The furnace used in the ece444 lab is a Lindberg-Tempress 8500 dual-stack furnace. It consists of 8 seperate 'quartz' furnace chambers where oxidation, dopant diffusion, and annealing are performed.

Each chamber is controlled by a Digital Temperature Controller (DTC). The DTC controls three heating zones with inputs from three spike (on the exterior of the furnace) and three paddle (integrated with cantilever loader) thermocouples for each chamber. It uses PID (Proportional, Integral, Derivative) control to quickly ramp the furnaces to the setpoint temperature and hold to within ±1 °C.

In addition, one half of the stack is fitted with cantilever autoloaders. Digital Process Contollers (DPC) control the loading, unloading, times, and process gas sequence of the autoloading chambers. The DPC can hold up to 16 individual recipes.

Temperatures

The chambers are typically operated at 950°C - 1100°C during oxidation and diffusion.

In general, the furnaces in the cleanroom are kept at a standby temperature of 600°C. A higher temperature standby condition is not utilized because the life of the furnace core windings decreases rapidly at elevated temperatures. The furnaces are not completely shut off when not in use because the cycling from room temperature, about 30°C, to above 1000°C devitrifies (crystallizes) the quartz tubes.

Chamber Construction

The chambers used in the furnace are fabricated from fused silica, often incorrectly referred to as quartz. Fused silica is amorphous silicon dioxide (SiO₂) and exhibits several attributes that make it the material of choice for furnace chambers.

• It is non-contaminating - fused silica is composed of the same materials used in IC production.
• It can be refined to high levels of purity (99.97% pure SiO₂)
• It has a low coefficient of thermal expansion (CTE) - it can withstand large temperature differentials without failing.

More precisely, the chambers in use in the 444 lab are stabilized - i.e. there is a controlled thickness of crystallized SiO₂ around the outer circumference of the chamber. This crystallized layer provides a compressive force to the chamber which minimizes sagging, allowing it to be heated and cooled repeatedly without failure.

Although this outer layer of crystalline quartz is beneficial to stability, uncontrolled crystallization at local areas of the chamber create areas of stress that can cause failure. Local crystallization is known as devitrification.

Devitrification is accelerated by crossing transformation temperatures (~250°C and 1100°C) repeatedly. Hence the standby temperature of 600°C and maximum temperature of 1100°C.

Processes

Oxidation

Three chambers are dedicated to oxidation in the furnace:

• Field oxidation - dry and steam oxidation
• Boron drive - dry and steam oxidation
• Gate oxidation - dry oxidation only

The furnaces are equipped to provide nitrogen (N₂), oxygen (O₂), and hydrogen (H₂).

N₂ is used to provide an inert environment in the chambers to prevent uncontrolled reactions within the chamber. It also helps maintain cleanliness of the system by providing a slightly positive pressure that minimizes backstreaming of air into the system.

The furnaces are also fitted with pyrogenic steam generators (hydrogen is burned with oxygen in the chamber) for steam oxidation. Proper interlocks and sensors make the process relatively safe.

Dopant Diffusion

Two chambers are dedicated to dopant introduction using Carborundum/St. Gobain Planar Diffusion Sources (PDS).

The PDS boron source wafers (boron nitride) are oxidized to form a B₂O₃ layer prior to a predeposition run (usually by a TA). It is the boric oxide which has a significant vapor pressure at the diffusion temperatures. B₂O₃ reacts with silicon to form SiO₂ with an extremely high concentration of boron.

The wafers used for phosphorus predeposition are made with SiP₂O₇ in a fine SiO₂ lattice. The SiP₂O₇ decomposes at diffusion temperatures to form P₂O₅ which vaporizes and reacts with silicon.

The student is encouraged to balance the predeposition reactions (or to read section 6.14 of Anner) since they are basic to understanding how you make ICs and are, therefore, excellent lab final material.