⚛️DIY Atomic Layer Deposition

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Preface

These pages will present the current proposed machine design plan and the work completed to date for our vertically aligned, cold-walled reaction chamber ALD machine for the Hacker Fab at CMU. The Fall 2024 semester's efforts at CMU are described here and the Spring/Summer 2025 efforts are described here. Please note that there were significant changes to the design of the chamber, precursor storage, and control systems from the Fall 2024 semester to Spring 2025; those aiming to replicate these efforts are recommended to review the Spring 2025 design changes. Once this project is complete by the end of Summer 2025, this page will be updated so as to present a finalized guide for the machine design and build.

Introduction

We are building a low cost Atomic Layer Deposition machine for the Hacker Fab to achieve improved gate dielectrics, which will help us achieve our goal of a 10 micron CMOS process as well as improve our capability in performing thin-film research. Our design work on it will be all open source, and we hope other labs can use our work to make their own ALDs at a fraction of the cost of commercial alternatives. We are building our ALD to fit 4" wafers so that it can be brought into the nanofab, which will help us lower the barrier to entry for researching thin film deposition and new materials. Although 4" is much larger than anything we are currently using in the Hacker Fab at CMU, this larger size will make it useful to a larger audience.

Our work thus far has been focused on machine design, largely drawing from two papers on “DIY” ALD machines: “Design Of Atomic Layer Deposition Reactors For The Deposition Of Nanoparticle Embedded Thin Films” by Michael Lubitz, and “Homebuilt Reactor Design and Atomic Layer Deposition of Metal Oxide Thin Films” by Pamburayi Mpofu. Each of these papers describes their machine design followed by some process development where they describe the settings (ie. temperatures and precursors deposition times) used in their initial depositions.

Bill of Materials

A complete list of parts and components used in making the DIY ALD system can be found here.

Precursor Selection

Oxide of interest

For the current system we aim to deposit Indium-Tin Oxide. Indium Tin Oxide (ITO) is a versatile material widely recognized for its excellent electrical conductivity and optical transparency. These properties make ITO a promising candidate for advanced applications, particularly as a channel material in thin-film transistors (TFTs). Its high carrier mobility and tunable electrical characteristics offer significant potential for improving TFT performance in display technologies and flexible electronics. Additionally, ITO thin films are of great interest in materials and thin-film research due to their unique combination of metallic and semiconducting properties. This makes them an ideal system for exploring novel deposition techniques, optimizing film uniformity, and investigating structure-property relationships.

The precursors for the metals were chosen based on their feasibility to react with water vapor as the oxidizer. The precursors chosen are Trimethyl Indium (TMIn) for Indium and Tetrakis(dimethylamino) Tin (TDMASn) for Tin with Nitrogen carrier gas. They have been shown to be used for ALD and CVD processes with water as the oxidizer in literature. As a starting point our aim would be to reproduce the results achieved in [Zhang et al.]. The process parameters highlighted in the paper are as follows (substrate temperature: -225C):

For the first attempts at deposition however, the research group at CMU will attempt to deposition Al2O3, as this process is well-documented in the CMU Nanofab. Then, the group will attempt to deposit more complex oxides such as ITO.

Safety Considerations

Most precursors and specifically metal organic precursors used for CVD and ALD processes tend to be pyrophoric (i.e. thermally unstable and spontaneously ignite on exposure to air) thus necessitating safety measures.

Material Considerations

Compatibility with the precursors and byproducts produced during reaction are important to consider when selecting components for the delivery system. For our given precursors the chemical groups for which we checked compatibility are: 1) ability to handle pyrophoric materials, 2) ability to resist corrosion due to water vapor, 3) compatibility with methane (by product of TMIn reaction with water) and dimethylamine gas (by product of TDMASn reaction with water)

The compatibility of o-ring materials was checked on the following sources: (1) (2)

Specific choices:

• Stainless Steel Tubing

• Aflas O-rings - conventionally used Viton O-rings are incompatible with dimethylamine

• Aluminum vacuum chamber

• Stainless steel bellow hose

• Vacuum pump - Although the vacuum pump manufacturer warns against using the pump with pyrophoric gases, this caution can be safely disregarded in our case. Since we will be working with extremely dilute concentrations of these gases, the risk of combustion or hazardous reactions is significantly minimized. The low concentration ensures that the gases remain well within safe limits, allowing for the pump's use without compromising safety or performance.

