Sample Manipulation & Motion

Sample Manipulation Technical Notes

Many different terms are associated with devices used to achieve sample manipulation:motion feedthroughs, sample transfer, translation devices, xyz manipulators, z-only manipulation, sample rotation, rotary drives, linear drives, etc. All cover different aspects of the same basic requirement— to mechanically move an object inside a vacuum chamber and under vacuum. Most fall under these general headings:

• Rotary Motion Drives
• Linear Motion Devices
• Sample Transfer
• Direction Adjustment Mechanisms
• XYZ Sample Manipulators
• Substrate Rotation, Heating, & Transfer Stages]

Some motion devices provide precise, repeatable movements of a few micrometers while others give coarse positioning over a few feet of travel. They may have rotary motion, linear motion, or a combination of both. The most complex devices move a sample in three orthogonal axes with rotation around two of them.

A movement device is 'sealed' using either of two mechanisms: a vacuum-tight seal on a continuous mechanical part that penetrates the air-to-vacuum wall or a magnetic coupling that transfers motion through an unbroken air-to-vacuum wall.

Rotary Motion

A rotary motion drive is essentially an air-side 'handle' that rotates a vacuum-side shaft or tube. At least five types of rotary drive exist, differentiated by actuator mechanism, vacuum sealing, compatible pressure range, and application.

Magnetic Coupling

An outer (air-side) rotating ring has a number of strip magnets (paralleling the ring’s center-line) mounted so one magnet’s poles are opposite in sign to its immediate neighbors. This outer ring magnetically couples through a stainless steel vacuum sheath to a vacuum-side ring with an identical number of strip magnets. The coupled device places an outer N-pole over an inner S-pole.

The inner magnet ring is on a shaft mounted on two dry (MoS₂ impregnated or ceramic) ball bearings. The lack of mechanical coupling and vacuum seal (the 'sheath' is a cup welded to a knife-edge flange) plus the selected construction materials allow this rotary drive to be baked at 250°C, making it UHV compatible.

The maximum torque is determined by the force that decouples the inner/outer magnets. For the larger rotary drives this is ~10Nm. If the magnetic coupling is not under high torque, this drive gives very precise rotary motion. It can be mounted in any orientation and has a long-life under continuous rotation (max. 500–1000 rpm). The trade name for this popular device is MagiDrive™.

Ferrofluid Seal

This rotary drive is basically a flanged cylinder with two roller bearings supporting a central rotating shaft. The shaft, a high magnetic-permeability material, is machined in a series of circumferential peaks and valleys (in section it looks like a cross-cut saw). A cylindrical ring magnet surrounds the shaft, creating a small gap between the shaft’s peaks and the ring. This gap is loaded with a ferromagnetic fluid—a low vapor pressure fluid in which nano-particles of magnetic material are suspended.

The field concentration effect of each peak causes the ferrofluid to form liquid o-rings seals between the shaft's peaks and the magnet. Each ring can sustain 70 Torr differential pressure. That is, designs with more than 11 peaks provide a non-wearing vacuum seal against atmospheric pressure.

With modest pumping speed from the chamber, outgassing from the ferrofluid usually limit these rotary drives to applications at pressures above 1x10^-8 Torr.

Some models are capable of high torque loading and high speed (10,000 rpm) with long life under continuous rotation. These drives can be mounted in any orientation and the continuous shaft means these drives can be precisely positioned. The trade name for one manufacturer is Ferrotec™.

Flex-Metal Bellows Seal

A vacuum-side shaft protrudes through a vacuum wall plate into the air-side space where it is bent at a modest angle. The whole shaft is sealed by surrounding it, on the air-side, with a flex metal bellows welded to the plate at one end and to a cap at the other.

The bellows cap, the air-side, is connected at a ball-socket joint and another bent shaft connects this ball-socket to the rotary drive's handle. Rotating the handle causes the bellows to 'wobble' about it central axis on the plate. This movement causes the vacuum-side shaft to rotate.

Bellows feedthroughs are bakeable to 250°C and are UHV compatible. However, the mechanism has limited torque, limited life (dues to bellows fatigue), and a maximum speed of 300 rpm.

