This invention relates to a mask blank and a mask and, in particular, relates to a mask blank and a mask for electron beam exposure. 
In the lithography technology for forming wiring patterns and so on, as patterns to be formed become extremely finer and finer, the pattern formation becomes difficult in the conventional optical lithography technology that is used as a general purpose technology.  Therefore, a serious investigation has been directed to an exposure technique using a short-wavelength beam, like a charged particle beam such as an electron beam or an ion beam, an X-ray, or the like so as to achieve further finer pattern formation.  Among them, the direct-write electron beam technique has been developed from initial point beam writing, then a variable shaping projection method that performs projection by changing the size and shape of a rectangular beam.  Thereafter, proposal has been made about a partial batch projection method that collectively projects or delineates a part of a pattern through a mask and repeats it in terms of improving pattern accuracy, shortening a projection time, and so on.  Then, following the partial batch projection method, a new electron projection system (SCALPEL system) has been proposed by S. D. Berger et al.,  in APPL.  PHYS.  LETTERS 57 (2) (1990) 153.  Thereafter, various proposals have been made about a similar projection system (PREVAIL system) and a transfer mask (reticle) structure for applying to those projection systems and its manufacturing method. 
For example, Japanese Unexamined Patent Application Publication (JP-A) No.  H07-201726 (corresp.  to U.S. Pat.  No.  5,466,904) relates to a PREVAIL system invented by H. C. Pfeiffer, wherein, schematically, preparation is made of a stencil mask formed with a through-hole (aperture) pattern of a predetermined size and arrangement in each of small regions and a charged particle beam is irradiated onto each small region so that a beam shaped through each through-hole pattern is applied onto an exposure-subject substrate formed thereon with a photosensitive material through an optical system, thereby transferring the through-hole patterns onto the exposure-subject substrate on a reduced scale.  In addition, a device pattern is formed on the exposure-subject substrate while connecting together predetermined patterns formed in a divided manner on the mask.  Transfer masks proposed for this system each use, as a main structure, a stencil-type mask having pattern portions formed by through holes that are uncovered at all (for example, see Japanese Unexamined Patent Application Publication (JP-A) No.  H10-261584 (corresp.  to U.S. Pat.  No.  5,935,739), Japanese Unexamined Patent Application Publication (JP-A) No.  H10-260523 (corresp.  to U.S. Pat.  No.  5,972,794)).  In the stencil-type mask, a pattern region is divided and reinforced from the back side by a strut (crossbar) structure to achieve a reduction in deflection of the pattern region.  This structure is effective to achieve an improvement in pattern position accuracy and so on. 
On the other hand, as a mask structure for the SCALPEL system, scattering masks (reticles) have been mainly proposed rather than the stencil masks.  These are specifically described in, for example, the above-mentioned literature by S. D. Berger et al.,  the above-referenced Japanese Unexamined Patent Application Publication (JP-A) No.  H10-261584, Japanese Unexamined Patent Application Publication (JP-A) No.  H10-321495, and soon.  According to the description thereof, the mask structure is such that a heavy metal layer is formed on a membrane (self-standing thin film) of SiN or the like and desired pattern formation is applied to this heavy metal layer.  Under the circumstances, when an electron beam is irradiated onto both layers, the electron scattering rate differs in dependence on the presence or absence of the electron beam scatterer.  Therefore, a method is proposed such that a reduced pattern is transferred by obtaining a beam contrast on a wafer by the use of the difference in a scattering rate. 
These exposure systems each satisfy high resolution that is a feature of the charged particle beam and enable pattern formation that is finer than 0.1 μm and, as compared with the partial batch method, achieve a large increase in shot size (e.g. the maximum shot size on an exposure-subject substrate increases from 5 μm to 250 μm) and an improvement in throughput in the manufacture of devices (e.g. exposure throughput of 30/hour or more at nominal in the case of 8-inch substrates and a minimum line width of 0.08 μm) because of the speedup of the system and so on.  The systems make it possible to realize an apparatus capability of enabling adaptation to production of general-purpose devices, and thus are highly practical systems. 
As described above, there have been released various proposals about the new exposure systems, about the transfer mask (reticle) structures for applying to those systems, and about the methods of manufacturing the masks.  However, the situation is that there are various problems about various proposed mask structures in terms of practicality.  Hereinbelow, those various problems will be briefly described. 
The mask structures hitherto proposed are roughly classified into the two types that are the stencil mask where a pattern has a through-hole structure and the scattering mask where an electron beam scatterer made of a heavy metal is formed on a thin film transmitting layer having a thickness of 100 to 200 nm.  Typical structure diagrams of them are shown in FIGS.  15 and 16. 
