Fine, uniform grains
Fully recrystallised grains, typically refined to well below 200 µm and often below 100 µm, give smoother erosion and better film uniformity than the millimetre-scale grains of cast or poorly worked material.
Thin-film materials guide
Aluminium and Aluminium Alloy Sputtering Targets
Aluminium sputtering targets are high-purity aluminium or aluminium-copper, aluminium-silicon and aluminium-silicon-copper discs and plates used in physical vapour deposition (PVD) to deposit metal films for semiconductor interconnects, bond pads, power devices, displays and optical coatings. Film quality depends on the target's purity, alloy uniformity, grain size, crystal texture and bonding. This guide explains each material system, the properties that matter and how to specify a target.
- Materials
- Al, AlCu, AlSi, AlSiCu
- Purity
- 5N (99.999 %) to 6N
- Construction
- Monolithic or bonded
- Key controls
- Impurities, grain, texture
What an aluminium sputtering target does
In magnetron sputtering, argon ions strike a target in a vacuum chamber and knock atoms off its surface; those atoms travel to a wafer or glass substrate and build up a thin film. The target is therefore the source of every atom in the film, and any impurity, inclusion or non-uniformity in the target can appear as a defect, a resistivity shift or a thickness variation on the substrate.
Aluminium was the original interconnect metal of the integrated circuit because it has low resistivity (about 2.7 µΩ·cm in bulk), adheres well to silicon dioxide, is easy to etch, and forms ohmic contacts. Copper replaced aluminium for the finest interconnect layers at advanced logic nodes from the late 1990s, but aluminium films remain essential: as the top metal and bond-pad layer above copper wiring, as the main interconnect in many mature-node, analog, power, MEMS and image-sensor devices, and in flat-panel displays, LEDs, solar cells and reflective coatings.
Because these devices run in very large volumes, aluminium-based targets remain among the most widely consumed PVD materials. Demands on them have tightened steadily: fewer particles, more uniform films across 300 mm wafers, and longer target life.
High-purity Al, AlCu, AlSi and AlSiCu targets
| Target material | Typical composition | Why it is used | Typical applications |
|---|---|---|---|
| High-purity aluminium | 99.999 % (5N) to 99.9999 % (6N) Al | Lowest resistivity, simplest process | Bond pads, power devices, displays, reflectors, MEMS |
| Aluminium-copper (AlCu) | Al with about 0.5 – 4 % Cu; 0.5 % is very common | Copper greatly improves electromigration resistance | IC interconnects and pads, power devices |
| Aluminium-silicon (AlSi) | Al with about 1 % Si | Silicon saturates the film and prevents junction spiking into the silicon substrate | Contacts to silicon in discrete and power devices |
| Aluminium-silicon-copper (AlSiCu) | Typically about 1 % Si and 0.5 % Cu | Combines spiking prevention with electromigration resistance | Legacy and mature-node IC metallisation, power semiconductors |
| Other aluminium alloys | AlNd, AlTi, AlSc and others | Hillock suppression, piezoelectric films (AlScN via reactive sputtering) | Display gate lines, RF filters, MEMS |
Electromigration is the gradual movement of metal atoms under high current density, which forms voids and hillocks and eventually opens or shorts a line. Small copper additions segregate to aluminium grain boundaries and slow this diffusion dramatically, which is why AlCu became the standard interconnect alloy. Junction spiking happens when silicon from the substrate dissolves into pure aluminium during annealing and aluminium penetrates the shallow junction; adding about 1 % silicon to the film removes the driving force.
The exact alloy content is set by the device process. Targets are produced to the composition and tolerance in the customer specification, and the certificate reports the measured values.
Purity grades and critical impurities
Purity is quoted in "nines": 4N is 99.99 %, 5N is 99.999 %, 5N5 is 99.9995 % and 6N is 99.9999 %. For alloy targets, purity refers to the aluminium base and impurity levels, excluding the intended alloying elements. Total purity alone is not enough; semiconductor users specify individual impurity limits, measured by glow-discharge mass spectrometry (GDMS) or similar techniques.
| Impurity group | Why it matters | Typical control |
|---|---|---|
| Alkali metals (Na, K, Li) | Mobile ions shift transistor threshold voltages | Very low ppm or sub-ppm limits |
| Radioactive elements (U, Th) | Alpha particles cause soft errors in memory and logic | Limits in the ppb range for sensitive devices |
| Transition metals (Fe, Ni, Cr, Mn) | Raise film resistivity and can form defects | Individual ppm limits |
| Gases (O, C, N, H) | Oxide inclusions cause arcing and particles | Controlled through melting and casting practice |
| Inclusions and pores | Trigger arcing and splash defects on the wafer | Clean melting, ultrasonic inspection |
High-purity aluminium is produced by refining primary aluminium, typically by three-layer electrolytic refining and fractional crystallisation or zone refining, before alloying and casting in clean conditions. Our high-purity aluminium guide describes the grades in more detail.
Grain size, texture and alloy uniformity
Two targets with identical chemistry can perform very differently because of their microstructure. Atoms leave each grain at a rate and angle that depend on its crystal orientation, so grain size and texture control deposition rate, film-thickness uniformity and how evenly the target erodes over its life.
