The ITRS roadmap indicates that significant improvements in photomask processing will be necessary to achieve the design goals of 45nm technology node masks. In the past, etch systems were designed to produce an etch signature that was as "flat" as possible to avoid introducing undesirable signatures in the final product. However, as error budgets are shrinking for all tools in the process line, the signatures produced by etch systems are used to compensate for some of the upstream CD issues. Process modifications have been used successfully in this fashion, but frequently process adjustment alone is not sufficient.
CD uniformity results from a complex interaction between the system and the sample. An etch system must be capable of adjusting radial, linear, and loading etch uniformity components to compensate for the specific needs of each sample. The adjustments should also be as independent of process as possible. Towards this end, experiments were conducted with various etch technologies to create specific, controllable etch signatures on demand without the need for hardware changes. CD data collected from binary chrome photomasks was used to verify performance of the uniformity adjustment technologies.
In any plasma etch process, there are slight variations in the output of generators, mass
flows, and pressure control systems that may sometimes contribute to run-to-run
differences in the final product. Even excluding material differences, endpoint times can
vary somewhat, requiring an accurate endpoint system to stop processing at the
appropriate time. The most widely accepted systems for plasma endpoint detection are
based on optical emission spectroscopy (OES). OES-based endpoint systems analyze the
visible and near-visible electromagnetic radiation emitted by the plasma in order to detect
subtle changes that occur when a film has been completely etched. A signal can be
constructed from this data and used to stop or otherwise modify the process.
Other methods exist for detecting endpoint. Laser reflectance is well known to
photomask etch engineers, but there are also lesser known methods that depend on
detecting changes in pressure, DC bias, or match network positions.
Each system has its own unique set of strengths and weaknesses. While all of these
systems are quite capable of detecting endpoint under normal circumstances, the
requirements of low load photomask etching are extremely demanding. Therefore, a
need exists to enhance endpoint detection on low load photomasks. Our proposed
method is a multi-sensor system that includes measurements of several process
parameters in addition to emission spectra to generate an endpoint signal that is more
robust than an endpoint signal produced by a single sensor.
The quartz dry etch is a critical step in the manufacture of Alternating Aperture Phase Shift masks (alt-APSM). In order to maintain uniform phase shift across the mask, the etch depth uniformity has to be strictly controlled. Both the radial and linear components of non-uniformity have to be minimized. The Mask Etcher IV developed at Unaxis USA reduces both the components of non-uniformity using unique hardware adjustments. Using a fluorocarbon based chemistry, etch depth variations between different feature sizes is also minimized. With good etch depth linearity, phase shift does not vary with feature size. To achieve this, etched quartz structures need to have good selectivity to resist / chrome and vertical sidewalls. Etch depth uniformity was measured using an n&k1700 RT and etch depth linearity was measured using an AFM. Etched quartz structure morphologies are observed using a SEM. After preliminary screening experiments, an optimized hardware suite and process conditions that produce good etch depth uniformity, linearity and quartz profiles with vertical sidewalls and minimum microtrenching is determined.
Alternating Aperture Phase Shift masks (alt-APSM) are being increasingly used to meet present day lithography requirements by providing increased resolution. The quartz dry etch is a critical step in the manufacture of these photomasks. Etch depth linearity, phase uniformity and minimum etched surface roughness are critical factors. To achieve this, etched quartz structures need to have good selectivity to resist / chrome, vertical sidewalls and good etch depth uniformity over the mask area. Using the Mask Etcher IV at Unaxis USA, a series of experiments were performed to study and identify the trends in quartz etching for photomasks. Etch depth uniformity was measured using an n&k1700RT and etch depth linearity from feature sizes ~0.4 micron to ~1.4 micron was measured using an AFM. Cross sections of the ~0.6 micron structure were obtained using a SEM to check for profile and any evidence of micro trenching. After several set-up experiments, an optimized process to minimize etch depth linearity and improve etch depth uniformity was obtained and is presented here.
To overcome the resolution limits of the current generation of steppers, mask makers are forced to include an ever-growing number of OPC features on 65 nm node masks. Although lithography techniques have improved significantly in the last five years, they have not kept pace with the needs of 65 nm technology. To produce viable OPC features at the 65 nm node, the etch process must be capable of accurately defining on the mask features as small as 100 nm. The etch must also show reasonable linearity to prevent distortion of the primary features. To this end, a four factor, irregular fraction factorial design was performed using a 4th generation mask etch system. The factors in this design include RIE power, RIE coupling efficiency, ICP power, and pressure. These factors were selected for their influence on CD bias, CD uniformity, and CD linearity. The results of this design will be presented, along with an optimized solution. This solution is demonstrated on an asymmetric test pattern representative of logic or ASIC devices, as well as an evenly loaded pattern more representative of memory devices.
