Novel methodology for postexposure bake calibration and optimization based on electrical linewidth measurement and process metamodeling

Extreme ultra-violet lithography (EUVL) has been prepared for next generation lithography for several years. We could get sub-22 nm line and space (L/S) pattern using EUVL, but there are still some problems such as roughness, sensitivity, and resolution. According to 2007 ITRS roadmap, line edge roughness (LER) has to be below 1.9 nm to get a 22 nm node, but it is too difficult to control line width roughness (LWR) because line width is determined by not only the post exposure bake (PEB) time, temperature and acid diffusion length, but also the component and size of the resist. A new method is suggested to reduce the roughness. The surface roughness can be smoothed by applying the resist reflow process (RRP) for the developed resist. We made resist profile which has surface roughness by applying exposure, PEB and development process for line and space pattern. The surface roughness is calculated by changing parameters such as the protected ratio of resin. The PEB time is also varied. We compared difference between 1:1 L/S and 1:3 L/S pattern for 22 nm. Developed resist baked above the glass transition temperature will flow and the surface will be smoothed. As a result, LER and LWR will be much smaller after RRP. The result shows that the decreasing ratio of LER due to RRP is larger when initial LER is large. We believe that current ~ 5 nm LWR can be smoothed to ~ 1 nm by using RRP after develop.

A new method for the measurement of acid diffusion parameters in negative-tone e-beam chemically amplified resists and its application for proximity effect correction is presented. It is based on the measurement of the diameter of lithographic features (pillars) obtained from single-pixel e-beam exposures. Generally, long post exposure bake (PEB) times or low post apply bake (PAB) temperatures lead to wider features compared with short PEB times and high PAB temperatures, due to enhanced acid diffusion. The acid diffusion coefficient was found to depend exponentially on cross-linking density. Additionally, proximity effect correction parameters (triple Gaussian parameters) are easily calculated. These parameters are t PEB dependent and in the limit (t PEB→0) are almost equal to the corresponding values for non-amplified resists. The method has been applied to SAL-601 (Shipley), and the calculated proximity parameters were used successfully to correct the proximity effect of complicated layouts with a 0.15 µ m critical dimension.

Model-based breakdown of resist and mask contributions to local CDU for sub-30-nm contact holes in EUV lithography

Negative SU-8 photoresist processed at several levels of lower temperature than conventional approach was investigated, and its low-temperature processing has been characterized in terms of delamination. According to two phases of statistical design of experiment (DOE), initially statistically significant variables of SU-8 photoresist processing were selected, and the selected variables were further investigated to find their effects on delamination of SU-8 microstructure. A three leveled factorial designed experiments were performed followed by a 2(6-1) fractional factorial designed as a screening experiment. Characterizing low-temperature SU-8 photoresist process, 27 samples were fabricated and the degree of delamination was measured. In addition, nine additional samples were fabricated for the purpose of verification. Employing a neural network modeling technique, a process model is established, and response surfaces are generated to investigate the degree of delamination associated with three process parameters: post exposure bake (PEB) temperature, PEB time, and exposure energy. From the response surfaces generated, two significant parameters associated with delamination are identified, and their effects on delamination were analyzed. The higher the post exposure bake (PEB) temperature at a fixed PEB time and the more delamination occurred. In addition, the higher the dose of exposure energy, the lower the temperature at which the delamination begins and the larger the degree of delamination. The results identified acceptable ranges of the three process variables to avoid the delamination of SU-8 film, which in turn might lead to potential defects in MEMS device fabrication.

Replicated pattern accuracy was investigated in synchrotron radiation (SR) lithography, using a chemically amplified (CA) negative-type resist and a highly accurate x-ray mask delineated by focused ion beams (FIB). The size of the hole resist pattern which is arranged in a lattice is affected by the proximity gap and the pattern pitch, but that of the dot resist pattern is hardly dependent on these variables. The dot resist pattern, which was replicated with the 0.25- micrometers -square hole mask pattern, became circular with a proximity gap of over 20 micrometers . In order to obtain highly accurate pattern shape as well as pattern size, the proximity gap should be less than 20 micrometers . The resist pattern size and shape depend on the exposure dose. This was confirmed through the simulation of Fresnel diffraction. The resist pattern was influenced considerably by the post-exposure baking (PEB) conditions, temperature and time. The PEB condition dependence of a replicated pattern was investigated. It is effective to extend the PEB time to improve the sensitivity without deforming the pattern shape.

