Source: http://www.google.fr/patents/US5480747?hl=fr
Timestamp: 2013-05-21 11:57:41
Document Index: 228725403

Matched Legal Cases: ['artz 16', 'artz 31', 'artz 36', 'artz 61', 'artz 61', 'artz 61']

Brevet US5480747 - Attenuated phase shifting mask with buried absorbers - Google�BrevetsRecherche Images Maps Play YouTube Actualit�s Gmail Drive Plus » Recherche avanc�e dans les brevets | Historique Web | Connexion Recherche avanc�e dans les brevets BrevetsAn attenuated phase shifting mask has absorbers embedded (buried) in the mask substrate, instead of on the surface of the substrate. The buried absorbers allow for controlling attenuation and phase shifting parameters. The material composition and the thickness of the absorber regions determine the amount...http://www.google.fr/patents/US5480747?utm_source=gb-gplus-shareBrevet US5480747 - Attenuated phase shifting mask with buried absorbers Num�ro de publicationUS5480747 AType de publicationOctroi Num�ro de demande08/342,939 Date de publication2 janv. 1996 Date de d�p�t21 nov. 1994 Date de priorit�21 nov. 1994 InventeursPrahalad K. Vasudev Cessionnaire d'origineSematech, Inc.Sematech, Inc. A Delaware Corporation Classification aux �tats-Unis430/5430/322430/323430/324 Classification internationaleG03F1/00 Classification coop�rativeG03F1/32G03F1/29 Classification europ�enneG03F1/29G03F1/32R�f�rencesCitations de brevets (7)Citations hors brevets (2) R�f�renc� par (37)Liens externesUSPTO Cession USPTO EspacenetAttenuated phase shifting mask with buried absorbersUS 5480747 A R�sum� An attenuated phase shifting mask has absorbers embedded (buried) in the mask substrate, instead of on the surface of the substrate. The buried absorbers allow for controlling attenuation and phase shifting parameters. The material composition and the thickness of the absorber regions determine the amount of attenuation that is to be achieved, as well as phase shifting in some instances. In other instances, offset distances of the absorbers from the surface of the mask control the phase shift. Light scattering and diffraction is reduced or eliminated by having the absorbers below the surface of the mask. By reducing light scattering and distortion, the mask of the present invention allows for PSM lithography techniques to be extended to ranges of shorter wavelength.
I claim: 1. An attenuated phase shifting photolithography mask for use in projecting an image pattern onto a target comprising: a mask substrate formed from a substantially transparent material for permitting light transmission therethrough; an absorber pattern, formed from a light absorbing material and buried a set distance below a surface of said substrate and not adjacent to said surface, for absorbing a significant portion of light transmission therethrough to form a phase shifting attenuator pattern in said substrate, such that light attenuation and phase shifting achieved in phase shifting regions of said mask are both obtained from said light absorbing material at a set wavelength of transmitted light; wherein having said attenuator pattern buried below said surface of said substrate but not adjacent to said surface, allows for light scattered from surface areas of said attenuator pattern to be reflected back into said substrate at a surface interface of said substrate, in order to improve image feature definition at said target; and wherein having said attenuator pattern buried below said surface at said set distance is of sufficient depth in order to provide for a maximum depth of focus of an exposure system being utilized to reside within said substrate, such that surface defects and contaminants at said surface interface are not imaged on to said target.
5. The mask of claim 4 wherein said attenuator pattern provides for a 180 6. The mask of claim 5 wherein said mask substrate is made from quartz.