The ITRS roadmap indicates that significant improvements in photomask dry etching will be necessary to achieve the design goals of 90nm and 65nm technology node masks. Although some existing dry etch systems are capable of R&D work on these masks, a new dry etch system is needed to achieve production worthy results. To this end, a new 4th generation mask etch system was designed and built by Unaxis USA, Inc. In early testing, the Unaxis Mask Etcher 4 has demonstrated significant improvements in CD uniformity and linearity compared to earlier systems. A designed experiment (DoE) was performed on this new system to more fully characterize its performance window. The results of these experiments are presented and compared to a standard process performed on a Unaxis Mask Etcher 3.
A mask patterning technology for the 90nm technology node has been developed using the FujifilmARCH resist FEP171 and the state-of-the-art mask making tools SteagHamaTech mask coater ASR5000, Leica 50kV variable shaped e-beam writer SB350, SteagHamaTech developer ASR5000 and UNAXIS Mask Etcher III. A resist resolution of below 100nm dense lines and 150nm contact holes was demonstrated. The line width shrinking due to chrome etching varies between 25nm and 50nm per feature and a corresponding resolution of 125nm dense lines in a 105nm thick chrome absorber has been achieved. The global CD-uniformity with a 3σ of 7.7nm and a total range of 10.8nm met the requirements of the ITRS roadmap. The local uniformity with a 3σ of 3.8nm and a range of 5.6nm offers potential for future application of the Leica SB350. Applying of a new correction method taking electron scattering and process characeristics into account provides a linearity of 6.1nm. In addition, the line width of different featurees was kept in a range up to 12nm when the local pattern density was changed. The composite placement accuracy of 12nm fulfills already the requirements of the 65nm node. A special investigation proved the excellent fogging depression of the SB350.
As the 193 nm generation of steppers reaches the limit of its capability, alternating aperture phase shift masks (altPSMs) are necessary to extend the lifetime of these tools. The fabrication of a production-worthy altPSM requires that the quartz dry etch satisfy many conditions. The etched quartz features must not only show excellent phase uniformity, but they should have near vertical sidewalls and good etch depth linearity across a wide range of feature sizes. Surface roughness must also be low enough that transmission is unaffected. To this end, Unaxis USA performed a series of quartz photomask dry etch experiments utilizing a Unaxis Mask Etcher III. Etch depth uniformity and etch depth linearity are studied for each experiment. SEM cross-sections of the etched profiles and AFM analysis of surface roughness are also provided. Various models were constructed by IBM that demonstrate the importance of some of the etch responses, and the results from the optimized Unaxis process will be shown.
Alternating Aperture Phase Shift Photomask (AAPSM) technology becomes more critical as the industry approaches 90nm design rules. AAPSM enables the lithographic process to extend the viability of deep ultraviolet (DUV) photolithography systems. However, manufacturing high quality AAPS masks is difficult due to the stringent requirements placed on the etch process.
Dry etch processes for alternating aperture masks must have good intra-mask phase uniformity, high Quartz/Cr selectivity, and vertical side-wall profile. While maintaining these parameters, it is also necessary for the process to achieve etch depth linearity across a range of feature sizes. Using a next generation Inductively Coupled Plasma (ICP), a series of experiments were performed to optimize the quartz etch process to improve feature size etch depth linearity and selectivity to resist. Etch results from an optimized solution are presented.
This work involved a demonstration of the infrastructure and the ability of mask-making equipment to produce 9 inch reticles. While the choices for this particular work made the timing and logistics long and complicated, we find that there currently exists adequate infrastructure to create 9 inch reticles and we have used this ability to produce several demonstration quality examples.
Positive tone chemically amplified resists CAP209, EP012M (TOK), KRS-XE (JSR) and FEP171 (Fuji) were evaluated for mask making. The investigations were performed on an advanced tool set comprising of a Steag coater ASR5000, Steag developer ASP5000, 50kV e-beam writer Leica SB350, UNAXIS MASK ETCHER III , STS ICP silicon etcher and a CD-SEM KLA8100. We investigated and compared resolution, sensitivity, resist slope, dark field loss, CD-uniformity, line edge roughness, and etch resistance of the evaluated resists. Furthermore, the influence of post coating delay, post exposure delay and other process parameters on the resist performance was determined.
A continuous improvement study of the Gen III ICP MoSi etch process is accomplished through the use of high resolution factorial experimental design (DOE). The main goal of this work is to more fully characterize the process space within a commercial GEN III MoSi plasma etch process reactor. Particular emphasis is placed upon the improvement of CD bias loss as well as isolated/dense feature linearity within the same mask pattern. CD uniformity is also monitored as well as MoSi etch profile. Several novel etchant gases are exported prior to the Designed Experiment to characterize the effect of alternate chemistries on MoSi etch performance; these results are reported. The Designed Experiment was utilized to optimize the most promising alternate gas chemistry in terms of CD performance, MoSi Etch Rate uniformity and Selectivity to Quartz. The novel gases included a known polymerizing etch gas as well as etch rate enhancement gases which have also historically been used within the silicon process industry to enhance selectivity to silicon dioxide and presumably, quartz.
