Atomic layer deposition
|Atomic layer deposition|
Atomic layer deposition (ALD) is a type of chemical vapor deposition (CVD) where the reactions are limited to the surface of the object being coated. Instead of flowing two or more gasses into the chamber and letting them react on or near the surface of the substrate as in CVD, in ALD the individual chemical components are introduced to the deposition chamber one at a time. Because the reaction is surface-limited, ALD creates extremely conformal films with well controlled thickness. Unfortunately, in order to limit CVD in the chamber, gasses must be pumped out completely between each dose, making the process very slow and only useful for very thin films.
- 1 Method of operation
- 2 Applications
- 3 Figures of merit
- 4 Parameters
- 5 Materials
- 6 Equipment
- 7 See also
- 8 Further reading
Method of operation
A wafer is placed on a substrate heater in a vacuum chamber and pumped to the mT range. The recipe is then run:
- Chamber purge: The chamber is purged with Ar to stabilize chamber temperature and pressure
- Dose 1: Precursor 1 dose valve is open - flooding the chamber with precursor. The precursor will stick to all open sites on the substrate
- Purge 1: Precursor 1 dose valve is closed and precursor lines are purged with Ar while the gas precursor is pumped away, leaving only the precursor that is reacted on the surface. No gas-phase precursor can be left in the chamber to avoid gas-phase CVD reactions.
- Dose 2: Precursor 2 dose valve is open - flooding the chamber with precursor 2 which reacts with precursor 1 to form a film.
- Purge 2: Precursor 2 dose valve is closed and again precursor 2 lines are purged with Ar while the gas precursor is pumped away, leaving only the precursor that is reacted on the surface.
- Repeat: Steps 2-5 are described as 1 cycle. Each cycle leaves a monolayer (or less) of material on the substrate. The cycles are repeated until desired thickness is met.
- Pump Out: Chamber is then pumped out and the recipe is ended.
Tool is then vented and substrate is removed.
An example of this is the common ALD process involving the reaction of Trimethyl Aluminum (TMAl) and Water to grow Al2O3:
This section requires expansion.
- gate oxide
Figures of merit
Deposition rate per cycle
Deposition rate is usually expressed in Å/cycle or nm/cycle as opposed to a rate per time such as Å/second or nm/minute. Once the timing is optimized for a fully saturated, ALD step, the amount deposited per cycle is self-limiting to a monolayer of material.
Number of cycles - Standard Process vs Nucleation Delay
ALD processes show a linear deposition rate vs number of cycles so the number of ALD cycles is set to establish the desired thickness. However it can take a few cycles or more for the material to begin to nucleate on the surface and form a continuous film that will promote growth of the next layers. Once it is established nucleation, the film will grow linearly with number of cycles:
Valve timing and Saturation
The timing of the 4 steps of the ALD process need to be set-up for each film and precursor process to optimize the process. With dose timing the goal is to saturate the surface with precursor while not overdosing and wasting precursor down the pump line. With the purges, the key is to pump/purge long enough to ensure full removal of any gas reactants but not waste any extra time. In most processes the dose times are extremely short (10ms-1s depending on precursor) while pump times can be longer (5-60s depending on how easy and "sticky" the material is to remove from the chamber.)
In a 2-material process (TMA and water for example) there are two dose steps and two purge steps that must be optimized but each one has the similar responses:
|Step||Too Short||Too Long|
|Dose||No Saturation - less than optimal deposition, not all sites are coated||Wasting Precursor, potential CVD|
|Purge||Not all material pumped - CVD - thicker material that does not adhere, coat evenly or reacts differently||Wasting time|
So a pressure vs time chart for an ideal ALD process is two independent pressure spikes separated by a purge time that is just long enough to purge out all the gas:
During setup of a new precursor, process window recipes are run to determine this ideal dose and purge times. These are usually called saturation curves. TMA dose on the LNF Oxford ALD at 150C show how the dose and purge times were determined for the recipe:
Each precursor has a temperature window in which it breaks down into reactive products. Below that temperature the material does not have enough activation energy to react and above that temperature the precursor may decompose or desorb. The window of temperature between these two is often referred to as the ALD window for the precursor.
Within this ALD temperature window, you can process at various temperatures. Generally higher temperature films will deposit less per cycle but can be slightly denser and more pure (although in many cases the films are similar within the temperature window.)
Plasma vs Thermal
Plasma ALD is popular for many materials. Instead of water dosing, an O2 downstream plasma is sent into the chamber to create the reaction. Typically this plasma is remotely generated so it doesn't see the "line of sight" issues associated with a plasma-generated-in-the-chamber PECVD processes.
Plasma has some advantages: First it can create a faster process when large doses of water are needed and the pumping time of the water is slow. The plasma also reduces nucleation delays in some materials and lowers the activation energy needed for some precursors. Finally, plasma can be used to create other chemistries of ALD by injecting Nitrogen, Hydrogen or Ammonia plasmas into the chamber instead of just Oxygen. The disadvantages to plasma are the expense and reliability of the plasma generation equipment and the potential for plasma damage to materials (carbon nanotubes can etch in Oxygen plasma for example.)
Oxford OpAL ALD
The Oxford OpAL is a plasma or thermal assisted ALD tool used to deposit Al2O3, ZnO, and HfO. It is an open loop (non-loadlocked) tool which had two channels for metal organic precursors, one channe1 for water and a plasma head.
- Wikipedia Atomic layer deposition
- Atomic Layer Deposition: An Overview Steven M. George, Chem. Rev. 2010, 110, 111–131
- Mikko Ritala, Marjju Leskelä, Chapter 2: Atomic Layer Deposition, In Handbook of Thin Film Materials: Deposition and Processing of Thin Films, Hari Singh Nalwa ed, Academic Pres