Process Latitude Research Articles

A 2 million transistor digital processor with 120 nm gates fabricated by 248 nm wavelength phase shift technology

Alternating phase shift technology has been shown to substantially improve focus latitude and resolution for several years. However, the use of phase shift masks to improve the process latitude in gate level lithography has been hindered by the lack of commercially available tools that can convert conventional gate layouts into phase shift mask patterns. A software package has recently become available that allows a user to create phase shift masks to reduce the gate length of features in existing circuit layouts. A digital signal processing chip with 2 million gates has been used as a test vehicle to evaluate the feasibility of phase shifting and processing a large number of devices in a complete circuit. Three wafer lots have been processed with a target feature size of 120 nm with a variation of 25 nm 3σ. The timing circuits of the chips have been tested; those with 120 nm gates showed a nearly fourfold improvement in speed when compared to 240 nm gate circuits at 1 volt operation.

Development of Photochemical Analysis System for F2-Excimer Laser Lithography Processes

A system for photochemical analysis of F2-excimer laser lithography processes has been developed. The system, VUVES-4500, consists of 3 units: (1) an exposure and bake unit that uses the F2-excimer laser to carry out a flood exposure and then post-exposure bake (PEB), (2) a unit for measurement of the development rate of photoresists, and (3) a simulation unit that utilizes PROLITH of profile simulation software to calculate the resist profiles and process latitude using the measured development rate data. With this system, preliminary evaluation of the performance of F2-Excimer laser lithography can be performed without the use of a lithography tool capable of imaging and alignment. Profiles for 150 nm lines are simulated for the PAR-101 resist (manufactured by Sumitomo Chemical) and the SAL-601 resist (manufactured by Shipley), a chemically amplified resist that has sensitivity at the F2-excimer laser wavelength. The simulation successfully predicts the resist behavior. Thus it is confirmed that the system enables efficient evaluation of the performance of F2-excimer laser lithography processes.

Level-specific lithography optimization for 1-Gb DRAM

A general level-specific lithography optimization methodology is applied to the critical levels of a 1-Gb DRAM design at 175- and 150-nm ground rules. This three-step methodology-ruling out inapplicable approaches by physical principles, selecting promising techniques by simulation, and determining actual process window by experimentation-is based on process latitude quantification using the total window metric. The optimal lithography strategy is pattern specific, depending on the illumination configuration, pattern shape and size, mask technology, mask tone, and photoresist characteristics. These large numbers of lithography possibilities are efficiently evaluated by an accurate photoresist development bias model. Resolution enhancement techniques such as phase-shifting masks, annular illumination and optical proximity correction are essential in enlarging the inadequate process latitude of conventional lithography.

Advances in X-ray Lithography at ASET.

In ASET, in cooperation with NTT, we have developed key systems for X-ray lithography (XRL) such as X-ray steppers and a 100-kV electron-beam (EB) lithography system for x-ray mask fabrication. Mask-to-mask overlay accuracy within 20nm (3σ) between the gate- and contact-hole-layers for 1-Gb DRAM has been achieved. Critical dimension (CD) control of ±14nm for the gate and ±10nm for the contact-hole-patterns have also been realized. The advantages of XRL over advanced optical lithography, in terms of both resolution and process latitude, were demonstrated through computer simulation. The outcomes of several experiments have convinced us that XRL is a particularly promising technology for mass-producing 0.1-μm or smaller devices.

Aberration Tolerance for 130 nm Lithography from Viewpoint of Process Latitude

In this paper, aberration tolerance of a projection lens for 130 nm patterning using ArF lithography is described. The influences of the three kinds of lens aberration were analyzed with aerial image simulation. Four mask types were evaluated, each mask has a vertical and horizontal isolated line as well as vertical and horizontal (V&H) dense lines. The common process latitudes for four patterns were calculated, then the aberration tolerance was evaluated for each mask type. From the viewpoint of the process latitudes, the aberration tolerance was about 0.14λ in case of the Levenson-type mask for the vertical isolated line only. When the common process latitude for V&H isolated and dense lines was considered, the aberration tolerance decreased to 0.02λ in the case of spherical aberration. We also found that the influence of aberrations differed depending on the mask type. Therefore, the mask types should be selected considering their characteristics with respect to the aberrations.

