Challenges for Non-Ideal Atomic Layer Deposition Processes and Systems

J.R. Gaines, Technical Director of Education for Kurt J. Lesker Company Another Introduction to Atomic thick. Presently the U.S. Department of Defense anticipates that the last process node for Layer Deposition semiconductor devices (the end of Moore’s) is 7 Introduction to Atomic Layer Deposition nm and will be achieved by 2020[2]. (ALD) has been described as a thin film deposition technology that can keep the The ubiquitous example of an ideal ALD process semiconductor industry on track per Moore’s is the creation of Al2O3 monolayers on silicon Law (or observation)[1] for a few more years. In substrates. Figure 1 depicts a four step process its most ideal form, it is a process that enables where a functionalized silicon substrate with monolayer, or sub-monolayer growth of certain hydroxyl group reaction sites is exposed to materials through the sequential exposure of a trimethyl aluminum (TMA). The precursor functionalized substrate to a pair of precursor molecules chemically bond with any available gases. If dosed correctly the gases attach at attachment sites to create an aluminum- specific surface sites and react to create a near containing monolayer. perfect film on the order of a few angstroms

8 2016 VACUUM EQUIPMENT RESOURCE GUIDE Next a purge gas is used to flush any extra, unreacted, or incompletely reacted precursor from the system. Oxidation then occurs through exposure to a mixture of carrier gas and H2O. After another purge step the process can be repeated and a stack of Al2O3 layers may result. Range of tool sets and processes This novel deposition technology is practiced in many forms. There are a few large-scale, state- of-the-art tools being deployed by companies like Applied Materials, a few hundred commercially manufactured research scale systems built by companies such as Oxford Instruments, Picosun, and the Kurt J. Lesker Company® and many home- grown systems made from scratch or adapted from existing tools. While systems from Applied may be focused specifically on a process, like Al2O3 and HfO2 for high-k dielectric applications, the research community continues to push the boundaries of film growth through the discovery and development of novel ALD chemistries. In the real world there is a predominance of non-ideal systems attempting to produce films from non-ideal materials pairings. See Figure 2.

Figure 2. Small-scale plasma-enhanced atomic layer deposition system (PEALD) at the University of South Florida. Source: http://transducers.eng. usf.edu/gallery.html

Basic vacuum and ALD It is appropriate to review some basic vacuum Figure 1. Atomic Layer Deposition utilizing two principles in order to illuminate the challenges distinct precursors (X, Y) sequentially dosed to the faced in the operation of ALD systems. substrate producing a chemical reaction www.avem.org 9 Molecular density and pressure Viscous flow ALD uses an inert carrier gas, typically argon or At a pressure of 10-3 Torr the ALD system is in nitrogen, to deliver precursor to the substrate and viscous flow (versus continuum or molecular also minimize reactant condensation on internal flow) with a carrier/precursor gas mixture of system components. These functions require a ~24 trillion molecules per cc rushing through continuous flow of gas at a pressure of ~10-3 Torr. the system. Flow in this regime is characterized At that pressure there is a mixture of nominally 2.4 by the development of vortices, dead zones, x 1013 molecules/cc of carrier and precursor gases. and unpredictable flow patterns. The viscous flow regime is given as an example of Chaos Flow regimes Theory[3]. While gas flow in an ALD system is There are three distinct gas flow regimes identified deterministic based on known conditions, it is for vacuum-based systems. The flow regimes still not predictable. Mixing of the carrier gas are based on the interaction between molecules and precursor during the dose cycle can also be at various molecular densities. Uninhibited, a an issue. The number of molecules involved are nitrogen molecule will travel in a straight line at heavily skewed toward the carrier gas with the about 500 meters/second. Small molecules like precursor molecules outnumbered by much more hydrogen move at 2,000 meters/second. Molecular than 100-to-1 (Fig. 3). behavior in the various flow regimes changes dramatically when collisions between molecules and internal surfaces are considered.