7. An attenuated phase shifting photolithography mask for use in projecting an image pattern onto a target comprising: a mask substrate formed from a substantially transparent material for permitting light transmission therethrough, said substrate having trenches on its surface corresponding to non-phase shifting regions and wherein said trenches have a depth corresponding to a set distance d.sub.2 ; an absorber pattern formed from a light absorbing material and buried at a distance d.sub.1 from said substrate surface, in which said distance d.sub.1 is greater than said set distance d.sub.2, and disposed below said surface of said substrate at phase shifting regions and not adjacent to said surface; said absorber pattern for absorbing a significant portion of light transmission therethrough to form an attenuator pattern in said substrate, such that light attenuation is obtained from said light absorbing material and phase shifting is determined by a combination of said light absorbing material and trench depth d.sub.2 ; wherein having said attenuator pattern buried below said surface of said substrate but not adjacent to said surface, allows for light scattering at said attenuator pattern to be reflected back into said substrate at a surface interface of said substrate, in order to improve image feature definition; and wherein having said attenuator pattern buried below said surface at said distance d.sub.1 is of sufficient depth in order to provide for a maximum depth of focus of an exposure system being utilized to reside within said substrate, such that surface defects and contaminants at said surface interface are not imaged on to said target.
8. The mask of claim 7 wherein said light absorbing material is comprised of a metallic film.
10. The mask of claim 9 wherein said offset distance for said attenuator pattern is set for a 180
12. A method of fabricating an attenuated phase shifting photolithography mask for use in projecting an image onto a target, comprising the steps of: forming a photoresistive layer over a mask substrate; patterning said photoresistive layer to expose portions of said substrate underlying said photoresistive layer; etching said exposed portions of said substrate to a predefined depth to form trenches in said substrate; removing remaining portions of said photoresistive layer; depositing a layer of light absorbing material over said substrate and filling said trenches; selectively etching back said layer of light absorbing material until only said trenches are filled with said light absorbing material; forming a dielectric layer of a predefined thickness over said substrate and said trenches to form an upper boundary region of said mask in order to bury said light absorbing material below a surface of said mask now formed by exposed surface of said dielectric layer; wherein said absorbing material forms an attenuator pattern below said surface of said mask and not adjacent to said surface, such that light attenuation is obtained from said light absorbing material and an amount of phase shifting is obtained from said light absorbing material at a set wavelength of transmitted light; wherein having said attenuator pattern buried below said surface of said mask allows for light scattered from surface areas of said attenuator pattern to be reflected back into said mask at a surface interface of said substrate, in order to improve image feature definition at said target; and wherein having said attenuator pattern buried below said surface is at a distance of sufficient depth in order to provide for a maximum depth of focus of an exposure system being utilized to reside within said substrate, such that surface defects and contaminants at said surface interface are not imaged on to said target.
13. The method of claim 12 wherein said absorbing material has a thickness t to provide both required attenuation and required phase shifting for said mask.
18. A method of fabricating an attenuated phase shifting photolithography mask for use in projecting an image onto a target, comprising the steps of: forming an oxide layer over a mask substrate; forming a photoresistive layer over said oxide layer; patterning said photoresistive layer to expose portions of said oxide layer underlying said photoresistive layer; etching said exposed portions of said oxide layer to expose portions of said underlying substrate; removing remaining portions of said photoresistive layer, but leaving patterned portions of said oxide layer; implanting doped ions into exposed portions of said substrate to a set depth below exposed surface of said substrate to form patterned light absorbing regions in said substrate and not adjacent to said surface; removing remaining portion of said oxide layer such that said substrate has a substantially planar surface with buried light absorbing regions forming an attenuator pattern; wherein said absorbing material forms an attenuator pattern below said surface of said mask, such that light attenuation is obtained from said light absorbing material and an amount of phase shifting is obtained from said light absorbing material at a set wavelength of transmitted light; wherein having said attenuator pattern buried below said surface of said mask allows for light scattered from surface areas of said attenuator pattern to be reflected back into said mask at said surface, in order to improve image feature definition at said target; and wherein having said attenuator pattern buried below said surface is at a distance of sufficient depth in order to provide for a maximum depth of focus of an exposure system being utilized to reside within said substrate, such that surface defects and contaminants at said surface interface are not imaged on to said target.
19. The method of claim 18 wherein thickness of said attenuator pattern in said step of implanting doped ions is determined by dosage of said doped ions implanted into said substrate.
