Advanced semiconductor logic devices are increasingly complex, typically composed of multiple layers of dielectric,
metal, and semiconductor materials. Laser micromachining is employed on these devices to form cut-outs, microvias,
and perform partial material removal, including scribing and dicing operations. The recent development of high average
power (> 10 W), < 20 ps, 1064 nm diode-pumped mode-locked solid state lasers, operating at pulse repetition
frequencies > 100 KHz, enables an attractive short pulsewidth laser process alternative to existing nanosecond process
technologies, particularly for laser micromachining of complex alloy structures. Emerging 45 and 65 nm node logic
devices may contain greater than eight metal layers, typically aluminum and copper. They may also contain advanced low K layers which have proven difficult to process using conventional mechanical techniques, such as dicing saws. Efficient operation at 355 nm was readily achieved using extracavity conversion by employing non-critically phasematched LBO for SHG and critically phase-matched LBO for THG. Over 3 W at 355 nm at 100 KHz was achieved with an input of 8.5 W at 1064 nm. Preliminary micromachining results on advanced logic devices containing multiple low k and Cu layers at harmonic wavelengths (532 nm and 355 nm) yielded micromachining rates of > 300 mm/s with good workpiece quality.
In this paper, we describe the studies conducted at Etec Systems, Inc., an Applied Materials company, using various resists from photoresist manufacturers. Etec undertook this investigation with the objective of creating 257-nm resist processes that would allow the continued use of precoated mask blanks from commercial suppliers. Successful delivery of this technology would allow the industry to adopt next-generation maskmaking systems without having to change current business practices, avoiding being forced to invest heavily in capital equipment, facilities, and the associated technology development programs necessary for local coating of mask blanks. The main criteria used to select an appropriate resist candidate were commercial availability, environmental stability, and across-plate critical dimension (CD) uniformity. The aging data provides an indication of shelf life-pre-exposure ranging from hours to months of delay. Methods for measuring the incoming plate quality are highlighted. The importance of a highly consistent, tightly controlled bake process-post-exposure bake (PEB) and post-apply bake (PAB)-is illustrated in detail. This paper also shows controlled CD structures below the wavelength: 257 nm. The authors describe the processing difficulties encountered while attempting to create a photomask manufacturing-capable process and the techniques used to address the difficulties. Bottom antireflective coatings, photobase, and chemical amplification are areas of focus, as are their resulting effects on the final photomask.
This paper discusses methods used for photoresist selection and etch processing for laser mask patterning tool characterization. A major requirement of a deep ultraviolet (DUV) resist is that is has a storage capability of more than 90 days. This means the material does not have to be coated on demand to deliver exceptional lithographic performance. Process difficulties in the development and implementation of an advanced DUV maskmaking solution and how they are being addressed is also described. The purpose of this paper is to provide a look at the resist, develop, and etch processes being developed at Etec Systems, Inc. for DUV maskmaking applications. Key topics are etch characterization and resist process optimization at 257nm associated with the migration to DUV from i-line manufacturing environments and turning from wafer to mask patterning applications. The paper also shows results of work being done to assess alternative resist chemistries in an attempt to maintain a precoated mask blank option for mask shop use. The paper points out issues to be considered when moving from diazoquinone (DNQ) chemistry to chemically amplified resists (CAR) processing in a mask manufacturing environment.
Photoactive compounds have been designed, synthesized and characterized for deep ultraviolet non-chemically amplified resist applications. These resist materials may have potential use in next generation 257nm mask fabrication. Mask fabrication requires stringent linewidth specifications over long post-coat and post-exposure bake delays. Lithography simulation and imaging experiments have been done to determine the lithographic performance of resists formulated with these new photoactive compounds. Previously studied chromophores, 7 substituted 3-diazo 4- hydroxycoumarin and N-substituted 3-diazo-2, 4-piperidione, both have the transparency, bleaching and exposure rate kinetics in the DUV that are analogous to those exhibited by the diazonaphthoquinone chromophore at 365nm. The sulfonate linkages attached to these photoactive compounds provide dissolution rate inhibition of novolak that is very similar to the diazonaphthoquinone-sulfonates. The trifunctional diazopiperidione that incorporates three sulfonate linkages provides more efficient inhibition per chromophore than the corresponding bisfunctional photoactive compound. The diazocoumarin based novolak resist demonstrates image reversal (negative tone) with the use of a post-exposure bake. The post-exposure bake causes the exposed photoactive compound to decarboxylate, which dramatically reduces its solubility in aqueous base. The trifunctional diazopiperidione provides the best overall imaging results due to almost complete bleaching and high contrast.