As critical dimensions and exposure wavelengths approach the physical limitations of optical lithography, the use of newer techniques such as Embedded Attenuating Phase Shift Masks become necessary to extend the viability of DUV lithographic tools. One of the more common EAPSM materials is MoSi and its analogues. Current MoSi processes are challenged to meet future requirements of improved CD and phase uniformity, vertical profiles, and reduced Quartz substrate damage. To this end, a Next Generation ICP (Inductively Coupled Plasma) hardware configuration has been adopted with improved plasma uniformity and larger process window. In this article, a description and a performance characterization of this new ICP source and Chamber Fixturing Hardware is presented. Additionally, the MoSi plasma etch parameter space is explored utilizing Design of Experiments and preliminary process optimization is offered. Finally, process results including etch rate uniformity, CD uniformity, and sidewall profiles are discussed.
As critical dimensions and exposure wavelengths approach the physical limitations of optical lithography, the use of newer techniques such as Phase Shift Photomask Technologies become necessary to extend the viability of DUV lithography tools. Alternating Phase Shift Mask technologies are challenging the capabilities of current quartz dry etch processes; as this phase shift technique is achieved by the precise removal of quartz, the need for ever improving phase shift uniformity across the photomask surface requires extremely uniform quartz etch depth. To this end, a Next Generation ICP (Inductively Coupled Plasma) hardware configuration has been adopted. In this article, the quartz etch parameter space of this new ICP source is explored. Finally, process results including, quartz roughness, sidewall profile, and most importantly quartz etch rate uniformity will be presented.
The use of Plasma Etch for the fabrication of Binary Cr Photomasks has become a mainstream staple for advanced Reticle fabrication. These Binary Cr Reticles are suitable for the 0.18 micron technology Nodes and beyond and have forced an appreciation of the varying exposed Cr loads of different Mask layers.
As on-glass line widths shrink and exposure wavelengths approach the physical limitations of optical lithographic printing, the adoption of newer technology such as Off-Axis Illumination and Phase Shift Photomask technologies will substantially expand the operating life of DUV lithographic tools. In this article, the dry etch processes and Inductively Coupled Plasma (ICP) hardware iterations associated with the etch optimization of Levenson-style hard shifters are explored. These Alternating Aperture Hard Shifters currently adopt a single material form, with Levenson-style photomasks making use of a precise removal of quartz material between the Cr lines of a standard Photomask, improving the resolution of the exposed features. This precise quartz removal is performed utilizing dry etch technologies, with the use of high density, de-coupled plasmas such as ICP preferred. We explore Inductively Coupled Plasma shaping techniques along with newer etch processes for these materials, offering a Next Generation ICP Source design. Process conditions are verified and on-mask results are reported.
The use of Plasma Etch in patterning Binary Cr layers for modern reduction reticles has seen dramatic increase in the past two years. The drive towards the 0.25 micrometer and 0.18 micrometer technology has rendered wet etch of Binary Cr inadequate for the demanding gate level designs of most advanced devices. The use of dry etch for these patterns is studied closely through the pattern loading within a mask (Exposed Cr Load). It has been seen that Cr Load strongly affects several plasma etch responses, e.g.: resist selectivity, Cr etch rate, overall CD Uniformity and within- Mask CD Uniformity pattern.
The plasma etching of Binary Cr films has become a true rate- limiting step in the recent production of high quality, 0.18 mm design rule Masks. The use of wet etch technology for Gate- level Poly definition Reticles is exceedingly difficult for 0.18 mm technologies and beyond; especially if the use of Optical Proximity Corrections (OPC's) becomes mainstream. The use of Plasma etch will significantly improve the Isolated/Dense Linearity as well as overall CD Uniformity. However, a recent issue is that the Cr dry etch parameters are sensitive to the overall Cr loading for the Mask pattern. It is well known that low Cr masks (e.g., Contact layer patterns) will require a different set of process conditions then more highly loaded parts (e.g., SRAM patterns). This study focuses on the resultant uniformity of Cr etch, both for the blanket etch of Cr as well as the etch of high and low load parts for CD Evaluation.
