Plasma-based Pattern Transfer Research Articles

Quasiordered, subwavelength TiO2 hole arrays with tunable, omnidirectional color response

Fabrication, optical characterization, and simulation of subwavelength TiO2 hole arrays exhibiting geometry-tunable, omnidirectional color response across the visible spectrum is described. Partially suspended TiO2 grating “membranes” (hole arrays supported by a high void-fraction, low-index underlayer) with quasiperiodic hexagonal order were created on an Si substrate using colloidal lithography, metal mask, plasma-based pattern transfer, and XeF2 etching. Optical measurements under specular and diffuse reflection conditions, along with finite-difference time-domain simulations, indicated that the omnidirectional color response of the hole arrays emerges from a broad distribution of Fano resonance states formed by coupling between guided and internal Fabry–Pérot (FP) cavity modes of the TiO2 layer. Higher-order FP resonances from the external cavity formed between the TiO2 layer and substrate control the apparent color when devices are viewed in direct light. The simulated modal behavior of arrays was found to be very sensitive to the degree of Si removal from the underlayer, in agreement with experimental observations. The fabrication methodology presented herein is substrate-agnostic and can be employed to fabricate suspended, subwavelength hole arrays in many material systems, with potential application to optical filters and reflectors, photocatalytic electrodes, photovoltaics, and sensors.

Fabrication and optical behavior of graded-index, moth-eye antireflective structures in CdTe

A simple and scalable method, based on dip-coat colloidal lithography, mask reduction, and plasma-based pattern transfer, is presented to create graded-index, moth eye-inspired antireflective features on II–VI semiconductors. Hexagonal arrays of isolated conical frusta with tunable geometry (top diameter = 200–1300 nm, pitch = 310–2530 nm, and height = 790–7100 nm) were realized by isotropic etching of various size silica colloid masks before pattern transfer into the underlying substrate. Substantial increases in single-side direct and total infrared (IR) transmission across the 4–20 μm range (9%–15% for CdTe thin films and 18% for bulk CdTe) were achieved, in excellent agreement with transfer matrix calculations and finite difference time domain optical simulations. The fabrication method presented can be used to enhance efficiency in multiple IR application areas including photovoltaics, optical system components, detectors, and focal plane array imagers.

Differences in erosion mechanism and selectivity between Ti and TiN in fluorocarbon plasmas for dielectric etch

Metallic masking materials are promising candidates for plasma-based pattern transfer into low-k materials for fabricating integrated circuits. Improving etching selectivity (ES) between the low-k and hardmask material requires a fundamental understanding of material erosion in fluorocarbon (FC) plasmas. The authors have previously reported on the erosion mechanism and plasma parametric dependencies of Ti etch in FC discharges. The present work focuses on elucidating differences in the erosion behavior between Ti and TiN hardmasks. The authors studied erosion of Ti, TiN, and organosilicate glass (OSG), a reference low-k material, in CF4/Ar and C4F8/Ar plasmas. Changes in surface composition, FC surface reaction layer thicknesses, erosion rates, and corresponding ES were established by x-ray photoelectron spectroscopy and in situ ellipsometry. The authors found that the erosion stages and plasma parameter dependent surface compositions were similar for Ti and TiN. The previously established dependence of T...

Studies of fluorocarbon film deposition and its correlation with etched trench sidewall angle by employing a gap structure using C4F8∕Ar and CF4∕H2 based capacitively coupled plasmas

A high-aspect ratio, small gap structure that provides a sample surface region without direct ion bombardment has been used to study surface chemistry aspects of fluorocarbon (FC) film deposition and to simulate FC film deposition on trench sidewalls during plasma-based pattern transfer. As on the sidewalls of microscopic trenches being etched, thin FC layers form by arrival of reactive neutrals on the shadowed surface portions of the small gap structure. The deposition rates, composition, and chemical bonding of FC films formed in the small gap structure were determined by ellipsometry and x-ray photoemission spectroscopy as a function of process conditions for C4F8∕Ar and CF4∕H2 discharges produced in a dual frequency (40.68∕4MHz) capacitively coupled plasma reactor. Actual trench features were also produced using photoresist patterned organosilicate films for the same plasma processes. Scanning electron microscopy of the trenches shows a characteristic sidewall slope angle for different process conditions. We find that plasma process conditions producing lower FC film deposition rates on the shielded surface of the gap structure yield more vertical trench sidewalls. This relationship confirms the relevance of the small gap structure approach to the examination of trench sidewall chemistry. Since the present approach produces macroscopic samples of gap-deposited FC films, it enables direct surface chemical characterization of a material that is analogous to sidewall deposited films and allows to avoid the difficulties connected with direct measurements of microscopic samples. The lack of ion bombardment for the shielded deposition increases the retention of the chemical structure of the FC film precursors for the deposited films, which promises to be useful for obtaining mechanistic insights on film precursors.