These drives are best used in non-continuous motion application, for example, as part of an XYZ manipulator. Some versions use precision bearings to maximize concentricity, minimize shaft run-out, and provide the best angular resolution and repeatability available in rotary drives. Any mounting orientation is permitted.

Elastomer Seal

The vacuum seal on this rotary drive is an elastomeric knife-edge or o-ring clamp to the stationary housing that rubs on the rotating shaft in what is called a 'dynamic seal'.

Gas leaks, permeation through the elastomer, and seal wear limit this drive to 10^-5 Torr or 10^-6 Torr range at best.

They are used in applications where: rotation is intermittent; rotation speeds are less than 100 rpm; there is little side-loading on the shaft; the base vacuum pressures are acceptable; but cost must be low. Any mounting orientation is permitted.

Rotary Platforms

While some rotary drives have hollow shafts, a rotary platform is distinguished by having no shaft. In essence, it is two bored flanges that rotate relative to each other. The vacuum seal is made by two Teflon® slip rings that are differentially pumped (through a radial port in the stationary flange) making rotary platforms UHV compatible. They are used in application where large through-bore are needed, for example, rotary platforms with internal diameters of 18" (457 mm) have been made.

Applications include inserting (into the chamber) large diameter sample holders, cryostats, or detectors that need rotation. For example, we built an 18" ID rotary platform for an application that examined the product compositions vs emergence angle at the intersection of two molecular beams. A quadrupole mass filter with all its cables was rigidly suspended below the platform in the vacuum chamber and moved to various positions around the beams' intersection.

Linear Motion

Linear motion devices, like rotary drives, are essentially air-side 'handles' that control some motion in the vacuum. Unlike rotary drives, distinguished by their sealing mechanism, linear drives are differentiated by what moves and how far it moves.

Linear Positioners

Positioners are bellows-sealed or magnetically-coupled rods that move along the rod’s axis. The mechanisms are either manually or pneumatically actuated, the latter allowing a push-pull motion between two stop positions.

Another mechanism is driven by a precise screw with manual or motorized actuation which can be precisely positioned at any intermediate position between its travel limits.

Positioners are used in applications needing straight-line, fixed distance movement, for example, beamstops, shutter actuators, substrate movements, etc., at all pressures between atmosphere and UHV.

Linear positioners sealed with elastomeric knife-edge or o-rings seals are available and perform the same functions. However, although less expensive than other forms of linear positioner, like other 'dynamic seal' devices, their vacuum compatible is typically between atmosphere and ~10^-5 Torr.

Linear Shifts

A linear shift is a pair of flanges connected by a bellows. One flange is free to move (in an axial direction) relative to the other. The motion is constrained by a rugged, precise slide mechanism so the flange faces are remain parallel during movement.

Linear shifts may be viewed as the linear version of the rotary platform; that is, they allow wide diameter devices to be inserted into the vacuum chamber and moved linearly between limits. The action is similar to the Z-only manipulator, but its positioning precision and travel length (and therefore cost) are lower.

Linear shifts are actuated by hand-wheels or, for higher precision, stepper motors. They are compatible with high vacuum and UHV, and find applications moving heavier, larger-diameter loads than linear positioners.

Sample Transfer

The term sample transfer covers two different motions:

1. Long travel between two chambers or a load lock and chamber, through vacuum isolation valves using equipment called transporters, probes, or transfer devices
2. 3-D motion needed to transfer a sample from a motion device or storage area to a sample stage/holder.

The latter is more mechanically complex, since it must: allow 3-D motion that is 'intuitive' during (one-handed?) operation; must 'hook' to the sample-holder' and have some degree of rotation to enable sample alignment. These devices are called wobble sticks or mechanical hands.

Of the long travel transporters there are three that are associated with sample-loading components such as load lock doors or sample entry ports. The fourth, the radial distribution center, combines long travel transportation with radial directional choice and is associated with moving samples between chambers rather than just sample entry. Unlike short-travel devices, long-travel linear drives may be subject to gravitational droop at their horizontal travel limits.