The continuing requirements for high resolution, critical dimension control and linearity on photomasks necessitates highly anisotropic and uniform etching of the absorber material. Plasma etching has seen strong increases in popularity to improve the above mentioned requirements. Also recently popular is the inclusion of Embedded Phase Shift materials such as Molybdenum Silicide (MoSi); these materials allow for an engineered 180 degree shift in the phase of the exposure light at the wafer pane, affording enhanced contrast at the edges of a line or feature. This article studies the effect of ICP-based plasma conditions on the CD Uniformity, MoSi etch rate and post-etch Quartz roughness of 6 X 6 DUV MoSi Embedded Phase Shift mask structures through use of carefully Designed Experiments. This Design of Experiment (DOE) makes it possible to screen plasma chemistry, optimize resultant plasma parameters and present an overlayed Simultaneous Solution which is used as a centerpoint for Device Plate etch tuning. The high plasma density, independent ion energy control and low pressure operation of Inductively Coupled Plasmas make this technology well suited to minimizing undercut of the MoSi and affords a vehicle for the realization of a zero-basis etch process.
The Quartz etch improvement program is established to technically understand and ascertain the limits, hardware implications and process conditions associated with the etching of Quartz Photomasks. This study explores several process parameters as well as salient hardware changes and ultimately analyzes results by etching test Photomasks as well as a test structure closely emulating a Levenson-style Phase Shift Photomask.
As device design rules continue to shrink, on-mask Cr structures must experience a corresponding reduction in size. Although 0.25 micrometer design rules require only 1 micrometer Cr features, the use of OPC structures, which may be needed to minimize line foreshortening and corner rounding, necessitate features to be etched into the Cr which are significantly smaller than this. This need, coupled with the demand for reduced CD bias and improved CD uniformities, requires the use of an alternate chrome etch technology. Plasma etching of Cr can be highly anisotropic, greatly reducing the etch under cut which is responsible for the CD bias typically associated with wet etching. Reactive ion etching (RIE) can provide significant enhancements in the capability of replicating micron and sub-micron features, but the Cr etch rate non-uniformity which is typical of this technique can translate into a CD nonuniformity. This is due in part to the relatively high pressure of operation (50 - 100 mTorr which is necessary to reduce the self generated dc voltage And which minimizes the photo resist etch rate. Recently, high density plasma sources, such as inductively coupled plasma (ICP), have become available which have the ability to operate both at low pressures and high plasma density while maintaining a low and controllable dc voltage. The low pressure operation significantly improves the etch rate uniformity and consequently the CD uniformity. In this study a design of experiment (DOE) is used to investigate the parameter space associated with the dry etching of Cr using an ICP source. The responses of Cr etch rate, selectivity to photo resist, CD uniformity and mean CD to target are studied, and from this an optimized parameter space is defined. Within this space the effect of overetch, dc voltage and pattern loading on the CD uniformity are also investigated. The role played by the photo resist profile in determining the Cr etch profile is also studied and preliminary measurements are made to understand the effect of the above parameters on the mask CD bias.
As device design rules continue to shrink, on-mask Cr structures must experience a corresponding reduction in size. Although 0.25 micrometer design rules require only 1 micrometer Cr features, the use of OPC structures, which may be needed to minimize line foreshortening and corner rounding, necessitate features to be etched into the Cr which are significantly smaller than this. This need, coupled with the demand for reduced CD bias and improved CD uniformities, requires the use of an alternate chrome etch technology. Plasma etching of Cr can be highly anisotropic, greatly reducing the etch undercut which is responsible for the CD bias typically associated with wet etching. Reactive ion etching (RIE) can provide significant enhancements in the capability of replicating micron and sub-micron features, but the Cr etch rate non-uniformity which is typical of this technique can translate into a CD non-uniformity. This is due in part to the relatively high pressure of operation (50 - 100 mTorr) which is necessary to reduce the self generated dc voltage and which, in turn, minimizes the photo resist etch rate. Recently, high density plasma sources, such as inductively coupled plasma (ICP), have become available which have the ability to operate both at low pressures and high plasma density while maintaining a low and controllable dc voltage. The low pressure operation significantly improves the etch rate uniformity and consequently the CD uniformity. By coupling this ICP plasma source with a non-contact backside photomask handling system and a true multi-station 'cluster tool.' In this study a design of experiment (DOE) is used to investigate the parameter space associated with the dry etching of Cr using an ICP source. The responses of Cr etch rate, selectivity to photo resist, CD uniformity and mean CD to target are studied, and from this an optimized parameter space is defined. Within this space the effect of overetch, dc voltage and pattern loading on the CD uniformity are also investigated. The role played by the photo resist profile in determining the Cr etch profile is also studied, and preliminary measurements are made to understand the effect of the above parameters on the mask CD bias.
The technique of dry etching as applied to the patterning of thin films is described, and compared to wet etching in terms of the etch precision and in terms of the usage and disposal of etch chemicals. The etch requirements of three representative display technologies (AMLCD, FED and EL) are outlined, and the range of plasma etch processes which are applicable to these requirements is described.
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