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Pulsed LASER Deposition System

Proposed Chamber Design

Harsh Purwar (07MS – 76) Student, Vacuum Systems Lab (PH – 424)

Indian Institute of Science Education and Research, Kolkata

Introduction

Pulsed LASER Deposition (PLD)

Pulsed laser deposition (PLD) is a thin film deposition (specifically a physical vapor deposition) technique where a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target (in a plasma plume) which deposits it as a thin film on a substrate (such as a silicon wafer facing the target). This process can occur in ultra-high vacuum or in the presence of a background gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films. While the basic-setup is simple relative to many other deposition techniques, the physical phenomena of laser-target interaction and film growth are quite complex. When the laser pulse is absorbed by the target, energy is first converted to electronic excitation and then into thermal, chemical and mechanical energy resulting in evaporation, ablation, plasma formation and even exfoliation. The ejected species expand into the surrounding vacuum in the form of a plume containing many energetic species including atoms, molecules, electrons, ions, clusters, particulates and molten globules, before depositing on the typically hot substrate. The process of PLD can generally be divided into four stages:

Laser ablation of the target material and creation of a plasma: The ablation of the target material upon laser irradiation and the creation of plasma are very complex processes. The removal of atoms from the bulk material is done by vaporization of the bulk at the surface region in a state of non-equilibrium and is caused by a Coulomb explosion. In this the incident laser pulse penetrates into the surface of the material within the penetration depth. This dimension is dependent on the laser wavelength and the index of refraction of the target material at the applied laser wavelength and is typically in the region of 10 nm for most materials. The strong electrical field generated by the laser light is sufficiently strong to remove the electrons from the bulk material of the penetrated volume. This process occurs within 10 ps of a ns laser pulse and is caused by non-linear processes such as multiphoton ionization which are enhanced by microscopic cracks at the surface, voids, and nodules, which increase the electric field. The free electrons oscillate within the electromagnetic field of the laser light and can collide with the atoms of the bulk material thus transferring some of their energy to the lattice of the target material with in the surface region. The surface of the target is then heated up and the material is vaporized.

Dynamic of the plasma: In the second stage the material expands in the form of plasma parallel to the normal vector of the target surface towards the substrate due to Coulomb repulsion and recoil from the target surface. The spatial distribution of the plume is dependent on the background pressure inside the PLD chamber.

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The dependency of the plume shape on the pressure can be described in three stages: o The vacuum stage, where the plume is very narrow and forward directed; almost no

scattering occurs with the background gases. o The intermediate region where a splitting of the high energetic ions from the less energetic

species can be observed. The time-of-flight data can be fitted to a shock wave model; however, other models could also be possible.

o High pressure region where we find a more diffusion-like expansion of the ablated material. Naturally this scattering is also dependent on the mass of the background gas and can influence the stoichiometry of the deposited film.

The most important consequence of increasing the background pressure is the slowing down of the high energetic species in the expanding plasma plume. It has been shown that particles with kinetic energies around 50 eV can resputter the film already deposited on the substrate. This results in a lower deposition rate and can furthermore result in a change in the stoichiometry of the film.

Deposition of the ablation material on the substrate: The third stage is important to determine the quality of the deposited films. The high energetic species ablated from the target are bombarding the substrate surface and may cause damage to the surface by sputtering off atoms from the surface but also by causing defect formation in the deposited film. The sputtered species from the substrate and the particles emitted from the target form a collision region, which serves as a source for condensation of particles. When the condensation rate is high enough, a thermal equilibrium can be reached and the film grows on the substrate surface at the expense of the direct flow of ablation particles and the thermal equilibrium obtained.

Nucleation and growth of the film on the substrate surface: The nucleation process and growth kinetics of the film depend on several growth parameters including:

o Laser parameters: Several factors such as the laser influence [Joule/cm2], laser energy, and ionization degree of the ablated material will affect the film quality, the stoichiometry, and the deposition flux. Generally, the nucleation density increases when the deposition flux is increased.

o Surface temperature: The surface temperature has a large effect on the nucleation density. Generally, the nucleation density decreases as the temperature is increased.

o Substrate surface: The nucleation and growth can be affected by the surface preparation (such as chemical etching), the miscut of the substrate, as well as the roughness of the substrate.

o Background pressure: Common in oxide deposition, an oxygen background is needed to ensure stoichiometric transfer from the target to the film. If, for example, the oxygen background is too low, the film will grow off stoichiometry which will affect the nucleation density and film quality.

