> Graphite Elements & Carbon Carbon Composite Elements. High density graphite and carbon carbon composite are ideal materials for in vacuum heating elements. Chemically the same, high density graphite and carbon carbon composite materials are very inert, gets stronger with temperature, has low expansion coefficient and will not seize after heating. High density graphite is brittle but inexpensive and machined conventionally from large blocks, therefore very large sized graphite elements can be produced in a variety of shapes and sizes. Our high strength ultra fine grained graphite enables small very intricate elements to be manufactured also. Graphite has a low expansion coefficient and is not degraded by constant heating and cooling, and also gets stronger as its temperature increases. Its low resistivity means it requires high current power supplies and therefore large feedthroughs and cables which can be expensive. It can operate over 2000 °C in an inert atmosphere or in vacuum and <500°C if oxygen is present. Graphite elements have the ability to take very high current density, and therefore very fast ramp up times can be achieved. Its relatively high specific heat capacity means that cool down times in vacuum can be quite long. Apart from reacting with oxygen from 500C, graphite is otherwise very inert and can therefore operate in very corrosive or aggressive atmospheres without degradation. Particle contamination and open porosity can be a problem with graphite, but this can be overcome with coatings, or impregnations detailed below. Graphite elements are suitable for UHV applications, but must undergo initial out-gassing process due to its open porosity.   Carbon Carbon Composite is much stronger than graphite and is not brittle due to its fibrous grain structure. Carbon Carbon Composite elements can therefore be made in very thin sections, typically 1mm thick, which overcome a number of the problems associated with high density graphite elements. Thin carbon carbon composite elements have a much higher resistance than high density graphite elements, allowing lower current, higher voltage power supplies to be used , and smaller power feedthroughs and cables, thereby reducing costs. The lower mass of the thin carbon carbon composite elements means that they heat up much quicker and also cool down much faster in vacuum. Carbon Carbon composite is produced in sheets (typically 1m²) of various thicknesses from 1.0mm to 30mm. Modern CNC machining techniques mean that carbon carbon composite elements can be produced very cheaply. We stock standard designs of carbon carbon composite elements from Ø1" to Ø6" which are manufactured in quantity and therefore very cost effective. Carbon carbon composite also has extremely low thermal conductivity which is beneficial in reducing heat loss through the power contact points, thereby increasing element uniformity. This low thermal conductivity also means that carbon carbon composite is ideal for use as a heat shield, both in vacuum and also in inert atmosphere.

For larger sized elements it will be necessary for the graphite element or carbon carbon composite element to be supported, as these materials do not have the rigidity to support themselves without sagging. Ceramisis can supply a range of ceramic materials suitable for supporting graphite elements and carbon carbon composite elements. Ceramisis can also manufacture ceramic bases with a recess machined to the same pattern as the element. A high thermal conductivity ceramic lid can then be fixed in position, completely encapsulating the graphite element. This not only supports the element but also protects it from deposition product, and eliminates high temperature arcing in low vacuum.

To electrically insulate graphite elements it is also possible to apply a SiC coating. This can be applied by CVD or by painting and firing. The CVD method has better and more uniform adhesion, and can withstand up to 1400C. It is however very expensive and the surface tension of the coating does apply a high stress to the graphite element meaning the element can distort and thin section elements are not strong enough to be coated. The paint on SiC coating is inexpensive and easy to apply but has a maximum operating temperature of 1100C. Other coatings can be applied to graphite and carbon carbon composite to seal the porosity and reduce particle count. These coatings are pyrolytic graphite, vitreous carbon, and pyrolytic boron nitride. These coatings can improve oxidation resistance, reduce particle count and improve chemical resistance of the graphite element. Pyrolytic graphite coating is the only coating that can be applied to carbon carbon composite. The table below details the properties and applications for each coating

Graphite Heater Controller and Heater Power Supply

Graphite elements require a special kind of temperature controller to accurately control them across the complete temperature range. Graphite has a lot of electrical peculiarities that occur at various points across the complete temperature range from ambient to 2000C+. Standard controllers and power supplies that use burst fire thyristors are not really suitable. Ceramisis has developed a heater controller specifically for graphite elements. The controller is 4U half rack mount or bench top mount, with a separate transformer box. Our heater controller and power supply offers the complete solution to powering and controlling graphite heating elements. Fitted with a Eurotherm 2216e PID temperature controller which enables the heater to be held at exactly the set temperature. A tuning facility enables the controller to accurately tune to the heaters environment and therefore avoid overshooting during temperature ramping. The control enables ramping and cooling between two set points at a specified ramp rate. The heater controller can also be controlled remotely and integrated in system computer control programs via modbus RS232, RS485 and other protocols. Our heater controllers have power on / off switch to the whole unit, and an output enable switch to isolate the output from the controller. A 24v interlock circuit is also included to enable the power supply to be integrated with interlocks into other systems, or safety interlocks to be attached directly to the unit; which will switch off the output from the unit if the circuit is broken. This enables safety interlocks via make / break switches for things such as chamber lids closed, vacuum interlocks, cooling water flow, over temperature sensors etc.