One of the delicate processes in semiconductor microfabrication is the photolithography. It is the process that sets the design dimensions on various parts of the device. In order to complete this process, two requirements need to be satisfied. First is to create, the exact dimensions and pattern as established in design phase, which in other word can be referred as the resolution of the images on the wafer. The second is the correct placement of the device pattern on the wafer relative to the crystal orientation of the wafer substrate. This is called alignment or registration of patterns in correct position. This registration requirement is similar to the correct alignment of the different floors of a building. It is easy to visualize that misalignment of elevator shafts and stair wells would render the building useless. In a circuit, the effects of misaligned mask layers can cause the entire circuit to fail. In this paper we have reported a couple of results that leads to photoresist development and optimization technique as a standard manufacturing process to form 1μm microbridge for later process of size reduction to form nanogap. Therefore, at the final stage of fabrication, a nano-diagnostic biochip device is developed to use it as a biomolecule detection biosensor. The development of biosensors is still an open field and much remains to be done before many of these bioelectronic devices become commercialized. In this research, the key factors such as resist thickness, post-exposure bake (PEB) time and developer concentration are taken into account to study the optimum measurements and process. The thickness of resist will affect the resolution of image transferred and developing time. Both PEB and developer concentration also has the tendency to affect the device pattern and developing time. As the result, the photoresist thickness is optimized at 1500nm, the developer RD6 concentration diluted at 10:25 (DI water: RD6) and PEB time optimized at 65s.

Statistically designed experiments for optimization in processing the high sensitive negative tone Hoechst resist RAY-PN (AZ PN 100) were used to establish robust processes for two different applications of the resist: (1) pattern replication on a wafer and (2) during the mask copy process under 40-mbar He environment via x-ray lithography. Minimization of linewidth change with respect to a 1:1 pattern transfer was achieved through manipulation of the following variables: exposure dose, post-exposure bake (PEB) time and temperature, and development time. A two-stage sequential strategy was employed. After specifying the response and the primary variables, the first stage of the sequential approach was based on a full factorial design. In the second stage, the influence of PEB parameters for wafers as well as for x-ray mask blanks were studied using a central composite design. Finally, the response surfaces for the pattern replication on x-ray mask blanks are demonstrated and the optimal process parameter set for a linewidth control within 50 nm for 0.5- to 3.0-μm feature size values are given.

In this paper, commercially available DUV resist for 'thick' resist applications were used to study the chemical and processing aspects of thin imaging layers. Two types of resist were used: a high Rmin resist and a low Rmin resist, both from Shipley. Input parameters were process conditions such as soft bake, post-exposure bake and development time. Also the influence of the resist thickness was taken into account. Output parameters are contrast, profiles and process windows. The most important parameters for optimizing thin imaging layers are Rmin and the post- exposure bake.

An ultra heat-resistant polyimide formulated with photo-base generator for alkaline-developable, negative-type photoresist

Effects of SU-8 Cross-linking on Flip-Chip Bond Strength when Assembling and Packaging MEMS

An alkaline developable and negative‐type PSPI with a high sensitivity and excellent mechanical properties based on a poly(amic acid) (PAA) and a photo‐base generator has been developed. The PAA was prepared by the polycondensation of p‐phenylenediamine (PDA) with an equimolar of 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA) and 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6‐FDA) and converted thermally to the corresponding polyimide, PI(PDA‐BPDA/6‐FDA). PI(PDA‐BPDA/6‐FDA) showed the high thermal and mechanical properties and the dimensional stability such as the thermal decomposition temperature of 530°C, glass transition temperature of 369°C, linear coefficient of thermal expansion of 28 ppm/K, ultimate tensile strength of 148 MPa, elongation at break of 25% and dielectric constant of 2.8. The PSPI was formulated directly from PAA(PDA‐BPDA/6‐FDA) with a photo‐base generator (PBG), (E)‐3‐(2‐hydroxy‐4‐methoxyphenyl)‐1‐(piperidin‐1‐yl)prop‐2‐en‐1‐one (HMPP) (10 wt% to PAA) and the optimized parameters for photolithographic process were investigated including the PBG content, post‐exposure bake (PEB) temperature, and PEB time. The PSPI based on PAA(PDA‐BPDA/6‐FDA) and HMPP (10 wt% to PAA) showed a sensitivity of 114 mJ/cm2 and contrast of 1.29 when exposed to 365‐nm light (i‐line), post‐exposure baked at 160°C for 5 min, and developed with an aqueous solution of 2.38 wt% tetramethylammonium hydroxide and iso‐propanol. A clear negative 8‐μm features pattern was obtained by contact‐printing and converted into the PI pattern upon heating at 250°C, confirming by scanning electron microscopy and infrared spectroscopy.