Sourcing Precursors

Given the safety considerations involved in handling the pyrophoric precursors, the sourcing of materials for ITO deposition is managed by the Claire & John Bertucci Nanotechnology Laboratory staff. They ensure that all necessary precautions are taken during the procurement, handling, and storage of the chemicals. The required precursors have been ordered from Strem Chemicals, a trusted supplier known for providing high-quality materials for advanced research and industrial applications. TMIn, TDMASn

Key Components

ALD precursors and co-reactants are self-limiting

1. Precursor and co-reactant selection

   1. Have to be reactive with the surface groups

   2. Volatility, thermal stability, and reactivity have to be high

   3. Different ways to deliver the precursor to the chamber

2. Chemical composition

   1. Checking if the intended materials are being deposited through X-ray photoelectron spectroscopy (XPS) or four-point probe conductivity measurement

   2. Optimization of the parameters

   3. Chemical composition and stoichiometry will determine the final material properties

3. Thickness control

   1. Want to deposit the same amount of material in each cycle

   2. Thickness is determined per cycle (growth per cycle or GPC)

   3. Keep track of material increase during deposition and deposit multiple samples with a varying number of cycles (Thickness vs. ALD cycles)

   4. Focus of thicker films over 15 nm

   5. Determining film thickness:

      1. Spectroscopic ellipsometry

      2. Number of deposited atoms (rutherford backscattering spectroscopy)

      3. Deposited mass (quartz-crystal microbalance or QCM)

4. Saturation

   1. Need to optimize precursor dosing time, precursor purging time, co-reactant dosing time, and co-reactant purging time (two-step process)

   2. Choose a relatively long time for three of the four while varying the fourth

   3. Confirm saturation when increasing the dosing or purging time does not increase or decrease the GPC any more (do with all steps of the procedure)

   4. Phenomena that deviate the saturation curve

      1. Precursor condensation and decomposition (precursor curve)

      2. Too short co-reactant dosing times (impurity incorporation)

      3. Too short purging times (parasitic components impacting conformality and uniformity

      4. Too long dosing and purging times can slow down the process

      
5. Material properties

   1. Optical properties (refractive index, absorption coefficient)

   2. Electrical properties (resistivity, carrier density, mobility)

   3. Surface morphology (roughness, crystallinity)

6. Temperature dependence

   1. ALD window where the GPC is nearly constant for reliability and repeatability despite slight change in temperature

   2. Constant GPC is not necessary but verifying saturation is important for all varied temperatures

   3. Temperature dependent GPC exist but show saturating ALD behavior over a wide temp. range

   
7. Uniformity

   1. Verified saturation in one area doesn’t mean dosing is sufficient everywhere

   2. Place the largest fitting substrate into the chamber and check for thickness variation as well as material properties

      1. Thickness uniformity may be good but the resistivity might vary significantly over the substrate

8. Conformality

   1. Ability to deposit on 3D structures with no variation in thickness (trenches or vias)

   2. Check for conformality

      1. Deposit a vertical trench or via with a certain aspect ration (AR = height/width)

      2. After deposition, calculate the ratio at different locations in comparison to the thickness at the surface

      3. There is also the Pillar Hall aspect ratio test

      
9. Nucleation behavior

   1. Different film growth behavior from beginning to end

   2. Linear growth, accelerated growth, and delayed growth

   3. Nucleation behavior can affect material properties (defects, pinhole density, crystalline structure, and film resistivity)

   4. Nucleation behavior depends on the growth mode

      1. Island growth forms isolated clusters that will eventually grow into a continuous film

      2. Layer-by-layer growth

   5. Growth mode depends on the difference in surface energy (substrate and deposited film)

   6. GPC increase after crystallization after a certain thickness

10. Other important aspects

    1. Stability of the films over time and sensitivity to the environment

    2. Reproducible film thickness and properties

    3. Avoid overdosing and required precursor dosing depends on the surface area of the substrate (larger when working with 3D substrates)

    4. Precursors can degrade after prolonged heating so turn off the heating when not depositing

References

Z. Zhang et al., "Atomically Thin Indium-Tin-Oxide Transistors Enabled by Atomic Layer Deposition," in IEEE Transactions on Electron Devices, vol. 69, no. 1, pp. 231-236, Jan. 2022, doi: 10.1109/TED.2021.3129707.

“Design Of Atomic Layer Deposition Reactors For The Deposition Of Nanoparticle Embedded Thin Films” by Michael Lubitz

"Homebuilt Reactor Design and Atomic Layer Deposition of Metal Oxide Thin Films" by Pamburayi Mpofu