The familiar long-travel transporters produce motion perpendicular to their mounting flanges. A less common
variety, called the bi-directional transporter, produces motion parallel to its mounting flange’s face in both left and right directions.

Linear Transfer

Two mechanisms are used for long distance linear travel: The LRP Transporter and the LMP Transporter. (Since the latter allows linear/rotary motion it is described in the next section.)

    LRP Transporter

The probe—the component that moves—has a rack gear along its length. It is supported on two groups of four roller bearings (dry lubricated) and is moved by a pinion gear which meshes to the rack gear. The pinion is mounted between the bearings and actuated by a rotary drive. The gears give the device its names, linear rack & pinion probe, LRP transporter, or just LRP.

The LRP's pinion's rotary drive is either wobble-bellows sealed or magnetically coupled for true UHV applications, or a ferrofluid type for high torque and heavy-load applications. The travel distance can be precisely controlled (particularly with motorized drives) with good position repeatability. Since the drive’s actuator is mounted close to the chamber, in the manual version one person can simultaneously move the probe and observe its progress through a viewport. The normal maximum for travel distances is 44" (112 cm) but examples have been built with travel distances up to 96" (244 cm).

Linear/Rotary Transfer

    LMP Transporter

As with the LRP transporter, the probe is supported on roller bearings (dry lubricated). At the probe's end farthest from the chamber is connected to an in-vacuum magnetic array. On the air-side of the vacuum envelope, a cylindrical magnetic actuator couples to the internal magnet. Sliding the external magnet along the tubular envelope moves the probe linearly. Unlike the standard LRP, however, the LMP's magnetic coupling allows rotary motion, leading to various names: linear/rotary magnetic probe, LRMP transporter (which can be confusing because to the similarity to the LRP), and LMP transporter. This probe is UHV compatible and has a maximum travel distance of 60" (152 cm).

In manual form, the LMP has one disadvantage—its actuator’s location. With the probe fully withdrawn, the actuator cylinder is at the furthest point from the chamber. This makes movement and the simultaneous observation of that movement difficult for one person. However, motorization overcomes this. The magnetic transporter’s major advantage, particularly for automatic docking/transfer, is its rotation through small angles to correct misalignment, or large angles to accommodate different mounting orientations.

The magnetic decoupling force for the standard LRM is 180N. For maximum load capabilities, “high power” LRMs have a (vertical) magnetic decoupling force up to 310N and rotary decoupling forces of 4.5Nm.

Bi-Directional Transporters

The direction of motion for the LRP and LRM transporters is perpendicular to the plane of its mounting flange. The bi-directional transporter’s motion is parallel to the flange’s plane. A small carriage or sample mounting stage runs left-right across the mounting flange on a sliding track pulled by wires attached to a rotary drive mounted on the flange. The rotary drive is either a wobble bellows or a magnetically coupled device, making the bi-direction transporter UHV compatible.

The bi-directional transporter’s track is contained in a central tube connecting two chambers. Typically, the tube's central position also includes a load lock for sample entry. The normal travel distance from the transporter's center point into a chamber is 24" (61 cm). However, travel distances up to 48" (122 cm) on either side of the center point are possible.

Wobble Sticks/Mechanical Hands

Short travel devices used for sample movement rarely involve a sample transfer step. By contrast, long travel transporters almost always require sample transfer from the probe to a sample stage/holder. The wobble stick (WS) and the mechanical hand (MH)are devices that transmits simple hand movements through the vacuum wall.

Both devices are two flexible bellows in series: a wide diameter short bellows that allows angular motion (by wobble) and a narrow diameter long bellows that allows linear motion (by compression-extension). These basic motions allow the end of the WS or MH to reach any position inside the devices 'action cone'. On the vacuum side, the probe terminates in a mechanism to attach/detach the sample such as hooks, forks, pincher clamps, screw-thread bosses, etc. The mechanism for actuating these devices are combinations of short travel bellows for tiny push-pull motions or magnetic coupling for rotation. The screw-thread sample attachment system is regarded as the most secure sample transfer available.

