Soft Bake Time Research Articles

The effect of soft bake on adhesion property between SU‐8 photoresist and Ni substrate by molecular dynamics simulation

AbstractSU‐8 photoresist is a negative, epoxy‐type, near UV photoresist and is commonly used in micro electro mechanical systems (MEMS) and other thick resist application fields. However, poor adhesion strength between SU‐8 photoresist and metal substrate makes it difficult to bind, even contributes to the lithography failure. This significantly restricts the improvement of image resolution and depth‐to‐width ratio. As soft bake temperature and time are important for the interfacial adhesion property, in this article, molecular dynamics (MD) simulation was performed to investigate the effects of these two parameters on adhesion property between SU‐8 photoresist and Ni substrate. According to the adsorption theory, the simulation results were analyzed and validated by experiments. It is shown that with increasing soft bake temperature the adhesional work firstly increases and then decreases, and with increasing soft bake time the adhesional work decreases. This study provides theory basis for the confirmation of soft bake parameters. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Photoresist Passivation Structures for Laser Lifted-Off Light Emitting Diodes

SU-8 photoresist passivation structures were used to protect vertical structure GaN-based light emitting diodes from explosive forces produced during the laser lift-off (LLO) process of a sapphire substrate. The soft bake time of SU-8, which determines the solvent content, was varied to measure the mechanical properties of SU-8 at different dilution ratios. The SU-8 attained a sufficient hardness of 0.23 GPa and a Young's modulus greater than 4.0 GPa, regardless of soft bake time and dilution ratio. The SU-8 passivation structures with good damping properties showed excellent protection of the weak GaN-based layers from the LLO process.

Enhancement on forming complex three dimensional microstructures by a double-side multiple partial exposure method

This study presents a novel batch process based on standard lithography technology, called the double-side multiple partial exposure (DoMPE) method, which enhances the fabrication capability of three dimensional (3D) photoresist microstructures. By incorporating gray-tone lithography and double-side exposure techniques, the proposed DoMPE scheme extends the multilevel morphology on both the front side and back side of the suspended photoresist microstructures. Back-side gray-tone lithography is achieved by depositing various appropriate thicknesses of Ti film on glass substrate that acts as the gray-tone mask. The process parameters, including metal film thickness, developed depth, exposure dosage, development time, and soft-bake time, are experimentally characterized. Different 3D photoresist microstructures with multiple levels on the front and back sides are successfully fabricated and presented here to show the enhancement effect using the proposed technique, including a microinductor structure and a vertical comb drive structure that demonstrate potential applications even on electrically conductive devices.

Study of stress and adhesion strength in SU-8 resist layers on silicon substrate with different seed layers

We present the details of our study on the internal stresses and adhesion strengths of SU-8 structures to different substrate seed layers. The effect of adhesion promoter-methacryloxy [propl] trimethoxysilane (MPTS), and OmniCoat-and different seed layer combinations (Ti/Cu/Ti, Ti/Cu, Cr/Au, and Cr/Au/Cr) was examined for internal stress and adhesion strength in 650-µm-thick SU-8 films. Increased stress and poor adhesion have led to the delamination of SU-8 in some cases. Adhesion and stress have proven to be the function of process parameters such as soft bake (time and temperature), exposure dose, post-exposure bake (time and temperature), and development time. We have found that a 100 silicon wafer containing a titanium-copper-titanium (Ti/Cu/Ti) seed layer with MPTS as the adhesion promoter yielded a thick SU-8 film with a lower value of stress and consequently better adhesion for processing in deep x-ray lithography (DXRL). A detailed correlation of the effects of adhesion and internal stress on the SU-8 film is discussed. An analysis of the possible chemical bonding interactions occurring between SU-8, and its adhesion promoter and/or substrate is also presented.

Study of crack formation in high-aspect ratio SU-8 structures on silicon

The high-aspect ratio capability of SU-8 photoresist led to the successful use of this epoxy based material in a diversity of microfabricated devices as a construction material as well as for micromolding purposes. Throughout the literature it was noticed that the thermal mismatch of SU-8 and the substrate material silicon generates high film stress in the spin-coated SU-8 causing crack formation in the microstructures. Using baking parameters this crack formation can be minimized but will remain a critical aspect of design. In this study the process was first optimised on non-patterned wafers. Secondly, we transferred this optimised process to a pre-patterned wafer containing deep silicon etch pits to account for a specific application in micromolding. We discuss the behaviour of film stress, number of cracks and crack length. The number of cracks as well as the length of cracks in concave corner designs can be significantly decreased, while round holes resulted even in crack-free microstructures. In the case of pre-patterned wafers no cracks appear around the features, however we observed unsatisfied development within the resist features caused by insufficient solidification in the deep etch pits during Soft Bake. Increased Soft Bake time can overcome these problems but will require more systematic investigations.

Optimisation of SU-8 processing parameters for deep X-ray lithography

The negative photoresist SU-8 has been recognised as an unique resist, equally useful for conventional UV lithography as well as deep X-ray lithography (DXRL) applications [2, 7, 12, 17, 18]. One of the major limitations in the use of SU-8 in lithographic processes is the occurrence of internal stress [15]. The processing parameters investigated for DXRL of SU-8 included resist thickness (450–850 μm), soft bake time (7–11 h), exposure dose (30–70 J/cm3), post exposure bake time (20, 40, 60 min) and development time. The effect of these parameters on stress was evaluated using wafer curvature measurements. Taguchi optimisation techniques have been used to asses the contribution of these parameters on the stress of the developed structures. This study shows that softbake time contributes the most to stress in the SU-8 film at 50%, followed by the exposure dose and post exposure bake with 30% and 15% respectively. Stress varied somewhat linearly with thickness. At higher thickness, the deposition process needs to be changed for very high aspect ratio structures. The main objective of this work has been to optimise the processing conditions of thick SU-8 films for DXRL.

KMPR Photoresist Process Optimization Using Factorial Experimental Design

MicroChem Corp. and Nippon Kayaku Co., Ltd. have jointly developed a new, photoimageable coating composition for use in fabricating a wide variety of micro-electromechanical systems (MEMS) and optical MEMS. This chemically amplified thick-film photoresist, known as KMPR, has many of the same high-aspect ratio photo imaging properties as solvent developable SU-8, but can be developed in a conventional aqueous alkaline developer (TMAH) and readily stripped from the wafer.Single-factor experimental methods rarely find the optimum performance of a photoimageable coating efficiently and often fail to detect the interactions so common in complicated photochemical systems. By combining the settings of several factors simultaneously in an array (experimental design), it is possible to separate out the main effects and their interactions.1This paper presents the optimization of KMPR for contact alignment and near diffraction-limited, thick film reduction stepper applications. A Sequence of Taguchi style, half-factorial experimental designs was used to evaluate the process parameters for a 5 and 50 Mm thick film exposed on a contact aligner followed by a third half-factorial design for a 10 Mm thick film of KMPR photoresist exposed on a 365 nm, 0.40 N.A. stepper. Independent factors, such as: soft bake time, focus, exposure dose, initiator activation time (post-exposure bake time), developer time and delay time were evaluated

A novel end point detection technique for in situ development of photoresist

The use of in situ measurements for development end point detection can drop process deviations caused by uncontrolled lithography parameters. Some of the process variations are temperatures, humidity, softbake time, process time delays between softbake and exposure, exposure parameters or developer temperature and normality. This paper will focus on an effort to identify process deviations from these parameters for different spray development processes on wafer tracks. We discuss the use of this end point detection techniques for process development and internal process automation.