Multi-zone resistive heater

Details Multi-zone resistive heater Multi-zone resistive heater

1999-05-19

2002-07-23

Paik, Sang (Department: 3742)

Electric heating

Heating devices

Combined with container, enclosure, or support for material...

C118S725000

Reexamination Certificate

active

06423949

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to heating mechanisms for process chambers, particularly, heating mechanisms for chemical vapor deposition chambers.
2. Description of Related Art
Chemical vapor deposition (CVD) is a popular process for depositing various types of films on substrates and is used extensively in the manufacture of semiconductor-based integrated circuits such as, for example, the processing of semiconductor wafers to form individual integrated circuit device. In typical CVD processing, a wafer or wafers are placed in a deposition or reaction chamber and reactant gases are introduced into the chamber and are decomposed and reacted at a heated surface to form a thin film on the wafer or wafers.
In general, there are single-wafer and multi-wafer CVD reaction chambers in use today. Multi-wafer reaction chambers typically resemble vertical furnaces capable of holding, for example, 25 wafers or more. For low pressure CVD (LPCVD), for example, 0.25-2.0 torr, for the deposition of Si
3
N
4
or polysilicon, a typical deposition time for a multi-wafer chamber might be several hours. Si
3
N
4
, for example, is formed at a temperature between 700-800° C. and a deposition time of 4-5 hours depending upon layer thickness in a multi-wafer chamber.
A second type of CVD reaction chamber is a single-wafer chamber in which a wafer is supported in the chamber by a stage or susceptor. The susceptor may rotate during the reaction process. For an LPCVD Si
3
N
4
deposition, for example, a suitable layer thickness may be produced at 700-800° C. in about two minutes.
In general, there are two types of heating schemes used in CVD systems: resistive heating schemes that utilize a resistive heating element localized at the wafer, and radiant heating schemes that use a radiant heating element such as a lamp or lamps usually placed outside the reaction chamber. Resistive heating schemes in a single-wafer chamber generally incorporate the resistive heating element directly in the stage or susceptor that supports the wafer in the chamber. In this manner, the reaction produced during the deposition may be generally more localized at the wafer.
In single-wafer resistive heating schemes that utilize a heating element within a stage or susceptor that supports a wafer, the heating element is typically a thin layer of conductive material, such as a thin coiled layer (about 2 mils) of a molybdenum (Mo) material formed in a single plane of the body of the susceptor. This design may be described as a “single-zone resistive heater,” the “zone” description referring to the location of the heating element in a single plane in the body of the stage or susceptor. The CVD reaction in which the resistive heaters are used typically has a temperature compatibility to approximately 550° C. At higher temperatures, temperature uniformity becomes problematic. One reason is that heat loss in a resistive heater increases with higher temperatures, particularly at the edges of the stage or susceptor. Single-zone resistive heaters typically do not have the ability to compensate for differences in heat loss across the stage or susceptor. The pressure in a chamber will also modify the temperature stability of single-zone resistive heaters.
In addition to providing the requisite temperature, the resistive heating element must also be amenable to the chemical environment in the reaction chamber including high temperature and chemical species. One solution to the compatibility consideration in prior art single-zone resistive heaters is to form the susceptor of aluminum nitride (AlN) with the heating element formed inside the susceptor.
Radiant heating schemes generally position lamps behind heat-resistant protective glass or quartz in the reaction chamber. Since the entire chamber is heated by the lamps, the CVD reaction occurs throughout the chamber.
Radiant or lamp heating schemes offer the benefit of generating a high chamber temperature and controlling that temperature better than resistive heating schemes. However, since radiant heating schemes utilize heating elements, e.g., lamps, placed outside of the reaction chamber, the ability to control the temperature in the chamber becomes more difficult as the chamber walls become coated with chemicals or other materials or reaction products used in the reaction chamber. Thus, as the materials used in the chamber deposit on the chamber glass or quartz, for example, the effectiveness of the heating is reduced and the process performance is effected.
In this regard, a reaction chamber used in a radiant heating scheme must be cleaned often. A typical cleaning agent is nitrogen trifluoride (NF
3
). In Si
3
N
4
CVD processes, for example, Si
3
N
4
reaction products form on the chamber walls and other components inside the chamber, such as a quartz window(s). Si
3
N
4
is difficult to clean from a reaction chamber with a cleaning agent like NF
3
. The cleaning temperature generally must be high in order to dissociate the NF
3
and provide enough thermal energy to clean Si
3
N
4
. If the cleaning temperature is high, the NF
3
will also attack components in the chamber, such as the susceptor. A remote plasma source used to energize the NF
3
can reduce the cleaning temperature but activated NF
3
species (particularly radicals) tend to attack quartz components. Therefore, currently there is no effective cleaning solution for radiant-based chambers. Since the walls of the reaction chamber are not easily cleaned with NF
3
, Si
3
N
4
material accumulates and shortens the lifetime of the chamber.
In LPCVD reactions, temperature uniformity is generally important. The surface reaction associated with a CVD process can generally be modeled by a thermally activated phenomenon that proceeds at a rate, R, given by the equation:
R=R
o
e
[−E
a
/kT]
where R
o
is the frequency factor, E
a
is the activation energy in electron volts (eV), and T is the temperature in degrees Kelvin. According to this equation, the surface reaction rate increases with increasing temperature. In a LPCVD process such as a Si
3
N
4
deposition, the activation energy (E
a
) is generally very high, on the order of 0.9-1.3 eV. Accordingly, to obtain a uniform thickness across the wafer, the temperature uniformity across the wafer should be tightly controlled, preferably on the order of ±2.5° C. or less for temperatures around 750° C.
Prior art single-wafer radiant heating schemes offer acceptable temperature uniformity even at higher temperatures (e.g., 750° C.) when the chamber is clean. However, as materials accumulate on the walls of the chamber, temperature uniformity becomes difficult.
It is also difficult to obtain a uniform high temperature (e.g., 700-750° C.) across a wafer with a single-zone resistive heater. As noted, in general, heat loss is not uniform across the surface of a susceptor at higher temperatures. A single-zone heater cannot compensate, for example, for a greater heat loss toward the edges of the susceptor than at its center. Thus, temperature uniformity is a problem.
A second problem with single-zone resistive heaters such as described above and temperatures of 750° C. is problems associated with localized heating. At high temperatures, single-zone heaters exhibit concentrated localized heating associated with high density power applied to the heating element at a localized area. Consequently, temperature uniformity is affected. A third problem with single-zone resistive heaters is that variations in manufacturing of the heating element can cause fluctuations in performance of a heating element which can lead to non-uniformity. The single-zone heater cannot be adjusted to compensate for the manufacturing variation. Further, at high temperature operation, single-zone heaters have shorter lifetimes due to the high power density applied at the power terminals and to the heating elements.
Still further, prior art resistive heaters and chambers that provide such heaters offer limited dynamic temperature measurement. In general

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