Characteristics for negative and positive tone resists with direct write electron beam and SCALPEL exposure systems

High acceleration voltage electron beam exposure is one of the possible candidates for post optical lithography, where tighter resolution and wide process latitude is required. The use of highly accelerated electrons instead of photons prevents optical related problems, such as standing waves, because the interaction mechanisms between the resist and photons and electrons are different. The present work will describe results obtained with a 50 kV direct write e-beam exposure tool and SCALPEL, which operates at 100 kV acceleration voltage on positive and negative chemically amplified resists from Tokyo Ohka Kogyo (TOK). TOK has designed a positive tone resist, EP-004, with 100 nm resolution for line and space patterns. The dose to print on SCALPEL is 6 μC/cm2 and process latitude is about 10%. The TOK negative resist, EN-009, has 100 nm resolution for isolated lines. The dose to print on SCALPEL is 7 μC/cm2 and process latitude is about 15%. These results are the first to meet high throughput requirements for SCALPEL in terms of resist sensitivity and process latitude, which are: dose between 5 and 10 μC/cm2 and process latitude greater than 10%.

Extension of x-ray lithography to 50 nm with a harder spectrum

To be competitive with the next-generation lithography technologies, synchrotron-based proximity x-ray lithography (PXRL) must prove to be extendible to produce minimum feature sizes of 70 nm and below. We present here a relatively simple and practical method to improve the PXRL system performance for the replication of features down to 50 nm with reasonable process latitude at large (g≈15μ) mask–wafer gaps. Contrary to previous conclusions indicating λ=1 nm as the best operating region, we find that a significant improvement can be achieved by a modest decrease in the effective wavelength of present PXRL systems, and by the use of non-silicon-based materials in beamline filters and masks. The proposed PXRL system requires a synchrotron storage ring with slightly higher energy than older rings such as Aladdin, but well within the design parameters of the newer generation of synchrotrons, and some beamline modifications. In addition, a diamond mask substrate is also utilized to eliminate the x-ray absorption due to the Si-absorption edge at 1.75 keV.

Focus drilling and attenuated phase shift mask for subwavelength contact window printing using positive and negative resists

We have investigated the process latitude and resolution limit of printing contact windows using a high numerical aperture KrF stepper and attenuated phase shift mask (APSM). We show that both positive and negative resists can be used depending on the size of the window on the mask. The advantages and disadvantages of using the positive and negative resists are presented. A combination of APSM and focus drilling are shown to extend the focus latitude of subwavelength window printing.

Low voltage electron beam lithography in PMMA

We have studied low voltage (1–2 kV) electron beam lithography processes in PMMA and compared them to conventional high voltage processing. We looked at the deposited metal after liftoff as well as directly imaging resist profiles by atomic force microscopy. As expected, the proximity effects were greatly reduced. The forward scattering was found to increase at low voltage. The study of developed resist profiles showed that linewidth versus dose has a single Gaussian functional form, proving that forward scattering plays the major role in line broadening. The effective Gaussian linewidth is 60 nm at 1 kV in a 50 nm resist layer. Modeling of the lithographic process showed a significant increase in resolution and process latitude for thinner resists.

A ULSI shallow trench isolation process through the integration of multilayered dielectric process and chemical–mechanical planarization

A multilayer thin film structure (SiO 2/SiN x /SiO 2) with a one-step chemical–mechanical polish (CMP) process is developed for shallow trench filling and planarization for ULSI devices in the quarter micron regime. By fine-tuning the plasma-enhanced chemical vapor deposition (PECVD) conditions we successfully modified the stoichiometry and other characteristics of the as-deposited SiN x and oxide films. As these film characteristics are changed, the CMP removal rate selectivity between sacrificial oxide and nitride stopper layer can also be adjusted. Correspondingly, the CMP process latitude for shallow trench isolation in 0.25 μm memory devices can be broadened by sequentially depositing multilayered oxide and nitride films with adjustable characteristics and CMP removal rates. Dishing-free wide trench areas with excellent planarity can be achieved through the integration of the proposed multilayered PECVD oxide/nitride scheme and the one-step CMP process.

