David L. Adler

Inspection of templates for imprint lithography

Masks of any next generation lithography (NGL), such as imprint lithography, must eventually achieve and maintain the very low defect counts of current production masks. This requires typically fewer than 10 or even no defects over the entire field. We describe an inspection methodology and how it can be applied to the imprint template. Special test patterns etched onto the template enable both a die to die comparison, to find nuisance defects, and also calibration of sensitivity to different types of preprogrammed defects. A state of the art deep ultraviolet photomask inspection system (KLA-Tencor model 526) can detect these rare events with about 70nm threshold for imprint masks with reflection mode contrast. Initial scans are made at various stages of the imprint process: the newly processed mask, after dicing, and after several imprints. The scans show mostly isolated point defects at a density of ∼10–100permm2. This is an encouraging start for a new NGL, and reductions are expected from better proces...

Three-dimensional simulation of top down scanning electron microscopy images

Low voltage scanning electron microscopy (SEM) metrology and inspection are performed by immersing the sample in an electric field; under this condition, when a scanning electron beam images a sample containing insulating features (like oxides and resist), a surface global charge builds up to offset the applied field and a transverse local field will form as a result of the scanning beam. The surface global charge is responsible for the voltage contrast and imaging properties, while local fields degrade image resolution. In this article we describe a simulation approach able to explain the imaging properties of charged surfaces and how resolution is affected by local fields. Using electron ray tracing in the column, the simulation follows both the emitted and primary electron trajectories outside the sample. In addition, Monte Carlo scattering simulation calculates the electron trajectory and charge deposition inside the sample. The resulting charge density is used to calculate the field inside and outsid...

Three dimensional simulations of SEM imaging and charging

SEM based CD control and wafer inspection has an increasingly active role in the semiconductor industry. Current design rules require a CD control with a precision in the nanometer range. In order to achieve this precision, a complete model of the image formation mechanism is desirable. For this reason we present a three-dimensional simulation of scanning electron microscope (SEM) images. The simulations include Monte Carlo electron scattering, charging in the substrate and electron ray-tracing in the column. We investigate some specific cases in CD-SEM metrology: We will describe the effect of scan orientation relative to the orientation of the imaged feature on the apparent beam width (ABW), the effect of magnification on contact imaging, and the effect of residue in resist trenches. Our results, regarding these examples, clearly indicate that a fully three-dimensional numerical simulation is needed to obtain an understanding of image formation and resolution limiting factors.

SEM voltage contrast simulations

High resolution inspection and metrology is an important part of current semiconductor technology and has an increasingly active role as miniaturization is pushed beyond 200 nm. Current and future semiconductor design rules require not only high resolution CD control and inspection but also the ability to image high aspect ratio structures patterned on complex layers. This goal is usually achieved by using e-beam based tools that exploit voltage contrast to form images of deep structures; although useful, such images are not obviously and uniquely determined by their topographical counterpart since other parameters may significantly affect image appearance. In this paper we present a simulation approach that explains the imaging properties of charged surfaces under different conditions. This approach shows that in order to get a physical description of an e-beam image formation process, the surface must be considered as an electrostatic optical element with its own properties.

Economic approximate models for backscattered electrons

The Monte Carlo method is widely used to simulate the signal from a scanning electron microscope. Although the results closely match the actual signal, the method is inherently slow due to the repeated calculation of random trajectories. The authors re-examine the alternative approach of using an approximate model to describe the signal. The authors develop a progressive approximate model which describes the spatial probability distribution of an electron along its entire path. The model also includes information about the angular and energy distribution of the electron. The model is compared to the Monte Carlo method by determining the yield of backscattered electrons as the beam is scanned across a step feature. For this simple feature the model-based approach computes the signal approximately two orders of magnitude faster.

Inspection and repair issues for Step and Flash Imprint Lithography templates

Step and Flash Imprint Lithography (S-FIL) 1X templates must eventually achieve and maintain the very low defect counts commensurate to current production masks. This requires typically fewer than ten or even no defects over the entire field and to minimize template fabrication costs and techniques must be identified to repair defects on templates when they do occur. We describe inspection and repair methodologies and how it can be applied to the imprint template. For inspection, test patterns etched onto the template enable both a die-to-die comparison, to find nuisance defects, and also calibration of sensitivity to different types of preprogrammed defects. A state of the art deep UV photomask inspection system (KLA-Tencor model 526) can detect these events with about 70 nm threshold for imprint masks using reflection mode contrast. Initial scans are made at various stages of the imprint process: the processed mask, after dicing, and after several imprints. The scans show mostly isolated point defects at a density of ~ 10 to 100 per mm2. To repair defects, studies were undertaken using RAVE’s nm650 tool which is essentially an AFM platform that relies upon a nano-machining technique for opaque defect removal. On S-FIL templates, the standard deviation for depth repairs in quartz from the target depth was found to be 3.1 nm (1σ). The spread in edge placement data for opaque line protrusions was 21.5 nm (1σ). Trench cuts through lines were successfully created with a minimum size of about 55nm. The repairs on the template were verified by imprinting the features on wafers. The range of depth offsets studied (-15 to +15) had no bearing on the imprinting process and the edge placement on wafers replicated the edge placement of the repaired templates. Trench cuts on the template were successfully filled with the imprint monomer and measured slightly larger than the minimum gap size. Finally, the imprinted wafers were used to pattern transfer features into 100nm of oxide.