, the topography is produced in the first layer 4 by means of lithography using either scanning tunnel microscopy (STM) or atomic force microscopy (AFM) .

EXAMPLE

A film of chromium 12 and 20-30 monomolecular layers of a polycyanoacrylate are deposited in succession onto a silicon wafer with a diameter of 76 mm. In particular, for the resist constituting the first layer 4 any one of the following compounds are used: poly (heptylcyanoacrylate) ; poly (allyloxyethylcyanoacrylate) ; copolymers of heptylcyanoacrylate; poly (2,2,3,3,3-pentafluoropropylmethylacrylate) ; polycyanoacrylates known respectively as CP-HCA-1, CP- HCA-2 and CP-BCA (see underlying formulae) .

An ordinary commercially available resist is used for the second layer 9, such as a novolac diazonaphthoquinone resin. However, the compound constituting the second layer should be determined in dependence on the desired sensitivity to a particular type of electromagnetic radiation, meaning that a broad range of commercially available resists could be used.

A commonly available source of UV rays was used to produce a topography in the first layer. The energy of the electrons and the dose of irradiation are worked out case by case in dependence on the thickness of the layers and the specific material being used. In the case of a first layer of polycyanoacrylate of a thickness of 15nm, this would provide protection in a "wet etching" operation for a chromium film of a thickness of 150nm.

The invention therefore achieves the objective intended, providing numerous advantages over arrangements of the prior art.

One substantial advantage is provided by the high resolution which can be achieved for topographies produced by the method of the invention. A second advantage of the method of the invention consists in the speed whereby extremely thin films can be deposited using the Langmuir-Schaefer method, with their thickness being controlled at a molecular level, and which are even and substantially free of faults .

Another advantage consists in the possibility of simultaneously exposing a large number of wafers to electron radiation, thanks to the ability accurately to control the thickness of the film, thereby achieving rapid production of the desired integrated circuits.

Furthermore, this rapid electron irradiation translates into a considerable reduction in manufacturing costs and calls for less complex equipment than is usually required in such operations .

Finally, by using the Langmuir-Schaefer method, it is possible to modify the Langmuir trays currently available commercially, in order to speed up considerably the production of treated wafers.

Claims

1. A method for producing a mask for high-definition lithography, which includes the steps of :

- providing a substrate;

- depositing on this substrate a first layer of a polymeric material sensitive to exposure to charged particles;

- depositing on the said first layer a second layer of a material sensitive to electromagnetic radiation;

- exposing the said second layer to the said electromagnetic radiation according to a predetermined topography, thereby defining first and second portions of the said second layer which are exposed or not exposed respectively to the said electromagnetic radiation;

- removing one of the said first or said second portions of the said second layer; characterised in that it includes the step of

- irradiating the said first layer, partially covered by either the said first or the said second non- emoved portion, by means of a source of the said charged particles diffused with low energy.

2. A method according to Claim 1, in which the said first layer is a Langmuir-Blodgett film.

3. A method according to Claim 1 or Claim 2 , in which the said second layer is a Langmuir-Blodgett film.

4. A method according to Claim 2 or Claim 3 , which includes the step of depositing the said first layer by means of the Langmuir-Schaefer method.

5. A method according to one or more of Claims 2 to 4 , which includes the step of depositing the said second layer by means of the Langmuir-Schaefer method.

6. A method according to one or more of the preceding Claims, in which the said first layer is insensitive to the said electromagnetic radiation.

7. A method according to one or more of the preceding Claims, which includes the step of selecting the energy of the said particles to be between 0.2 and 2 keV.

8. A method according to one or more of the preceding Claims, in which the said electromagnetic radiation includes x-rays.

9. A method according to one or more of Claims 1 to 7, in which the said electromagnetic radiation includes rays in the deep ultraviolet range.

10. A method according to one or more of the preceding Claims, in which the said charged particles are electrons.

11. A method according to one or more of the preceding Claims, in which the said first layer is composed of polycyanoacrylate .

12. A method according to one or more of the preceding Claims, in which the overall thickness of the said first and said second layers is between 10 nm and 50 nm.

13. A mask for high definition lithography produced by the method according to Claims 1 to 12.

14. A multi-layer element for high definition lithography which includes

- a first layer of a polymeric material sensitive to exposure to charged particles; a second layer of a polymeric material sensitive to electromagnetic radiation; characterised in that the said first and second layers are Langmuir-Blodgett films .

15. A multi-layer element according to Claim 14, in which at least one of the said first and second layers is deposited by means of the Langmuir-Schaefer method.

16. A multi-layer element according to Claim 15, in which both the said first and second layers are deposited by means of the Langmuir-Schaefer method.

17. A multi-layer method according to one or more of Claims 14 to 16, with a thickness of between 10 nm and 50 nm.

18. A multi-layer element according to one or more of Claims 14 to 17, in which the said first layer consists of polycyanoacrylate.

19. A multi-layer element according to one or more of Claims 14 to 18, in which the said second layer is sensitive to x- rays.

20. A multi-layer element according to one or more of Claims 14 to 18, in which the said second layer is sensitive to radiation in the deep ultraviolet range.

21. A multi-layer element according to one or more of Claims 14 to 20, in which the said first layer is sensitive to exposure to electrons.

22. A multi-layer element according to one or more of Claims 14 to 21, in which the said first layer is insensitive to the said electromagnetic rays .