Radial Distribution Centers (RDC)

Essentially, the radial distribution center (RDC) is a bi-direction transporter (BDT) mounted in an ovoid chamber and allowed to rotate in the horizontal plane through 360°. The BDT’s indented positions coincides with the centerlines of 4, 6, or 8 ports spaced around the RDC chamber’s circumference.

The BTD’s flange is mounted in the chamber’s top surface with the pumping port directly below, in the ovoid's lower surface. The RDC transfers samples between multiple chambers attached to its ports. A total system may consist of transfer between: load lock for sample entry/removal; plasma cleaning chamber; various thin film deposition chambers; an analytical chamber for ellipsometry; and a chamber for surface science techniques. The completed arrangement is not unlike process chambers surrounding the central robot in a semiconductor industry's cluster tool.

Most RDCs are UHV compatible and used to connect high vacuum and UHV chambers.

Direction Adjustment

Alignment errors are greatly magnified by long travel transporters. Some of these potential error sources are:

• Flange face not perpendicular to the tube axis on the transporter, load lock, gate valve, or chamber port
• Chamber port’s center line not aligned with sample transfer port's axial position
• Gravity causing the extended probe to droop

Unfortunately, alignment can only be effectively checked when the system is fully assembled and under vacuum. Clearly, to correct these errors, devices were needed that moved the probe’s position while the system was still under vacuum. Many such direction adjustment devices were developed and became known collectively as port aligners.

XY Stages

In the XY stage, a short flexible metal bellows joins two flanges. A cage holds the flanges at a fixed distance apart and parallel but enables one flange’s axis to move laterally (in the XY plane) with respect to the other. While XY stages can have same-size flanges, in most devices the fixed flange is larger than the moving flange.

Two common sizes are: 4-1/2" CF to 2-3/4" CF (DN63 to DN40) and 6" CF to 4-1/2" CF (DN100 to DN63). The moving flange’s motion is controlled by two vernier drives acting on two cross-roller slides set at 90°. This enables the XY position to be precisely set or re-set. The maximum travel for 4-1/2" CF to 2-3/4" CF version is 14 mm (0.55") in any direction from the 'aligned axes' position. The maximum travel for the 6" CF to 4-1/2" CF version is 31 mm (1.22") in any direction.

When used as a port aligner, the XY stage is mounted between the transporter and the chamber. If the extended probe’s position misses (in its XY co-ordinates) the desired point in space, corrections are made by adjusting the verniers on the XY stage.

Y-Shift Adjusters

Some long travel transporters are required to carry loads that exceeded the capabilities of the precise XY stage. For such applications, heavy-duty Y-shifts were developed. Their basic design is like the XY stage—two flanges (one of which moves) are joined by a bellows and built into a cage. However, travel is restricted to one direction so a much sturdier cage and motion actuator (handwheel or motor) construction is possible. Two versions are available (giving fixed flange OD first): 4-1/2" CF to 4-1/2" CF (DN63 to DN63) with a motion of ±7.5 mm (0.295") and 6" CF to 4-1/2" CF (DN100 to DN63) with a motion of ±31 mm (1.22").

Although this port aligner is called a Y-shift adjuster, its direction of motion depends on the bolt-hole orientation when the Y-shift is attached to the system. That is, if up-down is not the directional change needed, the shift is rotated 90° before mounting. It then gives left-right motion.

Tilt Mechanism

While a tilt mechanism has a bellows connecting two flanges, similar to the XY stage and the Y-shift, directional changes use a different mechanism. The flanges have three threaded rods, parallel to the bellows axis, mounted on their periphery. The rods are locked into the fixed flange and have a nut on either side of the moving flange. Tilt motion is induced by first loosening all the top nuts on the moving flange. The bottom nut on one rod is moved (toward or away from the fixed flange) and all the top nuts re-tightened. The result is, the two flanges are no longer parallel. This port aligner has identical flanges at both ends and is available in 2-3/4" CF (DN40), 4-1/2" CF (DN63), and 6" CF (DN100) flange sizes.