10 2016 VACUUM EQUIPMENT RESOURCE GUIDE Furthermore, the temperature window for the combined functions of effective gas transport and chemical reaction is modest (typically in the range of 100oC to 350oC) and, ideally, tightly controlled. At the operating pressures of ALD there is little heat transferred by conduction or convection, so the process relies on radiation and energy exchange from molecular collisions. Non-ideal processes In addition to critical equipment functionalities such as temperature and pressure control, as well Figure 3. The relationship of pipe diameter and as chemical resistance to the hostile precursors pressure to the transition point for Laminar, used in ALD, there are also unique peculiarities Transitional (Viscous), and Molecular flow regimes associated with many of the non-ideal chemical pairings required to make many desired thin films Heat transfer in vacuum using ALD[4]. To enhance the effects of a carrier gas, an ALD system may also be heated to further reduce unwanted film growth on internal hardware. The substrate is typically heated to promote a complete chemical reaction. Heat transfer in vacuum is a unique engineering challenge.

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www.avem.org 11 Selective substrate Incomplete substrate functionalization can be a source of film defects. With an insufficient number functionalization of reaction sites for precursors the resulting film One of the keys to achieving dense, defect free, growth may not be in monolayers. Some of the monolayer films is complete and perfect substrate substrate may be coated as planned, but films functionalization. This implies the comprehensive that nucleate in non-functionalized areas can be a siting of hydroxyls on the surface of a pristine source for non-conformity and surface roughness. silicon wafer or engineering the thickness of They may also be sources of unintended chemical an oxide layer for FinFETs[5]. Given the chaotic by-products during the ALD process where gas flow conditions in a reactor, it is highly unreacted or incompletely reacted precursors are possible that regions of a substrate may not see left to combine and form impurities. the functionalization gases uniformly, or that functionalized sites might be ablated away by For non-ideal chemical processes the exact the torrent of carrier gas molecules continuously method of surface functionalization may not bombarding the substrate. be clear. Robust functionalization requires an adsorptive process coupled with a non-reversible chemical reaction. Mere adsorption, in absence of a chemical reaction, may not create a bond site where true monolayer growth will occur. Films growing under non-ideal conditions may display what is called delayed nucleation. When considering the growth of a film across the entire topography of a substrate, it is convenient to use this term when using weight gain to monitor the evolution of a film. Multi-cycle Figure 4. AFM images of ALD-Al2O3 films delays in the accumulation of material (on a deposited with a variety of TMA/O pairings at 200oC quartz crystal monitor or QCM) are common. showing incomplete monolayer formation (Source: One considerable challenge is operating a QCM The University of Texas at Dallas, 2014. [12]) at the temperatures required for ALD; another is functionalization of the QCM’s surface so that it attracts precursor and undergoes the same chemical reactions as the substrate.

12 2016 VACUUM EQUIPMENT RESOURCE GUIDE Improper dosing to fulfill Thin film growth models reaction sites Deficiencies at the substrate surface cause regions For a given substrate there are a quantifiable of films deposited by ALD to divert from monolayer [6] number of functionalized reaction sites available growth models. Island growth tends to be the most to accept precursor molecules. While the number prevalent result reported for poorly functionalized or of sites can be calculated for a given substrate improperly dosed substrates. The creation of mixed [7] the calculation of a proper reactant dose requires material layers has also been reported . some accommodation for effects that may prevent receptors from meeting a proper mate. Doses are Non-IDEAL hardware systems typically calculated in Langmuirs, a unit defined As mentioned, there are a wide variety of ALD as an exposure of 10-6 Torr for 1 second. systems in the field, some designed from scratch and others adapted from or added to existing The chaotic nature of viscous flow, the ablative tools. In adaptation many necessary and/or ideal nature of swirling carrier gases, and time/ hardware functionalities may be lost. temperature considerations of exposure must be weighed against the cost of process materials, hardware degradation, and throughput.

