AVS2016 Session AS+SS-ThM: Depth Profiling, Buried Interfaces, and 3D Analyses

In 1956, Porous silicon (PS) was accidentally discovered by Uhlir at Bell Laboratories [1], and t he material was very much ignored. Later (70’s and 80’s) porous Si was found to be useful because its high surface area [2-5] for various applications, like microcavity, broadband AR coating, mid infrared LEDs, chemical Sensors, smart Dust, pressure Sensor, photonic crystal. Recent interest of porous Si is in the biomedical field [6] with wide range of applications, ex. drug delivery, cancer therapy, and tissue engineering. For diverse applications, single or multilayers porous Si stacks are required. In this abstract we present our metrology invention for single or multilayers porous Si stacks analysis.

We introduce a novel approach to characterize the different Si film stacks by using Model Based Infrared Reflectometry (MBIR). We believe, we are first group to apply the technique for analyzing various multilayer Si stacks. The film stack thickness has varied between 1 μm to more than 100 µm. Thick layers of silicon are opaque in the UV-VIS wavelength range, and IR wavelengths are ideal for measurements of such films. The ability to specify a film stack in the MBIR analysis model makes the technique more versatile, compared to traditional FTIR. We will present in the conference how the IR optical properties of PS can be described by Bruggeman Effective Medium Approximation (EMA), employing a standard multilayer reflectance model. To validate the MBIR results x-SEM and Gravimetric analysis have been used. The results have shown MBIR to be a suitable technique for characterization and production monitoring of the process steps associated with the new porous silicon applications field.

References

1. Uhlir, A., Electrolytic shaping of germanium and silicon. Bell System Tech. J., 1956. 35: p. 333-347.

2. Gupta, P., V.L. Colvin, and S.M. George, Hydrogen desorption kinetics from monohydride and dihydride species on Si surfaces. Phys. Rev. B, 1988. 37(14): p. 8234-8243.

3. Gupta, P., et al., FTIR Studies of H2O and D2O Decomposition on Porous Silicon. Surf. Sci., 1991. 245: p. 360-372.

4. Dillon, A.C., et al., FTIR studies of water and ammonia decomposition on silicon surfaces. J. Electron Spectrosc. Relat. Phenom., 1990. 54/55: p. 1085-1095.

5. Dillon, A.C., et al., Diethylsilane Decomposition on Silicon Surfaces Studied using Transmission FTIR Spectroscopy. J. Electrochem. Soc., 1992. 139(2): p. 537-543.

6. Porous Silicon for Biomedical Applications, Edited by: H.A. Santos ISBN: 978-0-85709-711-8

One of the aims in CMOS device development is to reduce power consumption while increasing performance. Pivotal to this, is the critical need to engineer dopant profiles, and to define the formation of the appropriate junctions. Tied to this is the increased severity of short channel effects (SCEs) as dimensions are decreased, hence the reason to move to 3D structures in the form of FinFETs. One type of SCE that is known to cause performance degradation is Drain Induced Barrier Lowering (DIBL). To reduce DIBL, dopant junction profiles are made more abrupt. This can be done through the introduction of Sigma/cavity, fully depleted silicon-on-insulator (FDSOI) structures and the modulation of stress through optimal engineered epitaxial buffer layers. To assess the quality of interfaces in these different structures over nanometer scale regions requires the use of analysis techniques such as Atom Probe Tomography (APT) and Transmission Electron Microscopy (TEM). This presentation will discuss the ability of APT to extract the critical information of interest to device engineering.

High conformality—the ability of a thin film to cover a complex three-dimensional substrate uniformly—is a key advantage of atomic layer deposition (ALD) compared to chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes. More than 700 ALD processes (with unique reactant pairs/activation) have been reported, as calculated from Ref. [1]. Conformality has been experimentally studied for a small minority of these, most likely because of the lack of easily available test structures and accessible methods of analysis.

When conformality is investigated, most typically, vertical trenches (or holes) etched into silicon, typically with aspect ratio (AR) up to around 50:1, are used and the results are analysed point by point by cross-sectional electron microscopy. Comparison of results obtained in different studies is difficult because of the lack of standard test structures and standard means of analysis, and the large variety of process conditions that are used. The theoretical framework for interpreting the conformality results is also underdeveloped.

To develop the conformality analysis, we have reported on macroscopic [2, 3] and microscopic [4] lateral high aspect ratio (LHAR) test structures. In contrast to vertical HAR, LHAR structures allow one to investigate thin film thickness and properties in very demanding aspect ratios (>10 000:1) and obtain accurate information of film thickness and properties along the feature by standard means of measurement such as ellipsometry and reflectometry. The purpose of the present work is to compare results obtained for different thin film processes in different test structures (our own + literature) using the highly studied [1, 5] Me3Al/H2O ALD process as baseline. We propose a classification scheme for how the thickness line profiles are expected to vary inside LHAR structures in different cases of characteristic governing growth chemistries.

Acknowledgements: This work has been funded by the Finnish Centre of Excellence in Atomic Layer Deposition, BOF-UGent, FWO-Vlaanderen and SIM-Flanders (TRAP-FUNC project). Feng Gao and Meeri Partanen are thanked for fabricating the microscopic LHAR structures.

References:

[1] V. Miikkulainen et al., J. Appl. Phys. 113 (2013) 021301.

[2] J. Dendooven et al., J. Electrochem. Soc. 156 (2009) P63.

[3] J. Dendooven et al., J. Electrochem. Soc. 157 ( 2010), G111.

[4] F. Gao et al., J. Vac. Sci. Technol. A, 33 (2015), 010601.

[5] R. L. Puurunen, J. Appl. Phys. 97 (2005) 121301.