The nominal maximum tilt angle depends on the distance between the flanges which, in turn, depends on the flange’s OD: 2-3/4" CF ±5°, 4-1/2" CF ±4°, and 6" CF ±3°. Tilt mechanisms can be baked to 250° C.

XYZ Manipulation

A device that precisely moves a sample to any point in space and any rotational orientation (within the design travel limits) is called XYZ manipulator or occasionally XYZ translator. There are six degrees of freedom for such movement, one along each X, Y, and Z axis, and one rotation about each axis. For most practical applications, no more than five (X, Y, Z plus two rotations) are necessary. In the description below, motions describes travel along the X, Y, or Z axes (when no specific direction is stated). Similarly, rotations covers generalized rotation.

The sample is typically mounted at some central position inside the vacuum volume, enabling access for instrumentation or processes. The actuators and devices controlling the motions and rotations are all outside the vacuum volume—motions sealed by flexible bellows and rotations sealed by wobble bellows, linear-acting bellows, or magnetic couplings.

Accessories/ancillaries used with XYZ manipulators include:

• Sample/substrate holders or stages
• Heating stages (to raise the sample’s temperature)
• Cooling stages (for cryogenic studies on samples)
• Tilt devices (to incline the sample’s support probe)
• Motorizing motions and rotation

    Terms Used in Motions: If a manipulator is viewed with its Z-travel bellows vertically in front of the observer and its support structure behind it:

• Y-axis runs through the bellows’ center and the support’s center
• X-axis runs left-right (through the Z-bellows’ axis)
• Z-axis is the bellows’ axis

Often these are identified as: 'sideways' (X-travel), 'fore-aft' (Y-travel), and 'up-down' (Z-travel)

    Terms Used in Rotations: Sample stages/holders at the probe’s end may have the sample»s surface on the Z-axis or displaced along the Y-axis, either in front of, or behind the Z-axis. This displacement is called offset. However, whatever the position of the sample's surface, if its (flat) surface is vertical and facing the observer standing in front of the manipulator, the rotations around the various axes are called:

• Z-axis R1-, primary-, polar-, theta-rotation
• Y-axis R2-, azimuthal-, phi-, alpha-rotation
• X-axis R3-, tilt-, third-, flip-, beta-rotation

XYZ Manipulators

XYZ manipulators are either 'single bellows' or 'dual bellows'. In both cases, the sample is mounted on a probe supported from the top flange of the Z-travel device. That means the distance from probe support point to sample may be long. Since the probe’s moment arm will amplify vibrations or motions, the probe has a large diameter to increase its stiffness and rigidity.

    Single Bellows manipulators must allow the probe to travel in X-, Y-, and Z-direction through the one bellows. That is, the bellows inside diameter (ID) determines the extent of X- and Y-travel and, for large probe movements, the ID must be large. To illustrate how large, consider a probe 3/4" (19 mm) OD in a bellows 1-1/2" (38 mm) ID. The maximum X- or Y-travel is ±3/8" (±9.5 mm). To substantially increase the XY-travel, wide-bore bellows are required. Combining that with long Z-travel means the bellows are not only wide, they are also long. Long, wide-bore bellows are exceptionally expensive.

    Dual Bellows manipulators separate Z-travel from X- and Y-travel. The Z-travel uses a small bore bellows that is as long as needed for the travel distance. For X- and Y-travel, one short, large bore bellows is used.

The manipulator's Z-travel bellows, its entire support structure, the Z-travel’s actuator, any rotary drives for rotating the sample, and the probe, are built onto a rugged top carriage.

The top carriage is connect to the lower carriage by the short, large bore bellows for the X- and Y-travel. The top carriage is mounted on the lower carriage by two vernier-actuated, cross-roller slides, mounted at a 90° to each other. These slides allow top carriage movement in X- and Y-directions relative to the lower carriage.

Despite the two bellows manipulator’s added complexity, it is a more practical and cost-efficient solution to large Z-axis travel within normal XY-axis travel than a long, widebore single bellows design with the same travel limits.

