This PDF file contains the editorial “Editorial: Microlithography and Aviation” for JM3 Vol. 5 Issue 04

In recent low-k1 lithography, the size of a mask pattern is becoming close to the wavelength of the light source. In a sub-100-nm pattern at wafer scale of 4× masks, transverse electric (TE) polarization light had higher transmittance of the zeroth order than TM polarization for a Cr mask according to rigorous model simulation of a finite difference time domain method. On the other hand, transverse magnetic (TM) polarization light had higher transmittance than TE polarization light for a MoSi mask. From the results of lithography simulation for a 45-nm pattern on the MoSi mask, TE polarization was better for wide exposure latitude, but TM polarization was better for large depth of field. The performance of a current MoSi mask is inferior to that of a Cr mask. However, a lower transmittance MoSi mask has better performance in the exposure defocus window under the dipole illumination. Also, rigorous simulation showed transmittance dependency of the light incident angle to the MoSi mask. The dependency was larger for TM polarization than for TE polarization.

The direct problem of optical microlithography is to simulate printing features on the wafer under the given mask, imaging system, and process characteristics. The goal of inverse problems is to find the best mask, imaging system, or process to print the given wafer features. In this study, we proposed the strict formalization and fast solution methods of inverse mask problems. We stated inverse mask problems (or "layout inversion" problems) as nonlinear, constrained minimization problems over a domain of mask pixels. We considered linear, quadratic, and nonlinear formulations of the objective function. The linear problem is solved by an enhanced version of the Nashold projections. The quadratic problem is addressed by eigenvalue decompositions and quadratic programming methods. The general nonlinear formulation is solved by the local variations and gradient descent methods. We showed that the gradient of the objective function can be calculated analytically through convolutions. This is an important practical result because it enables layout inversion on a large scale in order of M log M operations for M pixels.

It is well known that the refractive optics used in today's exposure tools are highly chromatic, meaning that small wavelength shifts will cause large focus shifts. Even a line-narrowed excimer laser has a large enough range of wavelengths that we can no longer think of an infinitely thin image plane. The concept of "focus blur" can be generalized to encompass the effect of laser bandwidth chromatic aberrations, vertical stage vibrations, and stage tilts, which cause the focus to change during the scan. We introduce a new parameter called mean absolute defocus that can characterize the focus blur and is shown to correlate with the lithographic effects. Focus blur can be incorporated into simulation models, in a manner similar to the way that stage vibration is modeled. New simulation results illustrate the impact of focus blur on modern lithographic processes. Process stability and machine-to-machine matching issues are discussed.

The chemical amplification provides high sensitivity of resists for deep-uv and extreme-uv (EUV) lithography. On the other hand, the chemical amplification involves photoacid diffusion that causes contrast degradation of the latent image or, in other words, resist blur. We study the influence of the resist blur in high–numerical aperture ArF immersion lithography by using an interferometric exposure tool. The contrast ratio between the resist latent image and the original aerial image was measured for half pitches from 45 to 80 nm. Acid diffusion length for a high-resolution ArF resist was determined as 11 nm in sigma (26 nm in full width at half maximum) assuming a Gaussian blur kernel. The results revealed that the influence of the resist blur is a significant issue for the 45-nm half-pitch node. The reduction of acid diffusion length is highly desirable. Given the tradeoff between the resist resolution and sensitivity, increasing illumination intensity in exposure tools can be an effective means to overcome the challenge of the resist blur. We also demonstrate resist imaging of 30-nm line-and-space pattern with a high-index fluid. While our focus is on ArF immersion lithography, our findings are also relevant to EUV lithography.

One of the well-known problems the industry faces concerning 193-nm resists is its shrinkage under scanning electron microscope (SEM) measurements. While different phenomena arising from electron-material interaction are assumed to take place (such as cross-linking and scission), the primary mechanism that brings about this shrinkage is still unclear. Three experiments were performed relating to three theories for the primary mechanism that brings about the shrinkage. The first experiment examined how the shrinkage is affected by 193-nm radiation and corresponds to a theory that the electron exposure induces an effect similar to that of 193-nm exposure. The second experiment deals with electron-beam curing, using parameters similar to those used in SEM measurements (curing entails a much lower power density). The third experiment addresses the theory of disassociation of carbonyl bonds in the resist, leading to cross-linking and eventual evaporation of CO₂ molecules. The results from the exposure and curing experiments lead us to believe that an exposurelike effect and resist local heating are not good candidates for the primary mechanism. The last experiment shows that slimming is related to the release of carbonyl bonds.

Measurement characteristics in scatterometry and critical dimension–scanning electron microscopy (CD-SEM) for lot acceptance sampling of inline CD metrology were investigated by using a statistical approach with Monte Carlo simulation. By operation characteristics curve analysis, producer's risk and consumer's risk arising from sampling were clarified. Single use of scatterometry involves a higher risk, such risk being particularly acute in the case of large intrachip CD variation because it is unable to sufficiently monitor intrachip CD variation including local CD error. Substituting scatterometry for conventional SEM metrology is accompanied with risks, resulting in the increase of unnecessary cost. The combined use of scatterometry and SEM metrology in which the sampling plan for SEM is controlled by scatterometry is a promising metrology from the viewpoint of the suppression of risks and cost. This is due to the effect that CD errors existing in the distribution tails are efficiently caught.

