Diffusion, Doping and Thermal Processing
What is a Diffusion or Oxidation Furnace
To make Semiconductor devices work electrically, chemical alterations to semiconductor material has to be made by adding atoms that have an excess or depletion of electrons. A term called "doping"
The chemical process usually involves heating the bulk material (usually silicon) in excess of 1000 Deg C then passing vapour or gases over the silicon for an extended period of time allowing the dopant atoms to diffuse into the semiconductor lattice.
The amount of dopant required or the "depth" to which the dopant atoms diffuse is a function of time and temperature as it depends on the mobility of the atoms inside the crystal structures involved. It is very important to keep the temperatures contant and highly controlled. These furnaces are normally called "Diffusion Furnaces".
Typical Horizontal Furnace with 4 process tubes
The very first step after silicon wafers are manufactured is to grow or deposit an electrically insulating layer.
This layer is usually "Oxide" (SiO2 - a form of glass). This requires temperatures in excess of 1200 Deg C with an oxidising gas injected into the process tube. Oxide grows on top of the silicon.
There are some mechanical and physical properties of "oxide" that make it ideal for the task in had - ie forming a dense conformal insulating layer with similar thermal properties to the silicon so stresses dont build up and cause failure.
Considering a new Furnace?There are lots of considerations to be taken into account which is why we often need on-site visits to discuss process and options
- Processes Required
- Gas Safety
- Heat Load on cleanroom
ProcessThis will determine how many process tubes you require. It will be a combination of process time, batch size, and chemicals. Uniformity of process across a wafer can be dependent on may factors such as gas flow and pressure.
Throughput and FootprintTroughput and process have a big impact on the decision to process wafers in the horizontal or vertical configuration. Vertical tools tend to take less cleanroom floor space if a single or dual process is required. Horizontal configurations allows stacking of upto 4 tubes (upto 200mm). As wafers are processed in boats (usually quartz) the loading of the carriers is also a consideration.
SafetyThere are some nasty gases and liquid sources used in this part of the process, from Hydrogen and Oxygen, to POCl3 , Silane SiH4, Ammonia NH3, Phosphine PH3, Teos, BBr3, B2H6, AsH3, Teasat, etc .... These materials linked with elevated temperatures means safety is a key issue and has to be high on the list of priorities.
Heat LoadOne of the biggest contributors to running costs of a cleanroom is air handling and air conditioning. Keeping a fabrication area (Fab) cool can be as challenging as keeping it warm. Global location of the factory can also be a factor, so additional heat entering the fab is to be avoided.
Furnaces normally require wafers to be processin "boats" that are thermally stable at these elevated temperatures. Specialist quartz manfucturers can design and build these carriert tio custom designs.
Quartzware SiC etc:
Exact quartzware depends on processing temperature and for extreme high temperature processing Silicon Carbide is often the materail of choice. THis refractory materail is very expensive, themally stable bu also very brittle. <
As the material is fragile speciailist cleaning companies are quite often located near to wafer fabrication facilitied to reduce the chance of transport damage.
From gas panel modification to MFC repair, to new and custom elements, thermocouples and carriers to system moves and installs - we and our partners can help. We are embedded in the diffusion and furnace area of many fabs around the world so have probably seen the problem before.
To see some of the products we provide in this technology please