EP1709484A2

EP1709484A2 - Protected pattern mask for reflection lithography in the extreme uv or soft x-ray range

Info

Publication number: EP1709484A2
Application number: EP05717492A
Authority: EP
Inventors: Jean-Louis Stehle
Original assignee: Societe de Commercialisation des Produits de la Recherche Appliquee SOCPRA
Current assignee: Societe de Commercialisation des Produits de la Recherche Appliquee SOCPRA
Priority date: 2004-01-30
Filing date: 2005-01-26
Publication date: 2006-10-11
Also published as: FR2865813B1; FR2865813A1; WO2005083516A2; US20070160913A1; US7763394B2; WO2005083516A3

Abstract

A mask (MM) with patterns (MF) for use in a reflection lithography device with a photon beam with a wavelength of less than about 120 nm. Said mask (MM) comprises a planar substrate (ST) fixed to a reflecting structure (SMR) comprising a front face provided with selected patterns (MF) made from a material which is absorbent at the given wavelength and further comprises protection means (SP) which are transparent to the given wavelength and arranged such as to maintain a distance (H) between the perturbing particles (PP) and the patterns (MF) greater than or equal to one of the values of the depth of focus of the lithographic device and the height associated with the percentage of photon absorption by the perturbing particles (PP) which is acceptable.

Description

PROTECTED PATTERN MASK FOR REFLECTION REFLECTION LITHOGRAPHY IN THE FIELD OF EXTREME UV AND SOFT RAYS

The invention relates to the field of patterned masks used in optical lithography.

Optical lithography is a well-known technique for reproducing on a resin layer, deposited on a substrate (or "wafer"), patterns present on one of the faces of a mask using a beam of photons and an optical projection device, most often by reduction.

As those skilled in the art know, the resolution of the lines of the patterns, formed (or "exposed") in the resin, is proportional to a critical dimension CD equal to kλ NA, where λ is the wavelength of the photons of the beam, k is a coefficient less than 1 representing the effect of the devices used to lower the theoretical limits (such as for example the non-linearity of the resin), and NA is the numerical aperture of the beam of photons at the level of reasons. In other words, the dimensions of the reproduced patterns depend on the wavelength of the photons used.

Due to the properties and advantages offered by very small electronic components, ever shorter wavelengths are used to achieve them. Thus, we gradually went from photons with a wavelength in the visible (436 nm) to photons with a wavelength in the near ultraviolet (UV) (365 nm or 248 nm), then to photons having a wavelength in the far ultraviolet (193 nm or 157 nm), using respectively the lines G and I of mercury, then lasers with excimer KrF, ArF and F ₂ .

As the masks are difficult to manufacture without defect, and particularly expensive, they must therefore be protected so as not to be "disturbed" by ambient particles. Such protection is relatively simple to implement when the mask is transparent to the photons used for the reproduction of its patterns. In this case, the mask can indeed be used in transmission, so that it is possible to place a thin protective membrane made of transparent and non-disturbing material, for example polymer, in front of its front face (where the patterns to be reproduced are placed), so that the particles " disruptive "(or undesirable) are kept at a distance from the patterns preventing their reproduction on the resin (typically 6 mm). This protective membrane (or film) can be cleaned, and in some cases it can be replaced after inspection.

When one tends towards the transparency limits of the mask, for example for a wavelength of 157 nm, the protection of its patterns is more delicate. First you have to purge the optical path with Nitrogen (N ₂ ) to remove gases, such as oxygen (O ₂ ), molecules, such as water (H ₂ O), and polymers absorbents. Then, a solid protective film must be placed in front of the patterns, made of special SiOF quartz or CaF ₂ , transparent at 157 nm, anti-reflective treatment, with very parallel faces and a thickness chosen so that it is part of the optical calculation of image formation of the patterns on the resin.