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AVEM_Half-Horiz_8-15.indd 1 8/26/2015 3:49:26 PM www.avem.org 13 ALD reactor designs In order to monitor thin film growth, some There have been many designs put forward for system designers have installed ALD reactors ALD reactors intended to mitigate some of the directly into the sensor heads used to provide issues described. On paper, ALD can be done in a in-situ characterization. These hybrids highlight soda can (that can withstand an external pressure the extreme pressure differential required for of 15 psi and the harsh chemistries required). deposition of films, at 10-3 Torr, and the pressure Practically, system designs have taken the required for XPS, nominally 10-6 Torr. Pristine shapes of tubes, spheres, and more exotic forms substrate surface preparation often requires UHV engineered to optimize gas flows which may conditions of about 10-8 Torr. Interfacing ALD include shower heads, the use of curtain gases, systems with high vacuum or UHV chambers and other schemes[8] (Fig. 5). requires specialized engineering including sample manipulation/transit and hardware protection[9] (Fig. 6).

Precursors Pumps

Electrode with showerhead Plasma gas HVDC Precursors

Figure 6. An advanced ALD reactor design, courtesy of the Kurt J. Lesker Company

Pump

Microwave cavity

Plasma gas Pump Precursors

Figure 5. Basic ALD reactor designs, including, from top, a cross-flow reactor, radical-enhanced using the microwave concept, and direct plasma enhanced using a showerhead concept

14 2016 VACUUM EQUIPMENT RESOURCE GUIDE Heat management and though unlikely, for a system to change temperature between half-cycles to accommodate temperature control of specific desorption, reaction, and decomposition gases in vacuum requirements of precursors which are not In addition to reactor chamber design there is the ‘thermally compatible’. management of surface temperature to minimize condensation. To promote uniform heating and Needs for pre/post processing temperature control, reactant delivery systems with heat/plasma are often equipped with localized heaters and Many popular thin film deposition processes engineered thermal masses to insure that heat result in the formation of a compound or a is evenly distributed and held at the designated mixture, often needing post deposition processing temperature (Fig. 7). to completely react the mix into a stable phase. Post-deposition processing is sometimes required to eliminate residual precursors and/or to complete a chemical reaction which, in-situ, may have exceeded the temperature budget of the system or one or more of the precursors. Adding functionality such as a plasma source may create issues with respect to system integration and protection from precursors. This step may also need to be done outside of the ALD module. The solid thin film battery electrolyte LiPON has been recently demonstrated using ALD from a complex quaternary precursor system[10]. Figure 7. Interior of an ALD system showing After successful deposition and oxidation of typical temperature management systems lithium and potassium-containing films, a nitrogen plasma was used to remove oxygen and This highlights an additional condition for insert nitrogen into the Li3PO4 structure to make precursor material pairings wherein, ideally, LiPON at nominally Li3PO3.95N0.05. The ALD/ both reactants have a similar non-condensation Plasma films exhibit an ionic conductivity similar temperature and reaction temperature such that to films deposited by reactive RF sputtering. they may be used successfully. It is possible, www.avem.org 15 Impact on equipment performance and life expectancy Unlike physical vapor deposition of thin films by thermal evaporation, where many users would suggest that ‘not a lot can go wrong,’ there are many system and chemistry related conditions which must be met in order for anything in ALD to go right. Unintended combinations of precursors, or unreacted precursor components, can prevent thin film growth, create inclusions or nucleation sites that inhibit desired growth patterns, or Figure 8. Deposits of Al2O3 on vacuum pump create hostile solids, like Al2O3 particles which internal components after failure in use during an can damage system components like valves and ALD system cycle vacuum pumps. As an example, even in ideal systems such as the widely practiced TMA/H2O pairing to produce Al2O3, excess precursors not adequately flushed from the system can react on the surfaces of vacuum pumps, quartz crystal monitors or in-situ analytical instrumentation to affect their rapid demise (Fig. 8).