Comparative study of AZPF514 and UVIII chemically amplified resists for electron beam nanolithography

Two commercial positive tone chemically amplified resists, AZPF514 and UVIII, were investigated and compared for their performance in e-beam direct write of wafers at sub-150nm resolutions. The resist sensitivity, process latitude and post exposure delay effect were studied. Both resists are of high sensitivity. They are insensitive to variation of post exposure bake conditions. The post exposure vacuum delay effect is much more pronounced in AZPF514. AZPF514 also suffers severely the ''T-topping'' effect. Feature dimension below 150nm cannot be achieved with AZPF514, while sub-50nm lines have been obtained with UVIII.

Can we do 0.15μm lithography with KrF?

Deep-UV lithography using 248 and 193-nm light will likely be the microlithography technology of choice for the manufacture of advanced memory and logic semiconductor devices for the next decade. Since 193nm lithography development has been slow, the extension of 248nm technology to 150nm and beyond is required. Advanced techniques, such as Optical Proximity Correction (OPC) and Phase Shift Masks (PSM) will be needed in order to maintain sufficient process latitude. This paper will discuss recent work to investigate the capability of 248nm lithography at 150nm. Imaging results using conventional and off-axis illumination (OAI) will be presented. Key resist performance parameters will be discussed, including process latitude, linewidth and line length control and full field critical dimension (CD) control. Although the performance appears to be adequate for early process and device development, further enhancements will be required for a manufacturable process at 150nm.

Metal wire definition by high resolution imprint and lift-off

Metal wires down to 25 nm width were fabricated using a simple imprint technology yet showing high resolution. The patterns were transfered with high geometric control from the silicon mold into a 100 nm thick PMMA layer by pressing the mold into the resist at a temperature of 140^oC for 20 min. Then the PMMA layer was thinned by Argon plasma etching and 10 nm thick Gold wires were defined by evaporation and lift-off. Similarly, by imprint in a three layer resist structure with an intermediate metal layer used as an etch stop 25 nm wide wires could be defined with larger process latitude.

A new e-beam method for grey scale 3D optical elements

A new e-beam lithography grey scale method has been developed for phase shift optical elements production. It could be used only for 3D elements with lateral dimensions of constant height regions much greater than the resist height and provides an extremely precise structure height control with a very high process latitude. Resist structure height in it is controlled not by the local dose but by the exposed cell area.

Optimization of an advanced positive DUV resist for 248 nm L/S pattern printing

This paper reports on the results of a NA-sigma optimization study of an advanced positive DUV photoresist, AZ^(R) DX 2058P, on silicon substrate. The contribution starts with the dose-to-clear (E0) swing curve on silicon and continues with a resist thickness selection (Emax and Emin) according to this swing curve. Dose-to-print versus dose-to-clear ratio is in the region of 2.4. Process windows with regard to dense (1:1 L/S) versus isolated CD 0.20 and 0.175 @mm patterns are determined and compared via top-down SEM analysis. Different NA and sigma settings are used in order to optimize the individual and overlapping process latitudes (exposure latitude, EL, and depth-of-focus, DOF). Cross-section SEM pictures will support the results.

Sub-100 nm KrF lithography for complementary metal–oxide–semiconductor circuits

Sub-100 nm metal–oxide–semiconductor field effect transistor gate structures have been fabricated using resolution-enhanced 248 nm optical lithography. We employed the chromeless phase-shift approach together with thin resists and optimized etches to achieve these results. Polysilicon gratings with critical dimension (CD)≈60 nm and TiN (damascene) gratings with CD≈35 nm have been fabricated with good process latitudes. We are applying this process to the fabrication of basic test circuits for an aggressively scaled silicon on insulator complementary metal–oxide–semiconductor technology.

