Specific Process Knowledge/Thin film deposition/ALD2 (PEALD)

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ALD - Atomic layer deposition

Picosun R200 PEALD, positioned in cleanroom F-2.

The ALD2 is a PEALD (Plasma Enhances Atomic Layer Deposition) tool. The machine is used to deposit very thin layers of different materials, one atomic layer at a time, by use of thermal ALD or plasma assisted ALD.

The ALD deposition takes place in a ALD reactor. In order to ensure that the ALD reactor has the same temperature everywhere, it has a dual chamber structure. The inner chamber is the reactor chamber, and the outer chamber is isolating the reactor chamber from room air. Both the inner and outher chamber are under vacuum. The space between the two chambers is called an intermediate space (IMS), and the IMS is constantly purged with nitrogen.

When a sample is loaded into the reactor chamber, it will take some time before it reaches the desired temperature. Thus, it is important to include a temperature stabilization time in the deposition recipes.

The ALD depositions take place under vacuum, thus a vacuum pump is connected to the bottom of the ALD reactor. The pump is located in the basement.

Different precursors are connected to the reactor chamber through separate gas lines. At the moment the available precursors are TMA, TiCl₄, SAM24, TEMAHf, H₂O and NH₃, and soon O₃ will also be available. These gas lines are all purged with a constant flow of nitrogen.

The precursor sources TMA, TiCl₄ and H₂O are located in a side cabinet on the left side of the machine. When these precursors are not in use, the manual valves have to be closed. The precursors SAM24 and TEMAHf are located in the big cabinet below the ALD chamber. These precursors are heated by a heating jacked, and users should not close the manual valves. O₂ is generated by use of an ozone generator that is located in the E-rack at the right side of the machine.

A remote plasma generator is connected to the upper part of the reactor chamber. Different precursor gasses are connected to this plasma generator through the same gas inlet. At the moment the available plasma precursor gasses are N₂, O₂ and NH₃. The plasma gas inlet is constantly purged with argon. The plasma gasses can also be used as normal precursors if the power to the plasma generator is not turned on.

The plasma generator is separated from the reactor chamber by a plasma cone. The argon flow through the plasma gas inlet will ensure that the plasma cone remains clean.

The precursor pulse time is controlled by special ALD valves that allow very short precursor pulses to be introduced into the reactor chamber and at the same time allow a constant nitrogen or argon flow. Thus, nitrogen and argon is always flowing through the ALD valves into the chamber, independent on whether a precursors pulse is introduced or not.

The plasma cone is not heated, and thus is will affect to temperature uniformity in the reactor chamber, and it will affect the gas flow and increase the necessary purge time. For that reason it is possible to mount a thermal lid between the plasma cone and the reactor chamber. However, it is quite time consuming to install the thermal lid as this requires that the machine is cooled to room temperature and vented. Furthermore, it is not possible to run a plasma process with the thermal lid installed.

If only the precursors connected to the gas inlets at the reactor chamber are used for a deposition, this is called a thermal ALD reaction. If the plasma generator is used to make one of the precursors, the deposition is called a plasma assisted ALD (PEALD) reaction.

At the moment it is possible to deposit Al₂O₃, TiO₂, HfO₂, SiO₂, AlN and TiN in the ALD. In order to deposit a good nitride layer with a low sheet resistance, the amount of oxygen has to be very low. Thus, the chamber has to be passivated before nitride depositions can be done, and oxides and nitrides cannot be deposited at same time.

Thermal ALD depositions are normally fastest and easiest to control, but the reactions can only take place at an elevated temperature. By PEALD plasma can be used instead of a high temperature in order to deliver the necessary activation energy for the ALD reaction to take place, and it is possible to deposit more different materials.

Samples are loaded through a load lock. 6" and 8" wafers can be loaded directly in the load lock, while 4" wafers and smaller samples have to be placed on a 6" carrier plate or a 6" Si dummy wafer with an etched recess. It is only possible to load one wafer or carrier plate at a time.

It is also possible to load a mini-batch holder into ALD through a door on the left side of the machine. However, this option is normally not available as it requires that the machine is vented, and thus a possible nitride passivation will be ruined. Furthermore, for plasma processes it is only possible to deposit on one wafer or sample holder at a time.

The ALD is controlled by use of a computer with a touch screen that is situated next to the machine.

A short presentation with some information about the ALD tool can be found here.

The user manual, the user APV and contact information can be found in LabManager:

!!ALD Picosun R200 info page in LabManager,!! ALD1 skal ændres når ALD2 siden bliver klar

Process information

• Standard recipes on the ALD2 tool
• Advanced recipes involving fabrication of multilayers done on ALD1]
• Al₂O₃ deposition using ALD1
• TiO₂ deposition using ALD1

Equipment performance and process related parameters

Equipment		ALD2 (PEALD)
Purpose	ALD (atomic layer deposition) of	Al2O3 TiO2 (amorphous or anatase) SiO2 HfO2 TiN AlN
Performance	Deposition rates	Al2O3: ~ 0.075 - 0.097 nm/cycle (Using the "Al2O3" recipe, depending on the temperature). Result from ALD1 TiO2: 0.041 - 0.061 nm/cycle (Using the "TiO2" recipe, depending on the temperature). Result from ALD1
	Thickness	Al2O3: 0 - 100 nm
Process parameter	Temperature	Al2O3: 150 - 350 oC
	Precursors	TMA - Trimethylaluminium TiCl4 - Titaniumtetrachloride H2O - Water O3 - Ozone NH3 - Ammonia SAM24 - Bis(diethylamono)silane TEMAHf - Tetrakis(ethylmethylamino)hafnium
	Plasma precursors	N2 O2 NH3 (4% H2 in N2) will be installed later
Substrates	Batch size	Small samples and single wafers are loaded through the load lock 100 mm and smaller are loaded on a carrier plate (150 mm) 150 mm or 200 mm wafer don't need the carrier plate Up to 5 wafers when doing thermal ALD of oxides wafer size 100 mm, 150 mm and 200 mm
	Allowed materials	Silicon Silicon oxide, silicon nitride Quartz/fused silica Al, Al2O3 Ti, TiO2 Other metals (use dedicated carrier wafer) III-V materials (use dedicated carrier wafer) Polymers (depending on the melting point/deposition temperature, use carrier wafer)