Subwavelength diffractive features etched into a substrate lead to form birefringence that can produce polarization sensitive elements such as wave plates. Using etched features allows for the development of pixelated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Form birefringence exhibits dispersion that can be advantageous to the design of wave plates with an achromatic response. Taking advantage of this dispersion, diffractive wave plates with good achromatic characteristics can be designed for the 2- to 5-μm spectral region. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. The fabrication processes for the subwavelength features will be discussed and fabricated devices with a measured average phase retardation of 80.6 deg and rms variation of 9.41 deg will be presented.

We developed an atomic force microscopy (AFM)-based technique to measure intrinsic material roughness after base development. This method involves performing an interrupted development of the resist film and measuring the resulting film roughness after a certain fixed film loss. Employing this technique, we have deconstructed the resist into component materials and established that the photoacid generator (PAG) is a major material contributor of film roughness and that PAG segregation in the resist is likely responsible for nanoscale dissolution inhomogeneities. Small differences in PAG concentration as a result of standing waves in the resist can lead to large changes in surface roughness due to PAG or PAG-photoproduct segregation and the resultant nonlinear change in nanoscale dissolution rates. The temperature dependence of the PAG segregation suggests that increased mobility of the PAG that occurs may be due to a lowering of the film T_g during the deprotection process.

A backlight panel (BLP) with a cold cathode fluorescent lamp (CCFL) light source, which is referred to as the backlight unit (BLU) is considered in the light module of a thin film transistor (TFT) LCD. A typical BLP is comprised of a light-guiding plate (LGP) and some optical components, such as reflective sheets, diffusive sheets, and prism sheets. We propose an integrated LGP that combines three functions of optical components by using microelectromechanical systems (MEMS) and hot-embossing techniques. The microprism structure, the micropyramid structure, and an aluminum thin film are used for the integrated LGP not only to eliminate the five optical components, but also to save the space and the cost of the BLU. From experimental illumination results, the maximum illumination is 426.1 lux and the minimum illumination is 370.4 lux. Thus, the integrated LGP achieves a uniformity of illumination of 86.9%. The new BLU could be simplified as only one integrated LGP and only one white LED light source without using any optical components. We anticipate that the integrated LGP could make the displays thinner and brighter for thin LCD application.

Micro-opto-electro-mechanical (MOEM) accelerometers using principle of optical coupling between two waveguide structures is proposed. It consists of an antiresonant reflecting optical waveguide structure combined with a cantilever beam. Under acceleration, the output waveguide optical power changes, which is a function of acceleration. The mathematical model of the mechanical sensing element in terms of proof mass, damping, and spring constant is formulated. Here, different aspects such as beam deflection, bending stress, and breakdown acceleration are incorporated into the formulation. Based on the analysis, the optimum mechanical structure for intensity modulated MOEM accelerometers are designed. We report for a typical uniform cantiliver beam of type I (3 mm×3 mm×0.38 μm) accelerometer, mechanical sensitivity of 0.15 μm/g, minimum detectable acceleration of 4.7 μg/√Hz at an optimum power of 500 μW, range of ±25 g, a bandwidth of 1.59 kHz, breakdown acceleration of 5.1×10⁴ g and cross-axis sensitivity of 0.001%. The results in the case of nonuinform beam cantiliver accelerations are sensitivity of 2.15 μm/g, minimum detectable acceleration of 0.27 μg/√Hz range of ±1.8 g, bandwidth of 0.411 kHz, breakdown acceleration of 2987 g and cross-axis sensitivity of 0.001%.

The reliability in the wafer bonding is highly dependent on the surface conditions present on each individual wafer. An emerging wafer bonding technique, referred to as transmission laser bonding (TLB), is examined and implemented for the bonding of Pyrex glass-to-silicon wafers using a solid-state pulsed neodymium: yttrium aluminum garnet (Nd:YAG) laser. Focusing specifically on the physical attributes of surface roughness and contact pressures, we explore their relative effects on the associated bond strength. The bonding strengths of the TLB bonded specimens are quantified using a microtensile tester, whereas the surface roughness and flatness are determined by atomic force microscopy and profilometry, respectively. The tensile results indicate that with appropriate surface roughness and contact pressure, the bond strength obtained is comparable with, if not better than, those using conventional wafer bonding techniques. Both the x-ray photoelectron spectroscopy and Auger electron spectroscopy are used to facilitate the study of the bonding mechanism of the delaminated surfaces of TLB specimens by evaluating the migration and diffusion of different atoms across the bonding interface. A thermal penetration analysis is also provided to validate the findings of the bond strength and the spectroscopic evaluations.

Molecular ruler nanolithography combines conventional lithography techniques with chemical self-assembly methods to improve the resolution of the existing lithography tools. We demonstrate the implementation of molecular ruler nanolithography using sacrificial multilayer host structures defined by electron-beam lithography. Using this process, 40-nm metal features are produced using host features spaced 100 nm apart. Using a thicker sacrificial layer, 100-nm-thick daughter metal layers are also produced, demonstrating the flexibility of this technique.