As shown in FIG.  15, since transfer pattern portions of the stencil mask are through holes 1 formed in a membrane 2, there is almost no energy loss of projection electrons while, because of a pattern with a high aspect ratio, there are a problem about pattern size accuracy and a problem about mask strength since a pattern region (thin film portion) is thin (e.g. 2 μm) and has a through-hole structure.  As a countermeasure for this, there is known a technique that improves the processing accuracy and strengthens the mask structure by forming struts (crossbars) (illustration is prepared) for supporting the pattern region (pattern field) from the back side of the mask.  However, in the case of the through-hole structure, a ring-shaped (donut-shaped) pattern or the like cannot be formed in a completely independent form.  As a coping method in this case, as H. Bohlen et al.  describe in Solid State Technology, September (1984) pp.  210, there has been proposed a method of preparing complementary masks for assembling a desired component element pattern and of forming the pattern by overlapping complementary patterns.  However, in this method, the masks are required twice at minimum and, further, the number of exposure shots increases to cause a large increase in exposure time, thereby reducing the exposure throughput.  Further, there is also a demerit that mask data having a pattern properly divided is required per device pattern. 
Further, when a reduction in thickness of the pattern region (thin film portion) is carried out for improving the processing accuracy (pattern size accuracy), a new problem arises.  The transfer pattern portions of the stencil mask are through holes.  No particular problem arises if a pattern formed in this case is only contact holes or a short line pattern.  However, for reasons of element pattern design, problems often occur in cases where a pattern support portion is a cantilever-shaped pattern (hereinbelow referred to as a leaf pattern).  In such cases, the leaf pattern is subjected to shrinkage displacement in a longitudinal direction (perpendicular to the surface of the mask) due to various conditions.  Further, in the case of a line pattern having a high pattern density (e.g. LS ratio is 1:1) and being a fine pattern, a mechanical strength in a lateral direction (horizontal or parallel to the surface of the mask) is also reduced.  In this case, if the Young's modulus of a material of the mask body is very large, it is possible to reduce the shrinkage displacement.  However, even by applying a polycrystalline diamond film having the largest elastic modulus currently known to the aperture body, it is difficult to reduce the shrinkage displacement of the pattern region to a practical level as long as the thickness of the pattern support portion is reduced.  In addition, in the SCALPEL or PREVAIL type apparatus, since the mask constantly moves at high speed in terms of the exposure system, when judging from a microscopic viewpoint, a very large force is exerted also on the mask pattern (including the leaf pattern) in the lateral direction.  That is, the viewpoint of mask rigidity becomes important not only in the longitudinal direction but also in the lateral direction (parallel to the surface of the mask).  However, since bending stress, torsional stress, and so on are applied to the leaf pattern portion due to high-speed movement of a mask stage and stress concentration occurs at the leaf pattern support portion, damage of the pattern might occur. 
On the other hand, in the SCALPEL mask (electron beam scattering mask), there arise, in addition to a problem of durability of the mask, a problem of a loss in transmitting electron amount due to electron beam scattering at an electron beam transmitting layer (film) (referred to as a pattern support layer (film) or a membrane) caused by the mask structure.  Description will be given on the basis of a sectional structure diagram (FIG.  16) of the foregoing electron beam scattering mask.  The electron beam scattering mask ensures a contrast by a difference in electron scattering angle, which is caused depending on the presence of an electron beam scatterer 5, and restricted apertures.  However, since a difficulty of film self-standing arises only by the electron beam scatterer 5 made of a heavy metal, it is necessary to form a pattern support layer 6 for the purpose of supporting the heavy metal scattering layer. 