When the photon wavelength is in the extreme ultraviolet (UV) range, or even soft X-rays (typically between about 120 nm and about 1 nm), the mask is no longer transparent, so that 'it should be used in reflection. Such a mask then consists of a planar substrate secured to a structure reflecting the wavelength of the photons used (produced, for example for a wavelength of 13.5 nm (located in extreme ultra violet) , in the form of a multilayer structure consisting of an alternation of layers of Silicon (Si) and Molybdenum (Mo)) and comprising a front face provided with selected patterns, made of an absorbent material at said wavelength ( for example in Cr or in TaN).

However, for this type of mask used in reflection lithography, there is no known means for protecting the patterns.

The invention therefore aims to remedy this drawback.

To this end, it proposes a patterned mask for a lithography device by reflection of a beam of photons of wavelength (λ) less than about 120 nm (and of numerical aperture (NA) chosen at the level of the patterns. ), comprising a planar substrate secured to a reflecting structure comprising a front face provided with selected patterns made of an absorbent material at the wavelength (λ).

This mask is characterized by the fact that it includes means of protection transparent at wavelength and arranged so as to keep the disturbing particles at a distance (H) from the patterns greater than or equal to one of the values taken by the focusing depth (doF) of the lithographic device and the height (h) which is associated with the tolerated percentage of absorption of photons by the disturbing particles.

For example, the protection means can be arranged so as to keep the disturbing particles at a distance (H) from the patterns greater than or equal to the greatest of the values taken by the focusing depth (doF) of the lithographic device and the height (h).

This distance (H) is for example between approximately 50 nm and approximately 5000 nm. However, it can be greater when a wavelength which deviates from the range of soft X-rays is used.

The mask protection means can constitute a structure having complementary characteristics which can be taken separately or in combination, and in particular it is preferable that:

• the structure has a maximum variation in optical thickness chosen so as to locally induce a negligible deflection of the beam in front of the placement precision of the patterns, ”the structure does not induce phase variation between the photons of the beam reflected by the mask ,

• the structure is hydrophobic,

• the structure has a front face, opposite the patterns, which can be cleaned of at least some of the particles which it maintains, • the structure is arranged so as to be able to be inspected, with a chosen contrast, using means of observation working in the visible or in the ultraviolet (V),

• the structure is suitable for thermophoresis, The structure is conductive so as to allow the implementation of an electrostatic effect, for example to repel the disturbing particles,

• the structure is non-diffractive and non-diffusing in the ultraviolet (UV).

Furthermore, this structure can take various forms, and in particular:

It can be deposited on the front face of the reflecting structure and parallel to it, and include at least one anti-reflective layer made of a chosen material,

• it can be in the form of a foam of a chosen material, • it can be made of a chosen material, be deposited on the front face of the reflecting structure, and define channels making it possible to reduce its density,

• it may comprise a membrane secured by pillars to the front face of the reflecting structure, and in a position substantially parallel to this front face, the thickness of the membrane and the height of the pillars then being chosen so that their sum is equal to the distance chosen,

• it can be composed of nanotubes, for example oriented in a chosen direction with respect to the normal to the front face of the reflective structure.

Among the materials which can be chosen to produce the structure, mention may in particular be made of polymers which are transparent at the wavelength (λ, for example equal to 10.9 nm or 13.5 nm), Carbon (C), Carbon nanotubes (or CNT), Silicon (Si), Berylium (Be), Ruthenium (Ru), Silver (Ag) and Zirconium (Zr).

Other characteristics and advantages of the invention will appear on examining the detailed description below, and the appended drawings, in which:

FIG. 1 schematically illustrates an exemplary embodiment of a lithography by reflection in the extreme ultraviolet (EUV),

• Figure 2 illustrates schematically, in a cross-sectional view, a first embodiment of a patterned mask according to the invention, • Figure 3 is a diagram illustrating the evolution of the parameter h as a function of the diameter d disturbing particles, for two different photon absorption values (1% and 4%), FIG. 4 schematically illustrates, in a cross-section view, a second embodiment of a patterned mask according to the invention,

• Figure 5 illustrates schematically, in a cross-sectional view, a third embodiment of a patterned mask according to the invention, • Figure 6 illustrates schematically, in a cross-sectional view, a fourth exemplary embodiment of a patterned mask according to the invention, and

• Figure 7 illustrates schematically, in a cross-sectional view, a fifth embodiment of a patterned mask according to the invention.