In some manipulator designs, the R1 rotation is continuous. But heating and cooling options limit the rotation to less than a full circle. R2 (and the more unusual R3) rotation is actuated by a push-rod movement and limited, normally, to a few degrees. When considering sample holders and rotation, system designers must ensure sufficient clearance (usually around the primary axis), so the sample holder’s sweep radius does not cause it to collide with chamber fixtures during the traverse.

Sample Holders

As their name implies, sample holders secure the samples so the manipulator’s motion is translated into an identical sample motion. The sample holder can be mounted directly to the XYZ manipulator’s probe but is frequently connected to the shaft of a rotary drive, mounted through a hollow probe, to provide R1 rotation. Where R2 motion is needed, the sample holder is designed to include a rack/pinion gear that causes rotation (around the pinion’s axis) when the rack is moved by a push-rod motion transmitted through the hollow probe.

Heating/Cooling

Heating and cooling options are also sample holder functions. Sample heating can be accomplished by electron bombardment (EB) or thermal radiation (IR). For EB, the sample holder is equipped with a filament capable of thermionic emission. Either the filament or the sample is electrically isolated and biased to make the filament a high negative potential with respect to the filament. This causes the thermionic electrons to hit the sample at high energy. For IR, a filament is mounted close to the sample’s backside and resistively heated. Thermal radiation heats the sample. Maximum sample temperatures depend on many factors, including: sample material, radiation shielding, surface emissivity, shape factors, etc., but range from 800°C to 1200°C.

Sample holders capable of cooling samples have heat-sinks on which the sample is mounted. The heat-sink is connected to a manipulator-mounted cryostat by a thick copper braid that both conducts heat (from heat-sink to cryostat) and enables rotation. The cryostat is cooled by flowing LN₂ or cold gaseous He through it.

Tilt Device

Using the mechanism described above (under the same heading in Direction Adjustment) tilt devices are added to XYZ manipulators. The probe C/L can be pointed in any direction, within a defined cone angle, relative to the manipulator’s axis.

Motorization

To achieve the highest positioning accuracy, resolution, and repeatability, XYZ manipulator motions/rotations are motorized with air-side mounted stepper motors. Motors are also used for long Z-travel-only manipulators, where making long travel movements by hand can be tedious (due to the exceptionally fine gear ratios), and on XYZ manipulators located at inaccessible points on large systems.

Substrate Rotation, Heating, & Transfer Stages

Why rotate a substrate? Rotation is often associated with thin film deposition. If the flux of material from a PVD source or the precursor gas’s local concentration from CVD inlet is non-uniform, the film’s growth on a static substrate will not have uniform thickness. One solution is to rotate the substrate and expose its entire surface area to the more uniform average flux.

Why heat a substrate? Heating is used for cleaning the sample but may, again, be associated with thin film deposition. For PVD techniques, the substrate’s temperature influences the surface migration of depositing atoms and the film’s morphology, which determines its porosity and grain structure. For CVD techniques, the substrate’s temperature is the reaction driver, converting the volatile precursor into the solid film.

What is a transfer stage? In some applications, the chamber is vented and opened to load the next substrate on the rotation/heating stage. For R&D applications and some (low base pressure) process applications, however, multiple substrates are stored under vacuum in the main chamber or an adjacent load lock. Substrates are exchanged while still under vacuum and, of necessity, the rotation/heating stage’s design must allow such transfer.

There are countless design variations of substrate stages. The simplest are fixed mounts without rotation, heating, or transfer. The next level provides rotation for a single disk about its axis. They are often called platens or turntables. Other platens carry multiple substrates that rotate about the platen’s axis or, with suitable gearing, about their own axis, as well as the platen’s axis. The basis of this last design is carried forward with hemisphere or dome 'planetaries' carrying multiple disk substrates. The substrate not only rotates about its own axes, but also rotates through different spatial positions on the dome. These devices are used in production batch coating tools for coating many, relatively small diameter substrates. Almost all rotating platens are driven by an externally mounted, motorized rotary drive.

Because heating and transfer only add to the confusion of variations, the characteristics the are examined below focus on one of the more sophisticated substrate heater-rotator-transfer stages commercially available, the EpiCentre™.

