摘要:

We propose a novel form of x‐ray projection lithography that: (1) requires no mask, and hence can be considered an x‐ray pattern generator; (2) is, in principle, capable of reaching the limits of the lithographic process. The new scheme utilizes an array of Fresnel zone plates, and matrix‐addressed micromechanical shutters to turn individual x‐ray beamlets on or off in response to commands from a control computer. Zone plate resolution is approximately equal to the minimum zone width, which can approach 10 nm. Zone plates are narrow‐band lensing elements: For a diffraction limited focus, the source bandwidth Δλ/λ should be less than or equal to the reciprocal of the number of zonesN. An undulator havingNumagnetic sections emits collimated radiation in a bandwidth Δλ/λ=1/Nu.Nuis usually in the range 35–100. We present a system design based on 25 nm lithographic resolution using λ=4.5 nm. ForN=100 the zone‐plate diameter is 10 μm. Each zone plate of the array would be responsible only for exposure within its ‘‘unit cell.’’ To fill in a full pattern, the stage holding the sample would be scanned inXandYwhile the beamlets are multiplexed on and off. A microundulator designed for installation on a commercial compact synchrotron can provide 87 mW within a 2% bandwidth around 4.5 nm in a divergence cone of 0.28 mrad. The calculated efficiency of first‐order focus for a zone plate operating at 4.5 nm is 31%, using 130 nm of spent U as the absorber/phase shifter. An exposure rate of ∼1 cm2/s at 25 nm resolution appears feasible.

ISSN:0734-211X    

DOI:10.1116/1.589044

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

This article discusses the effects different process flows have on the material and control requirements for refractory (subtractive) x‐ray masks. Our investigation of x‐ray masks shows that a film’s stress uniformity is more critical (for distortion) than the magnitude of the stress. In particular, we find for the wafer sequence the requirement on the membrane film’s stress uniformity is the most critical specification. For the membrane sequence, the requirement is for stress uniformity for films that are removed from the membrane (after pattern formation). We find that SiC reduces the distortions from the film stress gradient, however, the frame structure determines the distortions from a film’s mean stress. Finally, we conclude the best choice of an x‐ray mask flow should be decided on our ability to modify wafer equipment and processes, to control the stress uniformity of the membrane film, to control the stress and uniformity of the resist, hardmask, and absorber films, and finally to handle the lithography required to create predistorted mask patterns.

ISSN:0734-211X    

DOI:10.1116/1.589045

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

The x‐ray mask fabrication process we developed can be used to make and maintain essentially defect‐free masks consisting of Ta absorbers on SiN membrane. The surface of the deposited SiN substrates is polished to make them as smooth as possible. As Ta is chemically stable, mask fabrication processes are frequently followed with a wet‐cleaning process using a strong acid solution. Inspection of resist patterns printed on a wafer confirms that there are less than 20 printable defects per mask, and there are no defects on the back surface. The inspection and repair are repeated until the mask is defect free. Defects caused by mask handling and x‐ray exposure are immediately washed up by wet cleaning with strong acid. The resulting x‐ray masks have been used in large‐scale integrated circuits fabrication, and fully functional devices have been obtained.

ISSN:0734-211X    

DOI:10.1116/1.589046

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

A new method to correct x‐ray mask distortion was developed. This method uses the correction method of previous analysis of distortion and transformation of coordinates (PAT) and a simulator to calculate pattern displacement caused during backetching. The key advantages of using PAT with a simulator are that no extra time, substrates, or labor are required to fabricate send‐ahead masks. Using this correction method, it was confirmed that the 3σ value of the pattern placement error was reduced to below 76 nm. This indicated that PAT with a pattern displacement simulator was as useful for improving the pattern placement accuracy of x‐ray masks as PAT with send‐ahead masks. In addition, analysis of the remaining error made it clear that the correction error was mainly caused by the lack of e‐beam writing accuracy.

ISSN:0734-211X    

DOI:10.1116/1.589047

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

The belief that proximity x‐ray lithography is insensitive to contamination has been based on the assumption that contaminants are small hydrocarbon compounds. However, we will show that x rays may induce photoassisted processes that deposit saltlike films on the mask surface. We will also provide an explanation for the film geometry in the early stages of its growth and suggest some possible solutions to the issue.

ISSN:0734-211X    

DOI:10.1116/1.589048

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

We have evaluated and implemented a multiple‐pass writing scheme that significantly improves the image‐placement performance of masks for proximity x‐ray lithography. Masks were fabricated using a 75 kV EL‐3+ electron‐beam lithography system that separates the data into fields and subfields, and exposes the images by using a variable‐shaped beam. Multiple‐pass writing allows averaging of system noise between multiple exposure passes written at fractional doses [Jpn. J. Appl. Phys.32, L1707 (1993)]; stitching errors can also be averaged by offsetting the locations of the tool field and subfield boundaries for each pass [Jpn. J. Appl. Phys.32, 5933 (1993)]. Multiple‐pass writing was evaluated both with and without boundary offsets. Our experiments indicated that the offset method resulted in better image placement but negatively affected image size and defect performance because of the EL‐3+system limitations. The no‐offset method was optimized and implemented and achieved sub‐50 nm (3σ) image placement. The method was then transferred to the EL‐4 electron‐beam lithography system, resulting in image placement of sub‐30 nm (3σ).

