10/09/2018

Part 1: Metrology for Semiconductor Lithography

SERIES Advanced Packaging - Multi-Sensor Metrology for Every Process Step

The fabrication of an integrated circuit (IC) requires a variety of physical and chemical processes performed on a semiconductor (e.g., silicon) substrate. By creating structures of various components, millions of transistors can be built and wired together to form the complex circuitry of a modern microelectronic device. Fundamental to all of these processes is lithography.

Optical lithography is a photographic process in which a light-sensitive polymer, called a photoresist, is exposed and developed to form 3D relief images on the substrate. In general, the ideal photoresist image has the exact shape of the designed or intended pattern in the plane of the substrate, with vertical walls through the thickness of the resist. Thus, the final resist pattern is binary: parts of the substrate are covered with resist while other parts are completely uncovered. This binary pattern is needed for pattern transfer since the parts of the substrate covered with resist will be protected from etching, ion implantation, or other pattern transfer mechanism.

The general sequence of steps for a typical optical lithography process is as follows:

1.   substrate preparation,

2.   photoresist spin coat,

3.   prebake, exposure,

4.   post-exposure bake,

5.   development and

6.   postbake.

However, lithography as it is optimized for IC technology has a strong 2D nature. After all, the IC fabrication process flow comprises a sequence of stacking many virtually two-dimensional high precision process layers. The layout of each layer is designed in terms of XY positions and dimensions, defining either opaque or transparent areas.

Detailed information on the homogeneity of the photoresist layer deposited on a wafer is required under aspects of process control and optimization. Measuring the thickness and homogeneity of photoresists, which can be up to several hundred micrometers in thickness, is a major challenge for most non-contact optical techniques.

The MicroProf® with the interferometric sensor CWL FT offers the optimal solution to measure photoresist thicknesses.

The CWL FT is based on the spectral evaluation of the superposition of partial waves of a white light source reflected at the interfaces of a transparent layer. At a given layer thickness and refractive index, the intensity of the superimposed partial beams varies with the wavelength. The spectrum then shows a typical interference pattern. From this spectrum and the refractive index of the layer material, the sensor calculates the thickness of transparent layers that are measured at high resolution with vertical resolutions from 10 nm. With the CWL FT the layer thickness can be evaluated at a given position, along a profile or over the entire surface to evaluate its coating homogeneity.

The interferometric sensor IRT 80 can also be used for characterization of very thin photoresist thicknesses. The IRT is an interferometric film thickness sensor with an infrared light source that has been specially developed for measuring the thickness of products that are transparent in near infrared light. The measurements can be carried out at individual positions, in the profile or flat in the form of a mapping. In addition, the sensor is also suitable for the investigation of multilayer systems.

The MicroProf® uses the interferometric sensors for highly precise, spatially resolved coating thickness measurements. In a multi-sensor arrangement with an interferometric measuring head and a confocal chromatic topography sensor, the customer has an extraordinarily powerful measuring system for fast, spatially resolved layer thickness and topography measurements. With the MicroProf® a high-resolution 3D measurement of the homogeneity of the photoresist on wafers up to 300 mm can be carried out. The wafer as a whole is recorded quickly and non-destructively. The degree of automation ranges from manually operated measuring systems such as the MicroProf® 300, which automatically execute predefined programs, to fully automated wafer handling including automatic pre-alignment and fine alignment in the MicroProf® MHU.

The measurement evaluation is carried out with the analysis program Mark III. The powerful program is used in the FRT measuring systems for the display and evaluation of topography data as well as for layer thickness measurements. The results of the 3D mapping include the maximum, minimum and average photoresist thickness.

Overlay, the result of an alignment-exposure-development process, is one of the key benchmarks for a lithographic process. A precise characterization of overlay requires large amounts of measurements over the wafer hence the overlay measurement tools are usually automated. The measurement principle of optical overlay measurements is based on pattern recognition of customized features thus the alignment performance depends on the optical quality of the marker images.

A new software package for Acquire Automation XT offers fully automated measurement and analysis of the overlay offset in x- and y-direction as well as the rotation of microstructures. The function can be used on high resolution camera images as well as FRT's field of view sensors like the CFM (DT). This software feature combined with FRT's powerful sensor range efficiently helps manufacturers of modern 3D IC components to improve processes and increase production yield.

Whether for laboratory, development, quality assurance or production - FRT offers the right measurement technology for your application from Advanced Packaging. Do not hesitate to contact us if you have any questions. Our experts will be glad to support you in solving your measurement tasks by creating the best possible system configuration for you.