Page 336 - Sami Franssila Introduction to Microfabrication

31

Tools for Hot Processes

Thermal treatments constitute a major fraction of temperature-dependent). Heating can be achieved by
front-end processes. Traditionally, the horizontal tube inductive coils (as in epitaxy), by a susceptor/bottom
furnace (see Figure 13.1) has been the workhorse for electrode that is kept at a high temperature or by lamps
thermal processing (for oxidation, diffusion, annealing), (in rapid-thermal processing, RTP).
but more recently, vertical furnaces and rapid-thermal
processors (RTP) have entered the scene.
31.2 FURNACE PROCESSES
Thermal oxidation is the prototypical hot-wall furnace
31.1 HIGH TEMPERATURE EQUIPMENT: HOT process. Dry oxidation for a 25 nm oxide is shown in
WALL VERSUS COLD WALL Figure 31.1 and Table 31.1. The process consists of
ramp-up, oxidation, post-oxidation anneal (POA) and
Two main varieties of high-temperature systems exist: ramp-down.
hot wall and cold wall. Hot-wall systems remain hot Wafer cleaning before all high-temperature processes
constantly, usually by resistive heating as in horizontal is essential but in order to also guarantee tube clean-
furnaces. Cold-wall systems heat only the wafers and liness, chlorine cleaning can be done prior to thermal
the actively cooled system walls remain at room oxidation. This process reduces metallic contamination,
temperature. By analogy with kitchen equipment: an much like RCA-2 clean, which uses HCl; in fact, HCl
oven is a hot-wall system, a microwave oven is a cold- has been used as a furnace cleaning agent but today,
wall system. Warm-wall systems do exist: system walls organic chlorocompounds such as 1,2-dichloroethene
are heated unintentionally by the process but they remain (DCE) are used (see Figure 13.1). Alternatively, some
at a much lower temperature than the wafers. chlorine-containing gases can be used during oxidation.
Large thermal masses in hot-wall systems provide Open-tube furnaces are flushed with nitrogen during
excellent temperature uniformity but very slow temper- wafer loading, and this is usually effective in removing
◦
ature ramp rates: 0.1 C temperature uniformity and 5 residual water vapour. However, even 100 ppm of
◦
to 10 C/min ramp-up rates, and even slower cooling residual water vapour will change dry oxidation rates,
rates. New vertical furnaces have an order of magni- and 5 ppm of oxygen will lead to titanium silicide
tude higher ramp rates: tens of degrees per minute. deterioration. Double tubing is used if better atmospheric
Thermocouples are used for temperature monitoring. In control is required, but loadlocked systems must be used
hot-wall CVD systems, deposition takes place on all hot when exact atmospheric control is mandatory. It is useful
walls and successive depositions build up thick films on to have a small, controlled oxygen flow during ramp-up
walls. Film cracking and particle generation are espe- to prevent thermal nitridation of the silicon surface, and
cially probable when two different films are deposited accept minor oxidation instead, but of course this is not
at different temperatures. applicable for very thin oxides.
In cold-wall systems, only the wafers are heated, Actual oxidation time can be a very small fraction
and the rest of the system stays cool, which enables of total process time, as in the horizontal tube gate
faster temperature ramp rates and less deposition on oxidation example in Table 31.1. An optional POA den-
the walls (because chemical reactions are exponentially sifies the film, but does not, in the first approximation,

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)