The accompanying drawings may not only serve to complete the invention, but also contribute to its definition, if necessary.

The invention relates to a patterned mask intended for use in a reflection lithography device operating with a photon source whose wavelength (λ) is less than about 120 nm, that is to say which belongs in the field of extreme ultraviolet (EUV) and soft X-rays, in particular at 10.9 nm and 13.5 nm.

Reference is first made to FIG. 1 to describe a very schematic example of the production of a reflection lithography device using a mask with MM patterns according to the invention.

A lithography device mainly comprises an optical image-forming device M3-M8 (also called a projection device) and a source S of photons, coupled to collection mirrors Ml and M2, implanted in an ultra-vacuum enclosure E, in which are defined, very precisely, a positioning zone of a mask with MM patterns and a positioning zone of a wafer (or "wafer") W.

The MM pattern mask is intended to function in reflection. It will be described later in detail.

The wafer W generally consists of a planar substrate provided on one of its faces with a resin layer R sensitive to the photons delivered by the source S of the lithography device. The source S is for example responsible for delivering photons whose wavelength λ is equal to 13.5 nm. Such a wavelength is for example obtained with a discharge or laser plasma source, in Xenon (Xe) or Tin (Sn). But, of course, it could deliver photons having other wavelengths between approximately 120 nm and approximately 1 nm, and in particular a wavelength equal to 10.9 nm (which corresponds to another domain of emission of Xenon (Xe)).

In the example illustrated, the mirrors Ml and M2 and the filter FR are responsible for collimating the photons delivered by the source S, so that they reach the level of the front face of the mask MM (which includes the patterns MF (see Figure 2)) in the form of a beam having a digital aperture NA _; chosen. For example, NAj is equal to 0.064, which corresponds to an opening angle on the patterns of ± 3.6 °.

Furthermore, the optical image-forming device M3-M8 (also called optical projection device) is responsible for forming the image of the patterns MF of the mask MM at the resin R of the wafer W, with a reduction factor. chosen, for example equal to approximately 4. It is here made up of six mirrors M3 to M8, by way of example.

The angles of incidence of the photon beam on the different mirrors and the respective positions of the different mirrors are chosen so as to allow the reduction factor and the illumination of the resin layer R to be obtained under a chosen numerical aperture NA _f , for example equal to about 0.25.

The angle of incidence CC of the photon beam with respect to the normal N at the front face of the patterned mask MM (see FIG. 2) is generally equal to a few degrees, for example around 6 °.

Of course, this is only a very schematic illustrative example. Many other combinations of optical means can be envisaged to ensure collimation and image formation.

Reference is now made more particularly to FIG. 2 to describe a first exemplary embodiment of a mask with MM patterns, according to the invention. In this figure, as in the Figures 4 to 7, the relative dimensions of the different elements are not representative of their actual relative dimensions.

A mask with MM patterns, used in reflection, firstly comprises a planar substrate ST, one of the faces of which is secured to a structure SMR reflecting at the wavelength λ of the photons of the source S and comprising a front face provided with selected MF patterns, made of an absorbent material at the wavelength λ.

For example, the SMR reflecting structure is a multilayer structure consisting of a stack of 40 pairs of layers of Silicon (Si), for example 4 nm thick, and Molybdenum (Mo), for example 2.7 nm thick.

Buffer and protective layers may be added for technological reasons.