1. Introduction

Probing the sample chemistry beyond the surface region with ion beam sputtering is subject to practical limitations which include preferential sputtering, accumulated sputter beam damage, inclusions, and voids. These effects can result in a distortion or complete loss of the true chemical distribution as a function of depth.

In situ FIB milling and sectioning with TOF-SIMS chemical imaging (3D FIB-TOF tomography [1]) is an alternative approach to achieve 3D chemical imaging of complex matrix chemistries. The FIB milling can minimize or eliminate artifacts caused by sputter depth profiling from the surface.

For matrices with organics components, however, FIB beam-induced chemical or molecular damage may limit the detection of characteristic molecular signals. The characteristic molecular signals can often be recovered with cluster ion polishing to remove the organic FIB damage.

2. Method

The 3D chemical characterization of organic and organic/inorganic mixed composition structures was achieved utilizing FIB-TOF on a PHI TRIFT nanoTOF II (Physical Electronics, USA) imaging mass spectrometer equipped with the new parallel imaging MS/MS [2,3]. The spectrometer’s large angular acceptance and depth-of-field maintain high mass resolution and high mass scale linearity in this challenging geometry.

3. Results

Results will be presented for structures with mixed organic phases and mixed organic/inorganic phases. The FIB-TOF results will be compared with corresponding sputter depth profiling results to highlight the relative advantages of the two techniques along with potential complicating factors to the analyses. The high sensitivity of the TOF-SIMS technique is not limited to strong organic matrix peaks. Data will be presented showing the ability to probe the 3D distribution of polymer additives in a sample. The identity of the additives is confirmed using the newly developed parallel imaging MS/MS option for the nanoTOF II. The ability to study buried organic structures with FIB-TOF and then conclusively identify the detected species using MS/MS is a powerful new development for the field of TOF-SIMS.

[1] A. Wucher, G.L. Fisher and C.M. Mahoney, Three-Dimensional Imaging with Cluster Ion Beams (p. 207-246) in Cluster Secondary Ion Mass Spectrometry: Principles and Applications, C.M. Mahoney (Ed.), Wiley & Sons, N.J. (2013).

[2] P.E. Larson, J.S. Hammond, R.M.A. Heeren, G.L. Fisher, Method and Apparatus to Provide Parallel Acquisition of MS/MS Data, U.S. Patent 20150090874, 2015.

[3] G.L. Fisher, J.S. Hammond, P.E. Larson, S.R. Bryan, R.M.A. Heeren, SIMS XX Proceedings, D. Castner (Ed.), Wiley, N.J. (2016) DOI: 10.1116/1.4943568.

Introduction

Depth profiling of organic layers for optical and electronic devices can be ideally performed using gas cluster ion beams (GCIB) in combination with time-of-flight secondary ion mass spectrometry (TOF-SIMS). For optimum performance a dual beam approach is utilized, employing a lower energetic quasi DC sputter beam for material removal and a short pulsed small spot analysis beam for optimal mass spectral and imaging performance.

However molecular identification of unknown substances, e.g. contaminants, is usually hampered by constraints in mass resolution and mass accuracy of the TOF analyser. Furthermore ions generated in the sputter phase of the dual beam experiment are lost for the MS analysis. In order to overcome these limitations a TOF/Orbitrap^TM-SIMS hybrid mass analyser instrument was developed.

Methods

A prototype SIMS instrument with a hybrid TOF/Orbitrap mass analyser was utilized for acquisition of organic depth profiles. During sputtering with 5-20 keV argon clusters secondary ions can be detected using the Q Exactive^TM HF mass analyser. Selective ion gating was implemented to avoid artefacts from the crater walls. In situ tandem MS analyses of the most abundant peaks were used to confirm the mass assignments. Imaging TOF analysis with high lateral resolution was performed on the same instrument using short pulses from a 30-60 keV Bi-liquid metal ion gun (LMIG) and a dedicated TOF.SIMS V analyser for comparative measurements.

Preliminary Data

Molecular depth profiles were acquired using GCIB induced desorption in a single beam approach from organic test structures and organic LED materials. The high mass resolution of 240 000 of the Q Exactive HF mass spectrometer proved to be essential for separation of otherwise overlapping ion signals. Molecular assignments based on the high mass accuracy below 3 ppm were validated using tandem MS analysis. Up to 5 decades of dynamic range and a depth resolution below 8 nm were found to be possible with this approach leading to unprecedented depth profiling results.

Depth profiles and according spectra are compared to the classical dual beam approach. While depth resolution is similar, differences in the relative signal intensities were observed in spectra from the two different ion beams. Implications for the analysis of biological samples will be discussed.

In historical art objects, binding-medium degradation involves complex chemistries that can occur at the surface or interface of a paint layer. These can propagate inward toward the bulk material, caused by inherent impurities within the paint that migrate throughout the paint layer. These effects can cause mechanical failure due to binding-medium degradation, primarily observed as paint-layer flaking, spalling, and fracture. Our prior research performed on Renaissance-era artwork has indicated two major correlations to the severity of binding-medium degradation: i) depletion of long-chain fatty acid components within the binding medium of a paint layer, and ii) alteration of the amino-acid composition of proteinaceous materials comprising the binding medium. In this study, the effects of controlled aging factors (i.e., heat, humidity, and UV exposure) on thin films of egg tempera were observed through the use of x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The newly available technology of gas cluster ionization sources (GCIS) allows for the ability to depth profile through some soft organic and biological materials with no little or no ion-induced damage. By using an argon-cluster ion beam to depth profile through a degraded thin film, a 3-dimensional analysis of short- and long-range degradation effects, followed by XPS and ToF-SIMS, respectively, was performed. Since ultramicrotomy is an established sample-preparation technique in the art conservation field, results of GCIS will be compared to ultramicrotomy as sample preparation method for organic and biological thin films.