Chemically amplified resists are sought that have optimum performance at 257 nm while providing the post-coat and post-exposure stability required for mask fabrication. An acetal based resist, AZ DX1100 (Clariant), was explored due to its post-coat and post exposure delay stability. A photodestructable base is present in the resist to capture acid that transported into unexposed regions and to minimize the effects of environmental base contamination. Lithography simulation was used to simulate the acetal resist process at 257 nm. These simulations demonstrated some of the effects of standing waves and resist absorption. Reflectivity simulations were conducted to evaluate the influence of the photomask substrate on standing waves. Bake optimization experiments (PAB/ PEB) were performed to establish conditions that minimize standing wave effects while maximizing the resist sidewall angle. Acetal resists are low activation energy systems that deprotect at room temperature. The time scale and extent of the deprotection reaction at room temperature was quantified over the time frame (hours) required for typical mask writing strategies. The deprotection reaction was measured indirectly by monitoring the resist thickness change using a development rate monitor (DRM). This analysis provides insight into the amount of deprotection that occurs at room temperature and during the post-exposure bake.
Consumer demand for faster computers, increased data storage space, and higher density memory arrays has driven development efforts in photomask production the way of the wafer fab. This drive has pushed mask fabrication toward smaller and more uniform features. It has thus inspired the creation of a deep ultraviolet, or DUV, (257 nm) optical pattern generator and an attendant resist process for mask fabrication. A DUV photoresist process based on precoated photomask blanks is presented herein with a demonstrably robust resist, both in terms of plate shelf life and latent image stability. Benchmark lithographic performance in resolution, depth of focus (DOF), linearity, and iso/dense bias are presented. In addition, the ambient atmospheric stability of the resist system is explored. Traditional maskmaking has, until now, excluded a post-exposure bake (PEB) step. Equipment requirements and resist critical dimension (CD) performance as a function of PEB are also presented. Photoresist process parameter space is defined and discussed herein. Precoated mask blank post-apply bake (PAB) stability is also examined as it relates to after-develop inspection (ADI) CDs. Dark erosion tests are employed to study photoresist contrast uniformity as it relates to ADI CDs. First-pass specifications for incoming raw materials are also proposed.
The demand for smaller and more uniform features on photomasks has inspired consideration of a DUV (257 nm) resist process for optical pattern generation. Chemically amplified resists require storage and exposure in carbon filtered environments, and they require post-exposure bakes. Few mask facilities are set up to handle chemically amplified resists commonly used in deep UV wafer fabrication process. Hence, it is appropriate to explore the lithographic performance of non-chemically amplified resist materials for 257 nm laser photomask lithography. Resist characterization and lithography simulation were used to formulate a 257 nm resist from DNQ/novolak materials provided by a commercial resist supplier. Diazonaphthoquinone (DNQ)/novolak resists have not been used for DUV Integrated Circuit (IC) applications mainly due to the low sensitivity and the strong absorbance of the DNQ photoactive compound (PAC) at 248 nm. However, a 2,1,4 DNQ based resist has been characterized that bleaches at 257 nm and inhibits novolak. The photoproduct of the 2,1,4 DNQ PAC is much more transparent at 257 nm than 248 nm. Novolak resin also has an absorbance minimum in the DUV at 257 nm that provides transparency similar to poly (hydroxystyrene). Traditional photoresist formulation requires tedious, iterative, and expensive manufacturing trials. Resist characterization and lithography simulation can be used to relate lithographic performance (resolution, sidewall and process latitude) to resist formulation parameters (PAC concentration, developer concentration, etc.), thereby supporting the formulation optimization. An exposure system using a 257 nm frequency doubled argon laser system has been constructed to study the resist photokinetics. Dill exposure parameters (A, B and C) have been extracted for a 2,1,4 DNQ/novolak based resist. Dissolution rate measurements have been made with a development rate monitor developed at the University of Texas at Austin. Simulation using the exposure and development rate models were used to determine the resist formulation that maximizes the sidewall angle and exposure latitude for isolated resist spaces. Preliminary experiments reveal that a DNQ/novolak resist is capable of resolving 0.30 micrometer linewidths using a 257 nm optical pattern generator.
Semiconductor manufacturers continue to look for better techniques to create salable devices. As with any other manufacturing entity, cost effectiveness without sacrificing quality is the key. In photolithographic manufacturing, the elements that drive the cost are raw materials, process time (throughput), and process complexity (number and iterations of process phases). The specific area being addressed by this paper is the implant layers of the semiconductor fabrication process.
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