Controlled texture
The ratio of (200), (111), (220) and (311) orientations at the target face affects the angular distribution of sputtered atoms. Specifications often set a target texture and its permitted variation across the face and through the thickness.
Alloy distribution
In AlCu and AlSiCu, copper and silicon must be evenly distributed, with fine precipitates and no coarse particles, so the film composition stays constant as the target erodes.
Thermomechanical processing
Grain size and texture are set by forging, rolling or extrusion followed by controlled recrystallisation annealing, sometimes with severe plastic deformation for ultra-fine grains.
Through-thickness consistency
A target is used until a deep erosion groove forms, so the microstructure must be consistent at depth, not only at the surface.
Inspection
Grain size to ASTM E112 methods, texture by X-ray diffraction or EBSD, and ultrasonic scanning for internal defects.
Monolithic and bonded target construction
| Construction | Description | Advantages | Considerations |
|---|---|---|---|
| Monolithic | Target and mounting features machined from one piece of aluminium or alloy | No bond line to fail; good heat transfer; simple recycling | Pure aluminium is soft, so large monolithic targets can bow under cooling-water pressure |
| Diffusion bonded | Target plate joined to a backing plate (aluminium alloy such as 6061, or copper alloy) by solid-state diffusion | Very strong bond, high power capability, stiff assembly | Process and backing plate add cost |
| Solder bonded | Target soldered to a backing plate with indium or another low-melting solder | Lower cost, backing plate reusable | Power and temperature limited by solder melting point |
Bond quality is checked by ultrasonic C-scan, with the specification setting the minimum bonded area and the maximum size of any single unbonded region. Flatness after bonding, sealing surfaces for the cooling circuit, and the cleanliness of the finished surface are also inspected. Finished targets are cleaned, dried and double-bagged in clean-room packaging to keep particles and moisture out until installation.
Target shapes and dimensions
- Circular planar targets for wafer tools, with diameter, thickness and mounting features defined by the PVD chamber; 200 mm and 300 mm wafer tools use targets significantly larger than the wafer
- Rectangular planar targets for in-line and display coaters, sometimes tiled from several segments
- Rotary (cylindrical) targets for large-area glass and display coating, offering higher material utilisation than planar targets
- Custom profiles with thicker erosion zones to extend target life where the magnetron erodes unevenly
- Research targets in small diameters (25 – 100 mm) for laboratory sputter systems
Every chamber model has its own target interface, so the drawing, or the equipment make and model plus the original part reference, is the starting point for any quotation. Dimensional tolerances, surface roughness of the sputtering face and the O-ring sealing surfaces are critical for vacuum integrity and arc-free start-up.
How to specify an aluminium sputtering target
- Material and compositionPure aluminium or alloy, nominal composition and tolerance (for example Al-0.5 wt % Cu ± 0.05).
- Purity and impurity limitsBase purity grade and individual limits for critical impurities, including alkali and radioactive elements if relevant.
- MicrostructureMaximum average grain size, uniformity requirement and any texture requirement.
- Construction and drawingMonolithic or bonded, backing plate material, bond coverage, flatness and the full dimensional drawing.
- DocumentationCertificate of analysis with GDMS results, grain size and texture data, ultrasonic bond map, and packaging requirements.
References and standards
- ASTM E112 – Standard Test Methods for Determining Average Grain Size
- US Patent 6,835,251 – High-purity aluminum sputter targets and method of manufacture
- US Patent 5,456,815 – Sputtering targets of high-purity aluminum or alloy thereof
- M. Ohring – Materials Science of Thin Films (Academic Press)
- ASM Handbook, Vol. 2 – Properties of Pure Metals: Aluminum
Aluminium (aluminum) sputtering targets: frequently asked questions
Why is copper added to aluminium sputtering targets?
About 0.5 % copper greatly improves the electromigration resistance of aluminium interconnect lines, so they survive high current densities for much longer without forming voids or hillocks.
Why is silicon added to aluminum targets?
Silicon in the film, typically about 1 %, stops aluminium from dissolving silicon out of the substrate during annealing, which prevents junction spiking in shallow-junction devices.
What purity do aluminium sputtering targets need?
Semiconductor targets are usually 5N (99.999 %) to 6N (99.9999 %) aluminium base, with separate limits on critical impurities such as sodium, potassium, uranium and thorium.
Why does grain size matter in a sputtering target?
Grain size and crystal orientation control how atoms leave the target. Fine, uniform, controlled-texture grains give more uniform films, steadier deposition rates and more even erosion over the target life.
Should I choose a monolithic or bonded target?
Monolithic targets avoid bond failures and are simple to recycle, but soft pure aluminium can bow in large sizes. Diffusion-bonded targets on a stiff backing plate handle higher power and larger sizes.
What do I need to send for a target quotation?
The drawing or equipment model, material and composition, purity and impurity limits, any grain size or texture requirement, construction and quantity.
Need aluminium targets to your specification?
Send the drawing, material, purity, construction and quantity. We will reply with feasibility, documentation and delivery terms.