In PLD, a large super-saturation occurs on the substrate during the pulse duration. The pulse lasts around 10 – 40 microseconds depending on the laser parameters. This high super-saturation causes a very large nucleation density on the surface as compared to Molecular Beam Epitaxy or Sputtering Deposition. This nucleation density increases the smoothness of the deposited film. In PLD, [depending on the deposition parameters above] three growth modes are possible:

o Step-flow growth: All substrates have a miscut associated with the crystal. These miscuts give rise to atomic steps on the surface. In step-flow growth, atoms land on the surface and diffuse to a step edge before they have a chance to nucleated a surface island. The growing surface is viewed as steps traveling across the surface. This growth mode is obtained by deposition on a high miscut substrate, or depositing at elevated temperatures.

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o Layer-by-layer growth: In this growth mode, islands nucleate on the surface until a critical island density is reached. As more material is added, the islands continue to grow until the islands begin to run into each other. This is known as coalescence. Once coalescence is reached, the surface has a large density of pits. When additional material is added to the surface the atoms diffuse into these pits to complete the layer. This process is repeated for each subsequent layer.

o 3D growth: This mode is similar to the layer-by-layer growth; except that once an island is formed an additional island will nucleate on top of the 1st island. Therefore the growth does not persist in a layer by layer fashion, and the surface roughens each time material is added.

Each of the above steps is crucial for the crystallinity, uniformity and stoichiometry of the resulting film.

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Components

Vacuum Pumps

Turbo-Molecular Pump (P1): A variety of TMPs could be used to obtain ultra-high vacuum ( mbar) inside the chamber. Here I suggest the following product:

Company: TurboVacuum (High Vacuum Pumps) Model: Varian Turbo Pumps Model No.: V551 Inlet Flange: CF 100 Ultimate Pressure: mbar Rotational Speed: 42,000 rpm Backing Pressure: 0.1 mbar Outlet Flange: NW 25

Backing cum Roughing Pump (P2): Again a wide range of backing as well as roughing pumps can be used to match the requirements of the TMP. Along with the above suggested TMP the following backing pump fits very well. It is also being used for roughing in the proposed design.

Company: TurboVacuum (High Vacuum Pumps) Model: Oil Free Backing Pumps Model No.: XDS 10 Inlet Flange: NW 25 Pumping speed: 8 cubic feet per meter Ultimate Pressure: Torr or mbar

Note that the backing and roughing pumps should be completely oil free pumps.

Viewports

Viewport (V1): It’s the central bigger viewport attached solely for the purpose of viewing the interior of the chamber. This will also be used as a door to keep the samples and substrate in the arrangement inside the chamber. Technical details are as follows:

Company: MDC (http://www.mdcvacuum.com) Model: Fused Silica Dell Seal CF Part No.: 9722012 Mount Size: 9.97 inches (25.324 cm) Lens Size: 8.0 inches (20.32 cm) Temperature Range: -100 oC to 200 oC

Viewport (V2): It’s a smaller viewport attached at an angle of 45o measured from the vertical wall of the chamber in the anti-clockwise direction. The infrared pulsed LASER will be directed through this viewport to the sample kept inside the chamber. Technical details are as follows:

Company: Torr Scientific Ltd. (http://www.torrscientific.co.uk) Model: Sapphire Zero Length Viewport Part No.: VPZ38S-NM Nature of Flange: Non-magnetic Seal Type: Braze Max. Temp.: 450 oC Pressure Range: Torr Flange Type: NW35CF Flange Material: 316LN

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Following is the transmission curve for this viewport (V2).