The absorbent patterns are for example made of Chromium (Cr) or T N. However, any other absorbent material at the wavelength λ of the photons (here equal to 13.5 nm) can be considered. The thickness of the MF patterns is preferably reduced so as to avoid the edge effects of the masks, the dimensions of which are typically of the order of 152 mm × 152 mm (for an area of 104 mm × 104 mm reserved for the MF patterns). Furthermore, taking into account a reduction factor of approximately 4, the lines printed in the resin layer R have for example a width of 25 nm, 32 nm or 45 nm, which corresponds to MF patterns whose widths are 100 nm, 128 nm and 180 nm respectively.

As those skilled in the art know, and as mentioned in the introductory part, many very small disturbing particles PP (for example 30 to 60 nm) are likely to be inserted between the absorbent parts constituting the patterns MF, thus altering the integrity of the mask MM. The disturbing particles PP do not significantly interfere with image formation or absorption, when they are placed on the absorbent parts.

In order to prevent such an insertion from occurring, the invention proposes placing protective means SP transparent to the wavelength λ of photons in front of the patterns MF and responsible for keeping the disturbing particles PP at a distance H from the patterns, which is greater than or equal to one of the values taken by the focus depth doF of the lithographic device and the height h associated with the tolerated percentage of photon absorption by the disturbing particles PP.

The focusing depth doF is equal to λf NA; ² . For example, in the case of a digital aperture NA _j equal to 0.064 and a wavelength λ equal to 13.5 nm, a focusing depth doF equal to approximately 3296 nm is obtained. Of course, this is only an example, and doF can typically vary between about 50 nm and about 5000 nm depending on the values chosen for NAj and λ, or even more when using a length of wave which moves away from the soft X-ray domain.

In addition, it is recalled that the percentage of absorption of photons by the disturbing particles PP is defined by the factor (or percentage) of shade which is given by the formula:

Percentage of Shadow - h * 2NA _t

where d is the diameter of the disturbing particles PP, h is the parameter representative of the height separating the disturbing particles from the patterns MF.

If one tolerates an absorption of photons equal to approximately 1%, one then obtains the simplified relation h = 80 * d. On the other hand, if one tolerates an absorption of photons equal to approximately 4%, one then obtains the simplified relation h = 40 * d. The evolution of the parameter h (in nanometers (nm)) as a function of the diameter d (in nanometers (nm)) of the disturbing particles has been represented in the diagram of FIG. 3, for photon absorption values equal to 1 % (upper curve) and 4% (lower curve).

The smaller the diameter d of the disturbing particle PP, the less the parameter h intervenes in the choice of the distance H separating the front part of the protection means SP from the front face of the reflecting structure SMR (where the patterns MF are formed) . The value of H must therefore be chosen greater than or equal to one of the values taken by doF and h.

For example, at first order we can set the following conditions: if doF is greater than h, the distance H must be greater than or equal to doF, while if doF is less than h, the distance H must be greater than or equal to h. In other words, at first order, the value of H is chosen greater than or equal to the largest of the values taken by doF and h. Image simulation simulation calculations taking into account additional parameters, such as, for example, diffraction and / or index differences, make it possible to refine the aforementioned conditions.

The protection means SP may moreover have one or more additional characteristics which can reinforce their performance and / or advantages.

It is for example advantageous that the protection means SP do not (or very little) absorb the photons. It is indeed recalled that in reflection lithography the optical thickness T of the protection means SP is crossed twice. For this purpose, it is preferable to use a material with low, or even very weak, aging, resistant to the photon beam and with weak, even very weak, oxidation.

It will be noted that a disturbing particle PP can be crossed twice, but not by the same beam.

It is for example also advantageous that the protection means SP have a maximum variation in optical thickness T locally inducing a deflection of the photon beam negligible compared to the placement accuracy of the patterns MF.

It is for example also advantageous that the protection means SP do not induce (or very little) phase variation between the photons of the beam which are reflected by the mask MM.

It is, for example, also advantageous for the protection means SP to be hydrophobic. He recalled that the water molecules (H ₂ O) are absorbent at 13.5 nm.