Graphite, carbon carbon composite and solid pyrolytic graphite material specifications of grades suitable for elements.

For the majority of applications we would recommend using graphite grade GP25 or carbon carbon composite CCC27.

Heater design tips

We are happy to assist with heater element design but please first read the design tips below which gives the basic information we need in order to offer a design / quotation. If you can be flexible in your design please let us know as a standard design may then be suitable and reduce engineering costs.

Material Selection

Plain graphite elements- <500°C in Oxygen, >2000°C in vacuum or inert atmosphere, low cost but brittle with >12% open porosity.

Carbon Carbon Composite (Carbon Fibre) elements- properties are same as for plain graphite but is strong and not brittle.

Solid Pyrolytic Graphite (PG) elements-- <500°C in Oxygen, >2000°C in vacuum or inert atmosphere. Impervious, very high strength and rigidity.

Vitreous Carbon impregnation on graphite elements--<500°C in Oxygen, >2000°C. Low cost impregnation to reduce particle emission. Does not reduce porosity of base material.

Vitreous Carbon coating on graphite elements--<500°C in Oxygen, >2000°C. Low cost coating to reduce particle emission and reduce porosity of base material.

Pyrolytic Graphite (PG) coating on Graphite or CCC elements-<500°C in Oxygen, >2000°C in vacuum or inert atmosphere. This coating seals the porosity of the base material, eliminating particle emissions and greatly reducing out-gassing.

Silicon Carbide (SiC) coating on Graphite elements- <1400°C in Oxygen or in vacuum or inert atmosphere, seals porosity of the base material, and improves chemical resistance.

Pyrolytic Boron Nitride (PBN) coating on Graphite elements-<900°C in Oxygen, >2000°C in vacuum or inert atmosphere (with N2 present). Seals porosity of the base material, and improves chemical resistance.

 Heater Element Design (Flat elements)

To make element design and selection simple, we require that the following information to be specified:

1.      What path design is required:

1.1.   Meander path as per diagram 1

1.2.   Radial Path as per diagram 2a and 2b below

2.      What is the outer diameter of the heater

3.      What is the inner diameter, diag 2a, or is there no bore required, diag 1 & 2b.

4.      Are the power connection points within the hot zone diameter, diag 1 & 2a, or outside the hot zone, diag 2b

5.      What is the pcd (distance between centres) of the connection points

6.      What connection points are required- screw thread or clearance hole

7.      What maximum heater temperature is required?

8.      What atmosphere is the heater element working in, and is there any oxygen present.

9.      What power supply will power the heater, Power / Voltage / Current available

10.  What resistance must the element be

11.  How will the element be mounted- suspended or sitting on a substrate

12.  Are power connection rods required? If so what diameter and length.

13.  Where will the thermocouple be located

A typical heated stage design is shown below, to give an idea of how a heater can be mounted and power connected to it. Ceramisis can manufacture complete vacuum compatible hot stages as well as individual heater elements.

 To achieve good thermal uniformity across the substrate or wafer to be heated, it is important to realise that heat will be drained from the heating element at the following points:

§         where the power legs connect to the element

§         where ceramic supports touch the element

§         the outer edge of the element

§         the centre of the element, if a central shaft passes through the centre of the element, as in element type shown on diag 2a.

The effect of these heat drains, and thus cold spots on the element, can be reduced by taking the following steps:

§         Ensure the power leg connection points are outside the hot zone (diag 2b)

§         Minimise the contact area of any supports that touch the element, and make these supports from low thermal conductivity materials

§         Make the element larger than the substrate or wafer to be heated, therefore the colder outer edge will be outside the high uniformity hot zone.

§         Use a two or three zone heater to minimise edge or central shaft cooling effect. The outer zone can be controlled separately and run slightly hotter than the main high uniformity hot zone to eliminate the edge effect. This is a more expensive solution as each zone requires its own power supply and temperature controller.