Thin film imaging with CARL photoresist at the optical resolution limit.

Future lithography with exposure at 157nm, 126nm and 13nm require thin film resist techniques, because at these high exposure energies sufficiently transparent resist polymers for conventional singel layer resists are not available. Besides top-surface imaging dry developable bilayer resist systems offer excellent imaging properties (high resolution capability and wide focus windows) in a thin imaging photoresist layer that is applied on top of a thick light-absorbing and planarizing bottom resist. The high etch resistance of the underlying bottom coat provides unique process latitudes for the overall substrat treatment. Since 1995, Siemens has used the in-house developed CARL bilayer resist process [1, 2] as a from Clariant commercially available i-line version [3] in its high-volume DRAM and logic IC production [4]. The dramatic increase in focus latitudes at k1≤0.4, even with standard illumination and COG masks, allows shrinking down to 120nm with 193nm exposure or extending the lifetime of existing i-line and 248nm exposure tools. A chemically amplified CARL resist version is now investigated at 248 nm. With this material process development and optimization for the 193nm technology is possible because it is sensitive at both wavelengths (multi-wavelength characteristic). First results at 157nm exposure indicate that this process offers outstanding possibilities even with the methacrylate based polymers.

Amorphous carbon films for use as both variable-transmission apertures and attenuated phase shift masks for deep ultraviolet lithography

We describe the development of amorphous carbon (a-C) films grown by magnetron sputtering for use in optical elements for sub-0.25-μm deep ultraviolet (DUV) lithography. We have measured the transmittance of a-C films deposited onto quartz substrates as a function of film thickness, and find that the films are ideally suited for use in variable-transmission apertures that can be used to improve DUV process latitude: we can achieve essentially any transmittance (T) desired in the range 0<T<100% by controlling the film thickness (t) in the range 200>t>0 nm with subnanometer precision. We also find that the transmittance remains stable after prolonged exposure to high intensity DUV radiation. We describe a masked deposition technique to produce variable-transmission apertures using a-C films of various thicknesses, and also discuss the use of these films in attenuated phase shift masks, given that we can simultaneously achieve ∼6%–8% transmittance and a phase shift of 180° at either λ=248 nm or 193 nm.

Development Process for Chemically Amplified Resist by KrF Imaging

The development process for chemically amplified resist by KrF imaging was investigated. The effects of developer concentration, development time and surfactant on process latitude were examined. In order to extend the process latitude, the development condition with faster dissolution rate at the center of the exposed area and slower dissolution rate at the edge of the exposed area was found to be promising. It was also found that the addition of a surfactant to the developer extended the process latitude. The developer with the surfactant tends to suppress the slimmed resist linewidth in the overexposed area and to enhance resist dissolution in the underexposed area.

0.12 µm Optical Lithography Performances Using an Alternating Deep UV Phase Shift Mask

Manufacturing the next generation of devices will demand lithographic capability in the sub-0.18 µm range. The phase shift mask (PSM) is a key emerging technology which is thought to extend 248 nm optical lithography. Using the Levenson PSM technique allows us to improve resolution by as much as 50% at gate level. This paper describes the lithographic performances of the Shipley UV5 photoresist on SiOxNy bottom antireflective coating (BARC), using alternating PSM and an ASM/90 Deep UV stepper. Critical dimension (CD) measurement was done on an OPAL 7830i metrology scanning electron microscope (SEM). Results on sub-0.18 µm design rules are presented as follows. –The first part concerns experimental conditions: masks, process conditions, anti-reflective substrates, etching and metrology are discussed. –The second part concerns lithographic performances: process linearity from 0.12 µm to 0.18 µm, 0.12 µm isolated line process latitudes of 7% energy latitude for 0.8 µm depth of focus (DOF) have been exhibited. Finally we try to evaluate the proximity effect and the ultimate resolution of this technology.