21. The method of claim 20 wherein said dosage for implantation is in an approximate range of 3 /cm.sup.2.
Phase shifting techniques have been generally known as early as U.S. Pat. No. 4,360,586. One type of PSM that has shown considerable promise is the attenuated phase shifting mask in which an attenuator attenuates the transmitted light passing through it, at the same time shifting the phase of the light (typically, by 180 described in U.S. Pat. No. 4,890,309. Attenuated PSMs have emerged as one of the preferred, if not the most preferred, practical approaches to enhancing the lithography "process latitude" or "process window" (depth of focus
The attenuator is usually formed from a metallic like light absorbing film, such as chrome or chrome oxide, that has been thinned to allow for about 5%-15% of the incoming light to be transmitted through the material. This partial transmittance of the optical beam through the phase attenuator allows phase shifted light to be produced, which can "interfere" with the non-phase shifted light, thereby improving the edge sharpness and increasing the depth of focus. The thickness of the absorber, as well as the material composition of the absorber, is critical and determines the amount of attenuation and phase shift that occurs. For DUV wavelengths (193-248 nm), the attenuator thickness of Cr films becomes extremely small (&lt;100 Å) and poses a significant practical problem in controlling its uniformity across a mask plate and then etching patterns reliably with control.
SUMMARY OF THE INVENTION An attenuated phase shifting mask (PSM) using buried absorbers and methods for manufacturing such masks are described. The mask of the present invention utilizes buried absorber regions, wherein absorbers for the mask are embedded in the mask substrate and are not formed on the surface of the mask. Light scattering at rough vertical edges of absorbers of prior art masks are reduced or eliminated with the mask of the present invention, since much of the scattering is reflected back in to the substrate to improve the projected image.
Unlike the prior art masks, the properties (material composition and thickness) of the buried absorber are utilized to control the phase and attenuation parameter. The depth of the absorber regions from the surface of the mask surface establishes an offset distance which can be used to adjust the phase shifting parameter. Thus, the amount of attenuation and the amount of phase shifting to be achieved (usually 180 controlled independently of each other.
Methods are described for fabricating masks of the present invention. In one method, trenches are formed in a quartz substrate and filled with an absorber material. After planarization, a dielectric layer is formed over the surface of the mask and the filled trenches or another quartz layer is bonded over it and then etched back. The thickness of the trench determines the thickness of the absorber region and the thickness of the overlying layer determines the offset distance of the absorber region from the surface of the mask. If the absorber induced phase shift is insufficient to provide the required 180 self-aligned blanket flood exposure is performed from the backside to selectively expose the quartz regions where no absorbers are present. The quartz is then etched in the nonphase shift regions to a depth d equal to the offset parameter.
In an alternative method, a portion of a substrate surface is exposed by an overlying patterned oxide layer. Subsequently, ions are implanted into the exposed regions to form absorber regions within the substrate at a specified depth. The dosage of the ions determines the thickness of the absorber regions and the implantation energy determines the depth, which corresponds to the offset distance of the absorber region from the surface. If required, a selective quartz etch can be used to further adjust the phase shift to provide the required 180
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An attenuated phase shifting mask, having buried absorbers, for use in submicron optical lithography to fabricate semiconductor devices is described. In the following description, numerous specific details are set forth, such as specific structures, processes, chemical compositions, etc., in order to provide a thorough understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known processes and structures have not been described in detail in order not to unnecessarily obscure the present invention.
PRIOR ART Referring to FIG. 1, a typical imbedded, attenuated phase shifting mask (PSM) 10 well-known in the prior art is shown. Mask 10 is comprised of a quartz substrate 11 containing a patterned feature of metallic absorber film 12 formed on its surface. The PSM 10 is designed such that transmitted light rays 13 traversing through the absorber film 12 are shifted 180 degrees in phase as compared to light rays 14 traversing only through the quartz. The absorber material is specifically selected so that it provides both the required 180 degree phase shift and the required 80%-90% attenuation of transmitted light. While this approach appears simple, in most practical applications, it is difficult to make the absorber material flexible for operating over a wide range of wavelengths (for example, between 193 nm-365 nm). Because the absorber (attenuator) 12 controls the phase and transmission characteristics together, it is difficult to independently adjust either phase or attenuation of light transmitted through the absorber 12 without affecting the other.