Two methods to measure acid diffusion in positive acid catalyzed resists are described. The first method employs a spectrophotometric titration to determine the acid concentration ([H+]) followed by measuring the ion conductivity (σ) of the resist film to determine the diffusion coefficient (D). This method allows the diffusion coefficient of acid in the resist to be determined at different temperatures ranging from room temperature to different post-exposure bake (PEB) temperatures. The second method is based on the threshold acid density theory of image formation, which assumes that when a critical concentration of acid is reached, the developer solubility of the resist is changed. With this method, a constant level of acid can be followed at different PEB times and the diffusion coefficient determined. A comparison of the two methods to measure the acid diffusion coefficient will be made and the temperature dependence of diffusion for different types of organic acids will be presented. Based on a previously described reaction–diffusion model that predicts relative deblocking levels, evidence is presented for room temperature diffusion of acid to be a possible explanation for delay instability in positive deep ultraviolet resists.

Negative‐ and positive‐tone e‐beam resists have been exposed by focused beams of Si++ and Be++ ions. The resolution of the negative resist, Shipley SAL601‐ER7, was better than 0.2 μm and vertical sidewalls were achieved. A dose of was required to retain the full thickness of the resist after developing. Different postexposure bake (PEB) times and temperatures were examined in this work. A saturation in resist response was observed at moderately high PEB temperatures (115–125°C). This effect has not been reported previously for either ion‐beam or e‐beam exposure. Increases in developing time can increase the sensitivity and the effective contrast of this resist. The positive‐tone resist, OCG Microelectronic Materials P28, is at least two to three times more sensitive than PMMA. It also displays a wide range of doses for which positive‐tone imaging is possible. Present results show the resolution of P28 to be at least 0.3 μm, which is not as fine as that of polymethylmethacrylate (PMMA). Preliminary results on the behavior of the P28 resist as a mask for silicon etching are presented.

Post Exposure Baking (PEB) has been widely utilized in advanced lithography. PEB simulation is critical in the lithography simulation flow, as it bridges the optical simulation result and the final developed profile in the photoresist. The process of PEB can be described by coupled partial differential equations (PDE) and corresponding boundary and initial conditions. Recent years have witnessed growing presence of machine learning algorithms in lithography simulation, while PEB simulation is often ignored or treated with compact models, considering the huge cost of solving PDEs exactly. In this work, based on the observation of the physical essence of PEB, we propose DeePEB: a neural PDE Solver for PEB simulation. This model is capable of predicting the PEB latent image with high accuracy and >100 × acceleration (compared to the commercial rigorous simulation tool), paving the way for efficient and accurate photoresist modeling in lithography simulation and layout optimization.

Post exposure bake (PEB) Diffusion effect is one of the most difficult issues in modeling chemically amplified resists. These model equations result in a system of nonlinear partial differential equations describing the time rate of change reaction and diffusion. Verifying such models are difficult, so numerical simulations are needed to solve the model equations. In this paper, we propose a high speed 3D resist image simulation algorithm based on a novel method to solve the PEB Diffusion equation. Our major discovery is that the matrix formulation of the diffusion equation under the Crank– Nicolson scheme can be derived into a special form, AX+XB=C, where the X matrix is a 3D resist image after diffusion effect, A and B matrices contain the diffusion coefficients and the space relationship between directions x, y and z. These matrices are sparse, symmetric and diagonal dominant. The C matrix is the last time-step resist image. The Sylvester equation can be reduced to another form as (<i>I</i>&otimes;<i>A</i> + <i>B</i>^<i>T</i>&otimes;<i>I</i>) X =C, in which the operator &otimes; is the Kronecker product notation. Compared with a traditional convolution method, our method is more useful in a way that boundary conditions can be more flexible. From our experimental results, we see that the error of the convolution method can be as high as 30% at borders of the design pattern. Furthermore, since the PEB temperature may not be uniform at multi-zone PEB, the convolution method might not be directly applicable in this scenario. Our method is about 20 times faster than the convolution method for a single time step (2 seconds) as illustrated in the attached figure. To simulate 50 seconds of the flexible PEB diffusion process, our method only takes 210 seconds with a convolution set up for a 1240&times;1240 working area. We use the typical 45nm immersion lithography in our work. The exposure wavelength is set to 193nm; the NA is 1.3775; and the diffusion coefficient is 1.455×10^-17m²/s at PEB temperature 150&deg;C along with PEB time 50 seconds with image resolution setup to be 1240&times;1240.