16 2016 VACUUM EQUIPMENT RESOURCE GUIDE Enhanced and informed References: system designs 1. No Moore’s Law for Batteries, Fred Schlachter, Dual-trace systems,[7] which provide discrete gas Proceedings of the National Academy of Sciences, delivery paths and exhausts for each precursor vol.110 no.14, 5273 may be an approach which can accommodate 2. End of Moore’s Law: It’s not just about physics, competing thermal properties of individual Brooke Crothers and Robert Colwell, CNET August 28, 2013 precursor gases. The challenge of mixing a few billion molecules with a few tens of trillions of 3. Chaos Theory - https://en.wikipedia.org/wiki/ Chaos_theory molecules during half cycle dosing may also be reduced. This approach may also reduce the 4. Atomic Layer Deposition: An Overview, Steven M. George, American Chemical Society, Published on opportunity for residual precursors to mix in the Web 11/30/2009 gas phase to create CVD-style reactions. 5. Enhancement Mode Strained (1.3%) Germanium Some vacuum pump manufacturers have Quantum Well FinFET (Wfin=20nm) with High also invested in product modifications, such Mobility (uhole=700 cm2/Vs), Low EOT (~0.7nm) as increased rotational speed and operating on Bulk Silicon Substrate, Agrawal, Barth, Rayner, temperature, to make their dry pumps more suited Arun, Eichfeld, Lavallee, Yu, Sang, Brookes, Zheng, Lee, Lin, Wu, Ko, LeBeau, Engle-Herbert, Mohney, for extended service in ALD systems [11]. Yeo and Datta, 2014, IEEE, IEDM14-414 Technologies that promote cross-discipline 6. Real-Time Observation of Atomic Layer Deposition collaborations often produce the most exciting Inhibition: Metal Oxide Growth on Self-Assembled results. Like many advanced processes, ALD Alkanethiols, Avila, DeMarco, Emery, Farha, Pellin, continues to benefit from the collaboration of Hupp and Martinson, Applied Materials & Interfaces, materials scientists including chemists, thin film 2014, 6, 11891 - 11898 technologists, vacuum technology experts, and 7. An atomic layer deposition reactor with dose quan- great plumbers. tification for precursor adsorption and reactivity studies, Larrabee, Mallouk and Allara, Review J.R. Gaines, Jr. is the technical director of education of Scientific Instruments, 84, 014102 (2013); doi: for the Kurt J. Lesker Company. He has spent nearly 10.1063/1.4774042 40 years in the business of materials science As an educator and author, Mr. Gaines is in charge of the 8. Atomic Layer Deposition Apparatus and Process, US technical education outreach program for the Lesker patent application publication US 2012/0269968 A1, Company, worldwide, including its Lesker U suite of October 25, 2012 Vacuum Technology courses. 9. Integrating Atomic Layer Deposition and Ultra- High Vacuum Physical Vapor Deposition for In Situ Fabrication of Tunnel Junctions, Elliot, Malek, Lu, Han, Yiu, Zhao and Wu, The University of Kansas and Chinese Academy of Sciences. 10. Atomic Layer Deposition of the Solid Electrolyte LiPON, Kozen, Pearse, Lin, Noked and Rubloff, University of Maryland, Chem. Mater. 2015, 27, 5324-5331. 11. Improving the Reliability of Dry Vacuum Pumps in High-k ALD Processes, Mike Boger, Edwards Vacuum, Tokyo, Japan, January 2015, Solid State Technology. 12. Atomic Layer Deposition of a High-k Dielectric on MoS2 Using Trimethylaluminum and Ozone, Cheng, Qin, Antonio, Lucero, Azcatl, Huang, Wallace, Cho and Kim, ACS Appl. Mater. Interfaces 2014, 6, 11834 – 11838, July 15, 2014.