Plasma-surface interactions of nanoporous silica during plasma-based pattern transfer using C4F8 and C4F8∕Ar gas mixtures

We have investigated plasma surface interactions of nanoporous silica (NPS) films with porosities up to 50%, and SiO2 with C4F8∕Ar discharges used for plasma etching. The pore size was about 2–3nm for all films. In highly polymerizing plasmas (e.g., pure C4F8 discharges), the porous structure of NPS material favors surface polymerization over etching and porosity-corrected etching rates (CER) were suppressed and lower than SiO2 etching rate for the same conditions. The etching rates of NPS were dramatically enhanced in ion rich discharges (e.g., C4F8∕90%Ar) and the CER in this case is greater than the SiO2 etching rate. Both x-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (static SIMS) show that fairly thick (∼2–3nm) fluorocarbon layers exist on the NPS surface during C4F8 etching. This layer blocks the direct interaction of ions with the NPS surface and results in a low etching rate. For C4F8∕90%Ar discharges, little fluorocarbon coverage is observed for NPS surfaces and the direct ion surface interaction is significantly enhanced, explaining the enhancement of CER. We can deduce from analysis of angular resolved XPS data that the surface of NPS materials and SiO2 remain smooth during C4F8 etching. For C4F8∕90%Ar etching, the NPS surfaces became rough. The surface roughening is due to angle-dependent ion etching effects. These surface models were directly verified by the transmission electron microscopy. Depth profiling study of NPS partially etched using C4F8 or C4F8∕90%Ar discharges using dynamic SIMS indicates that the plasma induced modification of NPS was enhanced significantly compared with SiO2 due to the porous structure, which allows the plasma attack of the subsurface region. The modified layer thickness is related to the overall porosity and dramatically increases for NPS with an overall porosity of 50%. The distinct etching behavior of high porosity NPS (∼50%) in fluorocarbon-based discharges relative to NPS material with lower overall porosity is possibly due to interconnected pores, which allow plasma species to more easily penetrate into the subsurface region.

Investigation of surface modifications of 193 and 248nm photoresist materials during low-pressure plasma etching

Plasma-based pattern transfer of lithographically produced nanoscale patterns in advanced photoresist materials is often accompanied by photoresist surface roughening and line edge roughening due to factors which are not well understood. We have studied the evolution of surface roughening in prototypical 193 and 248nm photoresist materials during plasma processing as a function of plasma operating parameters. We used real-time ellipsometry and mass spectrometry, along with atomic force microscopy, x-ray photoemission spectroscopy and time-of-flight secondary ion mass spectrometry in an effort to understand the morphological and chemical changes of the photoresist materials as a function of plasma–surface interactions parameters, e.g., maximum ion energy, total energy flux, and plasma chemistry, and photoresist material. A comparison of 248nm photoresist with 193nm photoresist shows that significantly more surface roughness is introduced in the 193nm photoresist for most plasma processing conditions investigated. We also find a dramatic dependence of surface roughening on the chemistry of the plasma process, e.g., for Ar–C4F8 a modified photoresist surface layer with an extent of about 50nm is produced in 193nm photoresist, whereas for C4F8 discharges the surface modification is much less for otherwise similar conditions. We show that one important reason for these differences may be ion-enhanced selective volatilization of carbonyl groups of the 193nm photoresist polymer backbone which is absent for the 248nm material, along with modulation of the ion-interaction with the photoresist material by fluorocarbon surface passivation.

Surface science issues in plasma etching

Pattern transfer by plasma-based etching is one of several key processes required for fabricating silicon-based integrated circuits. We present a brief review of elementary plasma-etching processes on surfaces and within integrated-circuit microstructures—and an overview of recent work in our laboratory on plasma-etching aspects of the formation of self-aligned contacts to a polysilicon layer through a SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> layer and a Si <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> etch-stop layer. The work illustrates the richness of associated surface science issues that must be understood and controlled in order to most effectively achieve plasma-based pattern transfer.

Surface modification by plasmas: X-ray photoemission studies

Multilevel metallization schemes and ultra-large-scale integration of silicon devices require plasma-based pattern transfer processing. During plasma-based pattern transfer, the silicon surface is subjected to ion bombardment and can be coated with thin residue layers of species from the plasma, producing a damaged layer. The chemistry of the residue layer depends upon the plasma composition, system geometry, and substrate material. X-ray photoemission spectroscopy (XPS) has been used extensively to characterize plasma-induced residue layers. Various plasma etch and processing residues observed during silicon device pattern transfer will be discussed.