It is for example also advantageous that the front face of the protection means SP, which is opposite to the patterns MF, can be cleaned of at least some of the disturbing particles PP which it maintains. In this case, it is preferable that the means of SP protection can be inspected, with a selected contrast, by means of observation working in the visible or in uitraviolet, for example at 248 nm.

Instead of cleaning the front face of the protection means SP, it is possible to consider removing them from the mask MM in order to replace them. This removal can for example be carried out by combustion or by oxidation (in particular when they are made based on carbon nanotubes, as will be seen below) using carbon dioxide (CO ₂ ), which is then evaporated. and pumped to avoid residual deposits. Once this removal has been carried out, the mask MM can then be inspected and then new protective means SP deposited on its front face.

It is for example also advantageous that the protection means SP are suitable for thermophoresis.

It is for example also advantageous that the protection means SP are electrically conductive, in order to use them to implement an electrostatic effect, for example to repel the disturbing particles when they are ionized.

It is, for example, also advantageous for the SP protection means to be non-diffractive and non-diffusing in the ultraviolet (UV), including in the extreme UV, for the quality of the image, in particular when the wavelength is equal to 13.5 nm, and for the quality and contrast during the inspection.

Many structures can constitute the SP protection means presented above. These structures must always be secured to the substrate ST or to the reflecting structure SMR, and may include a part ensuring the maintenance of the disturbing particles PP, also called film (or membrane), which is either distant from the MF patterns, or in contact of these.

As is schematically illustrated in FIG. 1, the protection means SP can constitute a planar antireflection structure, preferably of the multilayer type, deposited on the front face of the reflecting structure SMR and parallel to it. For this purpose, it is possible, for example, to use Mo-Si layers. In a first variant, illustrated in FIG. 4, the protection means SP can constitute a structure composed of nanotubes oriented in a chosen direction relative to the normal N at the front face of the reflecting structure SMR. For example, it is possible to use carbon nanotubes or (CNT), preferably having walls of monoatomic thickness, spaced from each other by a distance less than the diameter of the smallest disturbing particles PP, and possibly possibly having a disorientation compared to normal N.

In a second variant, illustrated in FIG. 5, the protection means SP can constitute a structure comprising layers CL in 1 in which channels CX of defined dimensions are defined relative to those of the disturbing particles PP, in order to block them at level of their front ends. These CX channels being filled with "vacuum", they make it possible to reduce the density of the material. Such a structure can be defined using a lithography technique applied to a material such as silicon (Si) or a polymer, such as for example PMMA. It can also be obtained by adding a photonic crystal or porous silicon to the front face of the SMR reflecting structure.

In a third variant, illustrated in FIG. 6, the protection means SP can constitute a foam structure forming a membrane (or film), preferably secured to the front face of the reflecting structure SMR. As in the second variant, the foam contains numerous empty spaces which make it possible to reduce the density of the material. This foam can for example be made from a polymer, such as PMMA, or nanotubes, for example carbon (or CNT), or Berylium (Be), or Ruthenium (Ru), or Silver (Ag), or also Zirconium (Zr).

In a fourth variant, illustrated in FIG. 7, the protection means SP can constitute a structure made up of a membrane (or film) ME secured by pillars PS to the front face of the reflecting structure SMR. Certain ends of PS abutments can rest on the MF patterns, as illustrated. The membrane ME is placed substantially parallel to the front face of the reflecting structure SMR. Furthermore, the thickness of the membrane ME and the height of the pillars PS are chosen so that their sum is equal to the distance H chosen. The pillars can for example be obtained by growth of germs (or "seeds"). Such a structure can for example be produced from of a polymer, such as PMMA, or of Silicon (Si), or also in an anti-reflective material, such as Mo-Si. The material constituting this structure can also be in the form of a foam, as in the third variant presented above with reference to FIG. 6.