In FIG. 2, another prior art PSM 16 is illustrated. Similar to mask 10 of FIG. 1, mask 16 also has light absorbers (attenuators) 18 of a given thickness disposed on a quartz substrate 17. A dielectric film layer 19 typically formed from a dielectric material such as SiO.sub.2 is blanket deposited (by CVD or other process) over the surface of the absorber 18 and quartz 16. After a second aligned lithography step that selectively protects the deposited SiO.sub.2 over the absorber regions, the SiO.sub.2 is etched to remove the material from all the non-shifting regions. The dielectric layer 19 functions also to phase shift the light transmission through it so that now total phase shift is determined, not only by the absorber 18, but also by the thickness of the dielectric layer 19. Essentially, light rays 20 traversing though the absorber 18 and dielectric layer 19 should have a phase difference of 180 compared to light rays 21 traversing through the dielectric layer 19 only.
A hybrid of the above two prior art approaches is illustrated in FIG. 3. A PSM 23 is comprised of a quartz substrate 24 with absorbers (attenuators) 25 formed thereon. However, in this instance, the dielectric layer 26 is formed on the quartz substrate 24 first and then, the absorbers 25 are formed on the dielectric layer 26. Then, using the absorber as a mask, the SiO.sub.2 is etched away over the non-shifting regions, so as to leave the SiO.sub.2 only below the absorbers.
PRESENT INVENTION Referring to FIG. 4, a mask 30a of the present invention is shown. Mask 30a is an planarized structure having light absorbers 32a fully embedded (burried) within a highly polished quartz substrate 31a. The quartz substrate 31a is equivalent to that of the prior art quartz substrate or mask blank earlier noted, however, now the absorbers 32a are disposed completely within substrate 31a. Therefore, unlike the prior art masks 10, 16 or 23, the absorbers 32a are fully "imbedded" within the quartz 31a. Because the absorbers are no longer on the surface of the quartz, the surface 33a of the substrate 31a is essentially flat, thereby allowing for the mask 30a to have a planarized surface 33a. The image pattern formed by the absorbers 32a is equivalent to that of the prior art PSMs.
The mask 30a of FIG. 4 is equivalent in operation to the imbedded, attenuated PSM shown in FIG. 1. That is, the absorbers 32a function to provide both the required attenuation and the 180 absorber 32a material is selected so that it provides both the attenuation and the phase shift. This requires the choice of a material in which the phase shift is controlled by the imaginary part of the index and is, thus, not overly sensitive to thickness. That is, the complex index of refraction (n.sub.eff) of a material is given by the equation
n.sub.eff =n'-ik'
where n' is the real part of the index and k' is the imaginary part responsible for the absorption. For quartz, k�0. For most metals, k&gt;&gt;0 and thus the real part of n.sub.eff is given by
Re n.sub.eff =SQRT((n').sup.2 +(k').sup.2)
If k&gt;&gt;n, then the index and phase shift are primarily controlled by k. Thus, k essentially controls the phase shift, while the thickness "t" controls the transmission given by the equation
I=(Io)(e.sup.-&#8733;t)
where Io is the incident intensity, I is the transmitted intensity, ∝ is the absorption or extinction coefficient for a given material and t is the thickness. Essentially, the thickness of the absorber material provides the attenuation and the 180 to the PSM of FIG. 1.
The thickness t still pertains to the thickness of the absorbers 32b, but offset distance d.sub.1 now corresponds to the depth of the absorber 32b and offset distance d.sub.2 corresponds to the depth of the trench from surface 33b, which is the upper surface of substrate 31b residing in the phase-shifted regions (above absorbers 32b) of mask 30b. The additional thickness of quartz 36b above the absorbers 32b provides additional phase shifting for light transmitting through this phaseshifting region.