Substrate Rotation

Substrate rotation is variable up to 80 rpm using a DC motor or stepper motor. The former is preferred for smooth continuous rotation, and the latter if stage indexing or substrate positioning is required for in situ surface science (eg RHEED) measurements. The motor is connected through the vacuum wall to the rotating torque tube by magnetically coupled rotary drive. The absence of gears and use of well-proven (dry) ball races ensures hydrocarbon-free operation with negligible particulate generation. The DC motorized rotary drives are available as single, double, or triple stack arrangements to match the specific cradle inertias. The complete stage is bakeable to 250°C, and is compatible with UHV, high vacuum, and reactive gas environments.

Substrate Heating

Making a continuously rotating electrical connection that does not arc under vacuum is considered a huge engineering challenge. Disk platens are heated by adding stationary resistive heating elements or high-temperature quartz-halogen lamps to the “backside” of the platen structure. In-vacuum heating involves thermal radiation heat transfer. The sample's equilibrium temperature for a given heat input depends on the sample’s reflection, absorption, and transmission in the IR region emitted by the heater. That is, a 'maximum temperature' quoted without the material’s characteristics must be treated with caution. Based on a molybdenum block sample, the maximum temperature can be as high as 1200°C.

Heater Modules

The choice of heater module depends on the application. Pyrolitic graphite coated graphite heaters are robust, reliable, reach high temperatures, and provide good temperature uniformity. Pyrolitic boron nitride coated graphite heaters have a considerably smaller area of exposed graphite, which minimizes 'doping' for those applications sensitive to carbon. Pyrolitic boron nitride heaters have a layer of (grown) PBN covering an etched (patterned) graphite film deposited on a PBN disk, and are designed for long life, low current heating.

An interesting modification is the quartz-enclosure heater, which has its element surrounded by a vacuum-sealed quartz 'box' that separates the heater’s environment from the process environment. The box seals against an internal water-cooled flange, creating the secondary vacuum enclosure, which is separately pumped via a port on the stage’s service collar. This heater is used where: particulate cleanliness is critical, corrosive gases are present, or where high O₂ partial pressures are necessary. Because the element is farther from the substrate and the thermal radiation goes through the quartz, the maximum sample temperature is reduced to 1,000°C for a molybdenum block.

Temperature Measurement

Selecting the correct thermocouple type is essential to prevent corrosion or process contamination. The common types used in substrate heating:

• Type C (tungsten/rhenium)—non-magnetic, high maximum temperature, and is generally more resistant to chemical attack.
• Type “K”(Chromel/Alumel)—costs less, but has lower maximum temperature and is magnetic.

However, it must be noted that the correlation between the temperature indicated by the thermocouple's output and the sample's actual temperature depends on many factors including: emissivities of thermocouple junction, sample, and heater surface; shape factors between thermocouple-to-heater, sample-to-heater; thermcouple-to-chamber, etc.; thermocouple's contact with the sample; and many others. In-vacuum temperature measurements are best made with pyrometers or optical fiber thermometers. Often the thermocouple is no better than a repeatability indicator.

Substrate Transfer & Manipulation

A linear shift between the substrate stage and the chamber's mounting flange enables the entire stage, including wafer cradle and heater, to move as a unit along the stage’s axis (90° to the substrate’s surface). This motion is often used to vary the throw distance between substrate and a (PVD) source.

A linear shift between the heater’s service collar and the substrate stage’s body enables the heater to be moved away from the substrate cradle. This provides a gap through which the substrate can be manually extracted.

With both shifts installed, the stage may be manipulated in a typical 'wafer hand-off' routine. The heater is first retracted to provide access to the substrate and then the entire stage lifted and/or lowered as needed by the substrate hand-off motions. In addition to manually operated substrate hand-offs, the shifts may be motorized or even pneumatically actuated, enabling automated substrate exchange using PLCs.

Substrate Holders

The substrate holder, also called a substrate cradle, is made from refractory metal with various designs to hold a standard wafer 25mm–300mm (1"–12" diameter) or multiple substrate disks. The holders can be isolated to enable substrate bias up to 2kVDC (and a lower level of RF bias).