The invention is not limited to the embodiments of patterned mask described above, only by way of example, but it encompasses all the variants that a person skilled in the art may envisage within the framework of the claims below. after.

Claims

1. Mask (MM) with patterns (MF), for a device for lithography by reflection of a beam of photons of wavelength less than approximately 120 nm, comprising a planar substrate (ST) secured to a reflective structure (SMR) comprising a front face provided with selected patterns (MF), made from an absorbent material at said wavelength, characterized in that it comprises protection means (SP) transparent at said wavelength and arranged to maintain disturbing particles (PP) at a distance (H) from said patterns (MF) greater than or equal to one of two values taken from a focusing depth (doF) of said device and an associated pattern/disturbing particle height (h) at a tolerated percentage of absorption of photons by said disturbing particles (PP), depending on their diameter (d).

2. Mask according to claim 1, characterized in that said protection means (SP) are arranged to maintain the disturbing particles (PP) at a distance (H) from said patterns (MF) greater than or equal to the greater of the two values taken by the depth of focus (doF) of the device and the disturbing particle height (h).

3. Mask according to one of claims 1 and 2, characterized in that said protection means (SP) constitute a structure having a maximum variation in optical thickness chosen so as to locally induce a deflection of the beam which is negligible compared to the precision of placement of said patterns (MF).

4. Mask according to one of claims 1 to 3, characterized in that said protection means (SP) constitute a structure which substantially does not induce phase variation between photons of the beam reflected by said mask.

5. Mask according to one of claims 1 to 4, characterized in that said means of protection (SP) constitute a hydrophobic structure.

6. Mask according to one of claims 1 to 5, characterized in that said means of protection (SP) constitute a structure of which at least one front face, opposite said patterns (MF), is capable of being cleaned of at least some of the disruptive particles (PP) that it maintains.

I 7. Mask according to one of claims 1 to 6, characterized in that said means of protection (SP) constitute a structure capable of being inspected, with a chosen contrast, using observation means working in the visible or in the ultraviolet.

5. Mask according to one of claims 1 to 7, characterized in that said means of protection (SP) constitute a structure suitable for thermophoresis.

9. Mask according to one of claims 1 to 8, characterized in that said protection means (SP) constitute a conductive structure capable of implementing an electrostatic effect.

10. Mask according to claim 9, characterized in that said electrostatic effect is intended to repel said disruptive particles (PP).

11. Mask according to one of claims 1 to 10, characterized in that said means of protection (SP) constitute a non-diffracting and non-diffusing structure in ultraviolet.

12. Mask according to one of claims 1 to 11, characterized in that said distance (H) is between approximately 50 nm and approximately 5000 nm.

13. Mask according to one of claims 1 to 12, characterized in that said means of protection (SP) constitute a structure deposited on the front face of the reflective structure and parallel thereto, and comprising at least one anti-reflective layer made from a chosen material.

14. Mask according to one of claims 1 to 12, characterized in that said means of protection (SP) constitute a structure consisting of a foam of a chosen material.

15. Mask according to one of claims 1 to 12, characterized in that said protection means (SP) constitute a structure made of a chosen material, deposited on the front face of the reflective structure (SMR), and defining channels (CX) making it possible to reduce the density of said material.

16. Mask according to one of claims 1 to 12, characterized in that said means of protection (SP) constitute a structure comprising a membrane (ME) secured by pillars (PS) to the front face of the reflective structure, and in a position substantially parallel to said front face, the thickness of said membrane (ME) and the height of said pillars (PS) being chosen so that their sum is equal to said chosen distance (H).

17. Mask according to one of claims 1 to 12, characterized in that said protection means (PS) constitute a structure composed of nanotubes oriented in a direction chosen relative to the normal (N) to said front face of the structure reflective (SMR).

18. Mask according to one of claims 13 to 17, characterized in that said material is chosen from at least polymers transparent at said wavelength, Carbon, Carbon nanotubes, Silicon, Berylium, Ruthenium , Silver and Zirconium.