Thus, in this instance, the absorbers 32b need not necessarily provide the full 180 the absorber material is selected to provide the 85-95% attenuation and some phase shift. The offset distance d.sub.2 provides the rest of the phase shift. The depth of the trench 35b is designed so as to provide the remaining phase shift to bring the total phase shift to 180 significant advantage of this type of mask 30b is that less constraints are placed on the absorber 32b, since full 180 provided strictly by the absorber 32b. The remaining phase shift is provided by the overlying quartz region 36b. A suitable selection of the absorber material and quartz can be made over a very wide range using this technique, since the processing is independent of the absorber material.
It is to be appreciated that the offset distance d.sub.2 is determined by the depth of the trench 35b relative to the surface 33b. Thus, when an etching step is used to form trenches 35b, the quartz in the non-shifting region is etched to a distance d.sub.2. The depth of the trenches 35b can reach the level of the absorbers 32b, but should not exceed the depth where absorbers 32b are located. Thus, when constructing the mask 30b by utilizing the steps described in reference to FIG. 1, absorbers 32b need not lie at a precise distance from the surface of the substrate. However, the absorbers 32b must lie at a distance d.sub.2 or greater from the surface of the mask.
Essential, offset distance d.sub.1 is selected to be equal to or greater than distance d.sub.2. It is desirable to make distance d.sub.1 much greater than the depth of focus to eliminate imaging of surface defects. Trench depth d.sub.2, in combination with thickness t provide the necessary phase shift. In mathematical terms:
d.sub.1 &amp;gt;&amp;gt;Depth of Focus (DoF),
d.sub.1 &#8807;d.sub.2 and
Total&#966;shift=&#966;(t)+&#966;(d.sub.2)
For both types of masks 30a and 30b, the offset distance (d or d.sub.2) is calculated based on the equation
where λ is the wavelength, n is the refractive index of the shifter material and i is a multiple factor (i=1 for a 180 The offset distance will become smaller as the exposure wavelength shortens. For an exposure wavelength of 365 nm (nanometers), the offset distance is approximately 4000 Å for a 180
Thus, by using the buried absorber, attenuated PSMs of the present invention, a number of advantages are derived over the prior art attenuated PSM technology. Most notable is the ability to reduce edge distortion so as to permit masks for use at shorter wavelengths. The material and thickness of the absorber basically will control the amount of attenuation and perhaps the phase shift. Where the absorber material does not provide the required phase shift, the offset distance d.sub.2 is used to provide the flexibility of adjusting the amount of phase shift. Because these parameters can be adjusted , design latitudes allow for mask design over a wide range of wavelength. This also permits the use of a variety of materials for the absorber, since phase shifting parameters need not necessarily be built into the constraints imposed on the absorber with the second type of mask 30b. Also, pellicles are not necessarily needed if the absorbers are at a depth greater than the DoF.
Metallic materials, such as chromium, molybdenum and tantalum, are preferred for the absorber. Other metallic materials are Au, Ti, Mo, W, Ni, Sn, SnO.sub.2 and Ga. However, semiconductor, such as silicon and germanium can be used as well, if desired. In some instances, insulators, such as C, SiC and Si.sub.3 N.sub.4, can be used as well. It is to be appreciated that the invention is not limited to these elements or compounds and that other materials can be readily utilized as well. Finally, where the present invention utilizes a mask substrate of one material, typically quartz, and an absorber formed from another material, so that scattering due to interfaces of different materials are reduced to a minimum.
Referring to FIG. 11 A, a dielectric layer 55, such as a silicon oxide SiO.sub.2 layer, is deposited over the quartz substrate of FIG. 10. Since it is deposited on a planar surface, the topography of this layer 55 has substantial thickness uniformity. Layer 55 is deposited to a depth d, which corresponds with the offset distance d of the absorber from the surface of the substrate 51. Any of a wellknown prior art techniques, such as chemical vapor deposition (CVD) process, can be used to deposit layer 55. The absorber material 53 is fully encapsulated and has the offset distance d determined by the thickness of the dielectric layer 55.
