An Improved Method for Hg^sub 1-x^Cd^sub x^Te Surface Chemistry Characterization

Journal of Electronic Materials, Jun 2005 by Olshove, R, Garwood, G, Mowat, I, Bangs, J, Liguori, M

Hg^sub 1-x^Cd^sub x^Te surface chemistry has been studied extensively with well-known tools such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES) in order to advance detector array operability, performance, and yield. Raytheon Vision Systems has pioneered the first application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) as a Hg^sub 1-x^Cd^sub x^Te surface diagnostic tool to provide unprecedented analysis capability, including analyzing a 0.5-µm-diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. In this study, data are presented illustrating surface chemistry geometry effects and photoresist redeposition due to common Hg^sub 1-x^Cd^sub x^Te processing steps including photolithography, bromine etching, and photoresist stripping.

Key words: Time-of-flight secondary ion mass spectrometry (TOF-SIMS), surface analysis, HgCdTe

INTRODUCTION

The fabrication of high-lateral resolution, tightgeometry, world-class-performance Hg^sub 1-x^Cd^sub x^Te infrared focal plane arrays requires the judicious and reproducible application of well-understood processing technologies. The degree of understanding of these technologies rests in great measure on accurate and informative characterization of the materials and surfaces of products, related wafers, and substrates. This information has proven essential to ultimately enhance Hg^sub 1-x^Cd^sub x^Te detector array operability, performance, and yield through a more complete understanding of process effects on device surface chemistry.

There exists an impressive repertoire of physical and chemical analysis tools that have been successfully applied in the past to solve many process, materials, and device challenges. Subtle surface chemistry changes in Hg^sub 1-x^Cd^sub x^Te have been studied extensively with well-known tools such as total reflection x-ray fluorescence spectroscopy (TXRF), energy-dispersive x-ray spectroscopy (EDX), Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA, also known as x-ray photoelectron spectroscopy (XPS)), secondary ion mass spectrometry (SIMS), and Fourier transform infrared spectroscopy (FTIR).1-4 Similar surface chemistry nuances have recently been investigated with more exotic tools such as time-of-flight secondary ion mass spectrometry (TOF-SIMS) in silicon semiconductor processing fabrication centers.5,6

Raytheon Vision Systems has pioneered the first application of TOF-SIMS as a Hg^sub 1-x^Cd^sub x^Te surface diagnostic tool to provide unprecedented analysis capability to assess surface chemistry effects. Due to the unique capabilities of TOF-SIMS, i.e., capabilities of high lateral resolution, ultra-thin surface film sensitivity, high mass resolution, molecular and chemical composition scrutiny, and applicability to insulators, semiconductors, and conductors, it is an ideal tool for pixel-scale root cause failure analysis and process improvement. In addition, TOF-SIMS can generate the entire mass spectrum in one measurement and has detection limits ranging as low as parts per million (surface) and parts per billion (bulk).

This study demonstrated TOF-SIMS as a viable Hg^sub 1-x^Cd^sub x^Te surface chemistry characterization tool for process effectivity assessment. Accordingly, data will be presented showing chemistry effects due to common Hg^sub 1-x^Cd^sub x^Te processing steps and physical photolithographic window geometries. Analyzed process steps include photolithography, bromine etching, and photoresist (PR) stripping. Prior to describing the experiments and discussing the results, TOF-SIMS is discussed in conjunction with other common Hg^sub 1-x^Cd^sub x^Te surface analysis tools already used industrywide.

TOF-SIMS ANALYSIS TECHNIQUE

The TOF-SIMS method of extracting secondary ions from a sample surface is very similar to what is referred to as conventional SIMS. An incident ^sup 69^Ga primary ion beam desorbs and ionizes species from approximately the first one to three atomic layers through ion excitation and ejection, also known as sputtering (Fig. 1). However, due to the pulsed nature of the primary ion beam leading to a low level of ion bombardment on the sample (~10^sup 12^ ions/cm^sup 2^), [much less than]1% of the top surface will have the likelihood of being impacted by a primary ion.7 During ion impact, the damaged area extends

The similarities between these two methods stop at the point the ions are extracted from the sample surface. In TOF-SIMS, the extracted secondary ions are then accelerated to a constant kinetic energy, thus imparting different velocities to different masses (Fig. 2). The ions then enter a field free region, so heavier ions will travel more slowly and have longer flight times, whereas lighter ions will travel faster and have shorter flight times over a fixed distance. With the known distances and velocities, it is straightforward to determine individual ion masses indicating species of origin. This will provide a mass spectrum for each TOF-SIMS measurement. Note that the ability of TOF-SIMS to detect an ion and measure its mass-to-charge ratio is irrespective of its origin or type whether elemental, organic, inorganic, or molecular. TOF-SIMS is ultra surface sensitive with detection limits ranging from 10^sup 7^ to 10^sup 10^ atoms/cm^sup 2^. Other surface analysis techniques are typically many orders of magnitude less sensitive than TOF-SIMS (Table I)