ISSN:0734-211X    

DOI:10.1116/1.589049

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

We describe a technique to fabricate (1×) x‐ray masks utilizing an optical reduction tool (SVGL Micrascan II). A special 1× x‐ray mask chuck was constructed to fit the optical tool. The technique takes advantage of the relatively flat x‐ray mask substrates and the required low throughput which allows replication without compromising the printing tool depth of focus and the resist process. The potential advantages of this technology are: (1) improved placement accuracy relative to conventional e‐beam tools; (2) no (e‐beam) proximity effects; and (3) lower post processing data volumes. The placement distortion and the critical dimension uniformity obtained in the 1× daughter masks on a 22×32 mm field are similar in magnitude to x‐ray masks fabricated with e‐beam lithography utilizing product specific emulation techniques. Furthermore, the distortion seen is believed to be due primarily to the known distortion of the Micrascan tool lenses. This suggests that it may be possible to use the results of lens distortion characterization to adjust the pattern of the reticle so as to correct for the lens distortion. Lens correction in combination with product specific emulation may allow the fabrication of daughter masks with near‐zero placement errors, suitable for sub‐250 nm product applications.

ISSN:0734-211X    

DOI:10.1116/1.589050

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

This article presents a new analysis method and experimental results for the deflection and vibration of x‐ray masks, which are caused by the squeeze effect of the gas film between a mask and a wafer in x‐ray steppers. The deflection and vibration occur when a mask‐to‐wafer gap setting is executed or if the wafer vibrates in the mask direction during stepping motion with a narrow gap. They are not only detrimental to the throughput, but may also damage the mask membrane. Lees’ difference approximation method is applied to a compressible Reynolds’ equation and the equation of motion for the mask membrane. The main results of calculations and experiments, which showed good agreement with each other, are outlined below. (1) When the wafer approaches the mask at a constant velocity, the mask deflection increases in proportion to the velocity, and the mask deflection is larger for a smaller mask tensile stress. (2) When the wafer vibrates in the mask direction, the amplitude of the mask vibration increases with increasing the wafer frequency, and reaches a maximum value at a frequency that depends on the gap size. The maximum amplitude of a tested mask becomes 4 times as large as the wafer amplitude. (3) For a high frequency and narrow gap, the mask vibrates while deflecting convexly in the direction opposite to that of the wafer.

ISSN:0734-211X    

DOI:10.1116/1.589051

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

Image placement demands for x‐ray masks useable in sub‐0.25 μm applications have prompted a renewed effort to further reduce or eliminate sources of mask distortion. In this work we studied a phenomenon that so far had been overlooked among the contributors to poor image placement: the large change in stress of some resists upon exposure. For instance, it was found that the stress of PMMA films changes from ≂1×108dyn/cm2in the unexposed state to ≂1×107dyn/cm2after the exposure dose received during electron‐beam (e‐beam) writing. Becausee‐beam writing is a serial process, this stress change induces a dynamic distortion of the mask, since the force applied on the membrane by the resist film varies continuously as writing of the pattern progresses. The stress of PMMA films was determined as a function of dose for exposures usinge‐beams of 100, 75, and 25 keV energy, broadband UV light (from a Hg lamp), and to x rays from a synchrotron ring with λcharac.=10 Å. In all cases, the stress was found to be a very strong function of the exposure dose at low doses with a slow decay in stress change at larger dose values. The stress changes upon exposure are attributable to changes in molecular weight (MW) distribution, rather than to a reduction in MW only, since the stress of unexposed PMMA films was found to be the same for a wide range (25k to 920k) of initial MW resin. Model experiments demonstrated the correlation between the in‐plane distortion (IPD) of membranes and the lowered resist stress. Because of the drastic stress drop at low doses, fogging prior to writing reduces the distortion problem. Other methods thought to reduce resist stress (various annealing cycles, different solvents, plasticizers) proved unsuccessful. The effect of fogging on IPD was measured for several model cases and found to correlate with the lowered resist stress. Initial evaluation of the changes that fogging induces on the lithographic performance of the resist was performed.

ISSN:0734-211X    

DOI:10.1116/1.589052

出版商:American Vacuum Society    

年代:1996

数据来源: AIP

摘要:

Advanced x‐ray masks for giga‐bit DRAM require an optimization of the mask fabrication process and a stress‐free absorber. In this article, we will describe the performance of the x‐ray masks fabricated by a membrane process where back‐etch is performed prior to electron beam writing. With other techniques such as stress control by step‐annealing and electron beam (EB) multiple writing, mask‐to‐mask overlay was reduced to less than 40 nm. In spite of multiple EB writing, the pattern size accuracies of 0.14 μm critical dimension were about 10% and 20% for the line and two‐dimensional patterns, respectively. For further improvement in the uniformity of absorber stress and resultant higher placement accuracy, a space variant annealing method is proposed, where the nonuniformity of the absorber stress is corrected by spatially variant temperature distribution during the annealing. Under this method, the uniformity of the amorphous W–Ti absorber improved to about 10 MPa in a 50 mm diam region.

ISSN:0734-211X    

DOI:10.1116/1.589053

出版商:American Vacuum Society    

年代:1996

数据来源: AIP