However, since a CVD deposited dielectric material, even SiO.sub.2, will have an index of refraction different than that of quartz, precise matching of layer 55 to quartz will likely result in phase shifting problems. This is particularly true at 193 nm where SiO.sub.2 absorbtivity is high. Thus, an alternative (and slightly more complicated) process is described in reference to FIG. 11B. In this approach, a quartz plate 58 is bonded on to the quartz substrate 51 of FIG. 10. This second quartz layer 58 should be of sufficient thickness for etch-back. A thickness of approximately 20 mils is sufficient, although the actual thickness is a design choice. Although a variety of techniques could be used to bond quartz layer 58 onto the quartz substrate 51, the preferred step is performed in a rapid thermal processor (RTP) at a temperature of approximately 800 degrees centigrade in a nitrogen ambient. The higher temperature ensures an extremely strong quartz-to-quartz bond and the nitrogen ambient strengthens the bond so that subsequent delamination does not occur.
Referring to FIGS. 12-15, a quartz substrate 61 or mask "blank", equivalent to that of quartz substrate 51, is coated with an oxide layer 63, such as a SiO.sub.2 film layer, to a thickness of approximately 1 μm using a prior art technique, such as deposition by a CVD process. Next, the SiO.sub.2 layer 63 is coated with a photoresistive layer 62 using known techniques for depositing photoresists. The photoresistive layer 62 is then patterned using a known lithographic technique, such as the afore-mentioned e-beam or optical exposure process. After forming the pattern on layer 62, which is shown in FIG. 13, the underlying SiO.sub.2 layer 63 is patterned using an etch process, preferably a dry etch process using fluorine based chemistry, to expose portions of quartz substrate 61.
After patterning the SiO.sub.2 layer 63, the photoresistive layer 62 is stripped. Next, the substrate 61 is subjected to an implantation step in which metal ions are implanted into the exposed portions of the quartz 61 by high energy implantation as shown in FIG. 14. The SiO.sub.2 functions as a shielding layer so that the implantation occurs only into the exposed quartz. Implantation dosage in the range of 3 -2 implant metal ions to a depth ranging from 5000 Å to 2 μm. At these dose and energy levels, the metal ions will form a continuous metallic layer 65 in the quartz 61 matrix, wherein layer 65 will have a thickness in the range of 500 Å-800 Å. The actual depth of implantation, as well as the thickness of the implanted layer 65 is a design choice and will be determined by the application of the mask for which it is being fabricated. Furthermore, a variety of metal ions can be used for implantation. Although not limited to these ions, examples of metal ions are chromium (Cr.sup.+), gold (Au.sup.+), titanium (Ti.sup.+) and tantalum (Ta.sup.+).
Finally, as is shown in FIG. 15, the remaining SiO.sub.2 63 is stripped and the quartz substrate 61 is annealed at approximately 1000 degrees centigrade to remove any residual implant damage in the quartz 61. The substrate 61 is then cleaned and inspected. The implanted layer 65 is the buried absorber layer of the PSM 30a. Thus, the thickness of this layer 65 corresponds to thickness t of the buried absorber and the implant depth corresponds to the offset distance d of the mask. By utilizing this method, absorber thickness t and offset distance d can be tightly controlled.
In FIG. 16, a photoresistive layer 38 is formed over the planar surface of substrate 31. Then, the photoresist layer 38 is subjected to a backside flood exposure of ultra-violet light 39. The absorbers 32 operate as a self-aligned mask in order to selectively expose only portions of the photoresistive layer 38. Thus, only the photoresist overlying non-shifting regions are exposed. Next, with the photoresistive layer 38 patterned, the quartz in the non-shifting regions is etched using the afore-mentioned quartz etch technique, which is well characterized. The quartz is etched to the required offset distance d to form trenches 35b in the non-shifted regions. The remaining photoresistive layer 38 is stripped and the mask cleaned to obtain the mask 30b, which is shown in FIG. 17. The purpose of the overlying quartz region is to provide the additional phase shift necessary to obtain the required total phase shift (usually 180
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional diagram showing a prior art imbedded, attenuated PSM, having light absorbers on its surface.
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