Figure 1: Transmission Curve for V2

Flange

Flange is an important component of almost all vacuum systems. The proposed design uses mainly the following two types of flanges:

CF 100 (F1): Company: Vacom (http://www.vacom-vacuum.com) Model: CF 100 Bored Non-rotatable Flange Order No.: F100B104-316 Material: Stainless Steel 316L Bakeout Temp.: 450 oC No./Type of Bolts: 16 X M8

NW 25 (F2): Company: Swagelok (http://www.swagelok.com) Model: NW 25 Bored Non-rotatable Flange Order No.: JNWB2510 Tube OD.: 1.0 inch. Material: 304 Stainless Steel Bakeout Temp.: 204 oC

Figure 2: CF 100 Flange (A=152, RA=101, D=98, L=20, L2=9.5) (All dimensions in mm)

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Hydro-form Bellow

Hydro-form bellow is a flexible sealed tube as shown in the figure below. These are generally used with rotary pump for backing or roughing purposes. Technical specifications are as follows:

Company: Swagelok (http://www.swagelok.com) Model: KF Flexible Tubing Ordering No.: JNW2FH25-XX.00# Material: Stainless Steel Nominal ID: 25.4 mm Tube OD: 37.6 mm Wall Thickness: 0.25 mm

#Replace XX by appropriate tube length (in mm).

Figure 3: Hydroform Bellow (KF 25)

Pressure Gauge

Pressure Gauge (G1) is a combination of two gauges hot cathode and Pirani gauge. Moreover, hot cathode is controlled and protected automatically by the Pirani gauge. Technical specifications of the suggested product are as follows:

Company: Pfeiffer Vacuum (http://www.pfeiffer-vacuum.com) Model: Compact FullRange BA Gauge PBR 260 Part No.: PT R27 000 Inlet Flange: NW 25 (F2) Pressure Range: mbar to mbar Bakeout Temp.: 150 oC

Gate Valves

This design of PLD requires at least two gate-valves one to stop roughing and another to protect TMP as shown in the figure above. These two valves can be closed ones ultra-high vacuum is achieved inside the chamber thereby conserving a lot of power. Technical specifications of the two standard cycle gate valve are as follows:

CF 100 (V1): Company: HighVac (http://www.highvac.com) Model: Standard Cycle Gate Valve Model No.: 11110-0400 Operation: Manual Material: 304 Stainless Steel Bakeout Temp.: 250 oC

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NW 25 (V2): Company: MKS (http://www.mksinst.com) Model: Compact Vacuum Inline Valve Model No: CVNL-K2-MKKV-240 Operation: Manual Material: 304 Stainless Steel Pressure Range: Atm to Torr. Bakeout Temp.: 150 oC

Figure 4: MKS Compact Vacuum Inline Valve (Front View) (All dimensions in inches (mm))

Figure 5: HVA Standard Cycle Gate Valve (Front View) (All dimensions in inches [mm])

Substrate Manipulator (with 2’’ quartz lamp heater and gas shower ring)

PLD Substrate Manipulator (SM) shown above in the drawings comes with an inbuilt heater and gas blower. As mentioned earlier in the introduction section, to obtain good results out of the PLD Systems, the substrate has to be heated. And usually the substrate in the experiments is as large as a few centimeters. So here I suggest using a substrate manipulator with a 2 inches quartz lamp heater and gas shower ring (for carrying out experiments in the presence of gases like O2 etc.), whose brief specifications are as follows:

Company: Thermionics Vacuum Products (http://www.thermionics.com) Model: PLD Substrate Manipulator Max. Sample Heating: 700 oC Base Flange: CF 160 Substrate Sample Size Diameter: <2 inches X – Y Movement: inch Bakeout Temp: 230 oC Made from UHV Compatible Materials

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Figure 6: Substrate Manipulator with a quartz heater and gas blower ring.

Target Manipulator

PLD Target Manipulator (TM) is attached to manipulate the target or sample. At times it is tough to focus the LASER beam on the surface of the target. But it becomes pretty easy if the target can be manipulated inside the chamber from outside without disturbing the attained UHV. Technical specifications of the target manipulator are as follows:

Company: Thermionics Vacuum Products (http://www.thermionics.com) Model: PLD Target Manipulator Base Flange: CF 160 Max. X – Y Movement: inch. Max. Z Movement: inches Bakeout Temp: 230 oC Made from UHV Compatible Materials

Figure 7: Target or Sample Manipulator for inverted mount (gravity-held target holder).