Peter Pirogovsky

Next generation DUV ALTA mask patterning capabilities

The capability of the DUV ALTAÒ 4300 system has been extended by the development of two new optical subsystems: a 0.9 NA, 42X reduction lens and a high-bandwidth acousto-optic deflector based beam position and intensity correction servo. The PSM overlay performance has been improved by modifications to the software algorithms. Characterization data show improved resolution performance in line end shortening, through pitch CD bias and feature corner acuity. The AOD subsystem reduces stripe beam placement errors and random and systematic beam intensity errors. This has enabled local CD uniformity to be reduced to 4.3 nm (3σ) and global CD uniformity to be reduced to 5.8 nm (range/2). Second layer overlay performance is now 20 nm (max error). A split lot wafer evaluation has demonstrated the equivalence of unmodified ALTAÒ 4300 reticles to those printed on a 50 KeV electron beam system for a 130/110 nm device. Wafer lithography results show equivalent CD uniformity, depth of focus and pattern registration results.

Process Development for 257 nm Photomask Fabrication Using Environmentally Stable Chemically Amplified Photoresists

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.

Picosecond laser micromachining of advanced semiconductor logic devices

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.

Describing thin-film imaging with a Gaussian beam as potential flow

It is shown that patterning with a Gaussian beam on thin films is congruent to a diffusion process. In particular, the acuity of the image patterned is tied to a steady state (Laplace equation). For a small spot size, pattern acuity improves locally when the Laplacian approaches zero at the said region. However, when the spot size (blur) of the patterning beams is of the order of the critical dimensions of the geometry, the diffusion equation gives rise to a global steady state. As a corollary, a conformal mapping of the type w=z2 is applied, transforming a 45∘ rotated cross into a contact hole. We discuss the relevance of conformal mapping to corner rounding seen on raster beam pattern generators.

ALTA 4700 system mask patterning performance improvements for X-architecture and wafer electrical performance interchangeability with 50kV E-beam

The capability and performance of the production-proven DUV ALTA 4300 system has been extended by the development of two new optical subsystems: a 0.9 NA, 42X reduction lens and a high-bandwidth acousto-optic deflector based beam position and intensity correction servo. The PSM overlay performance has been improved by modifications to the software algorithms. The enhanced performance, delivered by these subsystem improvements, has been introduced as a new product-the ALTA 4700. Characterization data show improved resolution performance in line end shortening, through pitch CD bias and feature corner acuity. The AOD subsystem reduces stripe beam placement errors and random and systematic beam intensity errors. This has enabled local CD uniformity to be reduced to 4.3 nm (3σ) and global CD uniformity to be reduced to 6 nm (3σ). Second layer overlay performance is now 20 nm (max error). This paper also demonstrates superior X-Architecture performance delivered by the ALTA 4700. Characterization data show global CD uniformity in 0°, 45°, 90°, and 135° orientations better than 6.5nm (3σ); mean CD control in all 4 orientations less than 3.6nm; and smooth angled lines through a wide range of angles. A split lot wafer evaluation demonstrates the equivalence of wafers produced DUV ALTA system reticles vs. those produced with reticles from a 50kV electron beam system. The evaluation shows the interchangeability of these two systems for 90nm Metal 1 applications-with no changes to the wafer OPC (originally optimized for the 50kV system). Characterization data focus on final wafer electrical performance-the performance characteristic that determines ultimate integrated circuit device yield.

DUV ALTA system aerial image enhancement for improved pattern fidelity

The ALTA 4300 system has been used to successfully write many advanced design layers previously only feasible with 50kV vector shaped beam tools. In order to further enlarge the application space of this high productivity an aerial image enhancement technique has been developed to deliver mask patterns that more closely match pattern data for corners and jogs. This image enhancement is done in real time in the ALTA system’s rasterizer by modifying the gray level mapping of pixels near the corner vertexes. SEM measurements of corner rounding with standard rasterization and the enhanced rasterization show an improvement of corner rounding radius from ~205 to ~132 nm. A direct comparison of SEM micrographs show no qualitative difference between vector scan mask features and those written with aerial image enhancement. This convincingly demonstrates that the ALTA 4300 system with the new image enhancement can write many layers requiring vector scan corner acuity.

Rational design of bleachable nonchemically amplified DUV photoactive compounds

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.

DUV ALTA system aerial image enhancement for improved pattern fidelity: phase II

The ALTA 4300 system has been used to successfully write many advanced designs previously only possible with 50kV VSB systems. In order to further enlarge the application space of this high productivity system, an aerial image enhancement technique has been developed to deliver mask patterns that more closely match the pattern data for corners and jogs. This image enhancement is done in real time in the ALTA systems rasterizer by modifying the gray level mapping of pixels near the corner vertexes. SEM measurements of corner rounding with standard rasterization and the enhanced rasterization show a 35% improvement of corner rounding radius from ~205 to ~132 nm. A direct comparison of SEM micrographs show little qualitative difference between vector scan mask features and those written with aerial image enhancement. This convincingly demonstrates that the ALTA 4300 system with the new image enhancement can write many layers requiring vector scan corner acuity.

Conformal mapping in microlithography

We show that a conformal mapping of the type, W = z π/α, describes how a shape of a 45° rotated cross transforms into a contact hole. We discuss its relevance to corner rounding seen on raster beam pattern generators.