In the mask of this structure, there arises a contradictory problem between the thickness of the pattern support layer for supporting the electron beam scattering layer and the electron transmittance.  That is, a material of the known pattern support layer is a SiN-based material or a Si material and, other than it, a diamond film or the like has also been proposed.  As properties required for those pattern support layer materials, it is preferable that the material density be low and the material strength properties such as the Young's modulus be excellent.  In other words, it can be said to be more preferable as the electron transmittance at the pattern support layer becomes more excellent and as the elastic modulus or the like of the material increases.  In terms of only the electron transmittance, the problem can be solved by providing a higher acceleration voltage of a charged particle beam or reducing the thickness of the pattern support layer.  Since the acceleration voltage of an electron source used in the SCALPEL or the like is a high acceleration voltage like 100 KeV or more, electrons achieve a transmittance of substantially 100% in the case of a thickness (50 to 200 nm) of a pattern support layer described in, for example, U.S. Pat.  No.  5,260,151.  However, electrons are scattered in any substance.  Herein, it is noted that the scattered electrons pass through the pattern support layer and an electron outgoing angle from the pattern support layer has a predetermined range.  In this event, the electrons having outgoing angles outside the predetermined range cannot pass through restricted apertures provided at an upper portion of an exposure-subject substrate in the exposure apparatus.  This results in a reduction in rate of the electrons for exposure (referred to as exposure electrons).  In order to reduce the number of scattered electrons outside the predetermined range, in other words, in order to increase the number of electrons that are transmitted without scattering, there is no alternative but to reduce the thickness of the pattern support layer being the support body.  However, in the case of the heavy metal scatterer, for example, in the case of the scatterer of tungsten, a thickness of about 50 nm is sufficient for ensuring electron scattering of several times but, when, for example, a silicon nitride (SiN) based pattern support layer is applied, a SiN film having a thickness of about 100 to 150 nm is required in order to support the scatterer having the thickness of 50 nm in the case where the thickness of the film is set in terms of the material strength properties.  When the pattern support layer of this thickness is used, electron scattering due to the pattern support layer under the acceleration voltage of 100 KeV is very large so that the rate of electrons that can contribute to exposure decreases to several %.  If the thickness of the SiN pattern support layer is reduced, the pattern support layer is easily subjected to shrinkage due to self weight of the tungsten scatterer and, further, cannot bear a number of processing steps and, therefore, damage is liable to occur in the pattern support layer and so on. 
As described above, when the electron beam scattering layer made of the heavy metal is too thin, the excellent beam contrast cannot be obtained.  On the other hand, when the heavy metal electron beam scattering layer is increased in thickness for obtaining the excellent beam contrast, shrinkage occurs due to the self weight and damage and so on are liable to occur due to an increase in membrane stress change (change in warp) during the processing steps.  Further, in order to support such a heavy metal electron beam scattering layer, the thickness of the pattern support layer should be increased considerably so that there has been the problem that the loss of the exposure electrons increases.  Further, in the SCALPEL mask as described above, the reduction in thickness of the respective layers and the transmittance of non-scattered electrons are contradictory to each other so that it has conventionally been difficult to obtain a practical mask. 
Moreover, when a mask stage is moved at high speed while the mask is used like in the case of the stencil mask, the pattern region (thin film portion) including the electron beam scatterer might be damaged quite easily.  In addition, when the electron beam scatterer is made of the metal material such as tungsten, there is also a problem of surface oxidation.  In the case of a material that is easily oxidized with age, a membrane stress thereof shifts in a compression direction following oxidation.  As a result, it is presumed that it becomes difficult to ensure the mask performance over a long-term period. 
In order to solve the foregoing problems, the inventor of this application has proposed a SCALPEL mask where the thickness of an electron beam scattering layer is increased while the thickness of a pattern support layer is reduced (Japanese Unexamined Patent Application Publication (JP-A) No.  2001-77013 (corresp.  to U.S. Pat.  No.  6,812,473)).  The thickness of the electron beam scattering layer is increased (thickness 0.2 μm to 2 μm) to thereby achieve film self-standing of the electron beam scattering layer and, as a result, the thickness of the pattern support layer is reduced (thickness 0.005 μm to 0.2 μm) to thereby reduce loss of electron beams at the pattern support layer, aiming further to obtain optimal membrane stresses with respect to the respective pattern layers.  With respect to materials of the respective layers that enable such a mask structure, disclosure is made of, as the electron beam scattering layer, carbon element and/or silicon element, specifically, diamond like carbon (hereinbelow, DLC), a material obtained by doping at least one of B, N, Si, and P into DLC (doping amount is 0.1 to 40 mol %), or a material containing silicon as a main component.  On the other hand, as the pattern support layer, disclosure is made of DLC, a material obtained by doping at least one of B, N, Ti, Si, and Al into DLC (doping amount is 0.1 to 40 mol %), a material containing silicon as a main component, SiC, or TiC.
The pattern support layer disclosed in the foregoing publication has a problem that the membrane stress changes from a tensional stress to a compressive stress with the lapse of time and it has been found that a change amount thereof increases as the membrane thickness decreases.  However, the membrane thickness is preferably thinner in terms of the electron beam transmitting amount and so on so that it becomes necessary to form a pattern support layer having excellent membrane quality for reducing the time-dependent change of the stress with the thin membrane thickness.