Tuesday, April 14, 2015

Ozone for high quality High-k Capacitors by Atomic Layer Deposition

IN USA, Inc. is a leading manufacturer of commercial Ozone Instrumentation and Ozone Generators designed to produce ultra clean, high purity and very high concentration ozone gas that is ideal for a wide range of semiconductor process applications such as:
  • Atomic Layer Deposition ALD
  • Chemical Vapor Deposition CVD
  • Oxide growth
  • Surface conditioning
  • Ashing
  • Wet Processing
  • Particle Cleaning
  • Photoresist Removal
  • Epitaxy


The use of ozone based ALD processes for memory technologies like DRAM, a metal–insulator–metal (MIM) capacitor device, has been standard since the introduction of high-k materials in 2004 (Samsung 90 nm DRAM) [1]. Further improvement of deposition processes, material properties, and integration schemes has been crucial in order to meet the strict requirements of current and future devices.

One of the key challenges has always been to enhance the throughput of the ALD process for the high-k node dielectric, which has been a bottleneck in production since it was introduced, especially in the case of high aspect ratio devices like the DRAM capacitor cell. That is why most DRAM producers (Samsung, SK Hynix, Micron, Elpida, Winbond) have always used Batch Furnace ALD equipment from, e.g., Tokyo Electron or ASM, and in some cases multi-wafer process chambers like the JUSUNG's CYCLONE PLUS™ .

When it comes to the choice of Zr-precursor there are a number of options in the market. Initially, the dominating precursors were the Zr alkyl amides, first developed by Gordon Lab at Harvard [2] (e.g. TEMAZr) and later by more thermally stable heteroleptic Zr-Cp precursor like ZyALD™ from Air Liquide[3]. Both types have in common that they produce better performing material and process when used with ozone as a co-reactant instead of the more common use of water in, e.g., metal halide based ALD processes.

There are a number of reasons why ozone is a better choice than water:

1) Less impurities – Incorporation of impurities in the film is lower compared to water based processes, since the process runs in combustion like mode burning off ligands into highly volatile byproducts like CO, CO2 and H2O. Higher purity will also reduce the capacitance equivalent thickness (CET) at a given leakage current, which is very important for scaled DRAM capacitors.

2) Higher throughput – Ozone and the process by-products as described above can be more effectively purged compared to water and less volatile ligands that have not been broken down.

3) Process activation – There are a number of ALD processes that can only be activated in thermal ALD mode by ozone. Those are, for instance, many of the precursors for rare earth oxides that are sometimes used as dopants in a high performance high-k stack (e.g. La, Gd, and Er).

4) Fab facilitation – From a practical point of view no water bubbler refill is required - ozone is available on demand from a ring line supplied and monitored by ozone generators installed in the sub fab with the appropriate monitoring equipment.

5) Enhanced growth rate – Another trick for ozone based ALD is that some processes actually have “two ALD windows”, i.e., the process saturates in a step function – the first saturation lies at ~1 Å/cycle whereas the second and final saturation can lay as high as 2.8 Å/cycle [4], as described below.



Linear growth of ZrO2 thin films: ZrO2 thickness and standard deviation as function of reaction cycles at different deposition temperatures (TD). (graph used with permission, Ref. [4]) 
For TEMAZr/O3 ALD, the standard mechanism can be described in half-reactions as follows [3]:

O3 pulse: 2 surf-NR1R2 + x O3(g) --> 2 CaHbNcOd(g) + 2 surf-OH

TEMAZr pulse: 2 surf-OH + Zr(NR1R2)4(g) --> surf- Zr(NR1R2)2 + 2 HNR1R2(g)


The standard process as described above produces a maximum growth rate of 1.1 Å/cycle determined from atomic models and experimental data. To explain the high growth rate seen experimentally in “the second saturation”, additional reactions must have substantial influence on ZrO2 growth. For instance, the enhanced growth rate can be explained by the presence of “active oxygen” at the surface after the O3 pulse that is created by pulsing high concentration of O3 into the reactor. [4]

In addition, the crystallization behavior of the enhanced ZrO2 process has been investigated. Up to a film thickness resulting from 47 cycles of ALD growth (8.4 nm), deposition of an amorphous ZrO2 occurs. After passing this critical film thickness, an increase in the growth rate to 2.8 Å/cycle has been determined which can be connected to the surface roughness and the density of active surface sites of crystalline ZrO2 films. The enhanced ALD ZrO2 process has been applied in fully integrated MIM capacitors that show very good electrical performance. The capacitors yielded a high quality dielectric with a k-value of 39.4 and a leakage current of below 10-8 A/cm2 for +/-1 V after full crystallization meeting typical requirements to be considered for a DRAM storage capacitor application. [4]

To conclude, this example shows the importance of having a high performing Ozone Technology that is not only designed to supply ozone on demand and reliably but also to produce ozone at as high concentrations as commercially possible in order to derive the most out of the ALD process and material properties.


IN USA offers a wide range ozone generators and ozone delivery systems that are carefully designed to meet the most demanding ALD applications. All of IN USA’s ozone generators are based on its proprietary thin gap silent corona discharge technology.

IN USA’s wide range of Ozone Generators, whether the AC Series of Air Cooled Ozone Generators for entry level applications, or the OG Series of Water Cooled Ozone Generators for more advanced applications, are designed for high purity processes delivering the highest concentration of ozone commercially available in their class.

All of IN USA’s Ozone Generators are available as standalone or as part of a custom turnkey Ozone Delivery System (ODS) that would be configured to meet any requirements and any budget constraints. They all could include IN USA’s cutting edge Instrumentation and Servo-Loop Control technology to interface to the tool while meeting the most stringent safety requirements.

For more information on IN USA's Ozone Equipment, please complete our Information Request Form, or contact IN USA via e-mail

Please come and visit IN USA Inc.’s stand to discuss your application at the upcoming ALD Conference in Portland Oregon from June 29 to July 1st.


References:

[1] “2004 -The Year of 90-nm: A Review of 90 nm Devices”, Dick James, Chipworks Inc. Advanced Semiconductor Manufacturing Conference and Workshop, 2005 IEEE/SEMI, Munich, Germany.
[2] “Atomic Layer Deposition of Hafnium and Zirconium Oxides Using Metal Amide Precursors”, Dennis M. Hausmann, Esther Kim, Jill Becker, and Roy G. Gordon, Chem. Mater. 14, 4350 (2002)
[3] “Novel mixed alkylamido-cyclopentadienyl precursors for ALD of ZrO2 thin films”, Jaakko Niinistö, Kaupo Kukli, Maarit Kariniemi, Mikko Ritala, Markku Leskelä, Nicolas Blasco, Audrey Pinchart, Christophe Lachaud, Nadia Laaroussi, Ziyun Wang and Christian Dussarrat, J. Mater. Chem., 18, 5243 (2008)
[4] “TEMAZ/O3 atomic layer deposition process with doubled growth rate and optimized interface properties in metal–insulator–metal capacitors”, Wenke Weinreich, Tina Tauchnitz, Patrick Polakowski, Maximilian Drescher, Stefan Riedel, Jonas Sundqvist, Konrad Seidel, Mahdi Shirazi, Simon D. Elliott, Susanne Ohsiek, Elke Erben and Bernhard Trui, J. Vac. Sci. Technol. A 31, 01A123 (2013)

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