Gas dispersion head

Details Gas dispersion head Gas dispersion head

1999-09-20

2002-11-05

Bueker, Richard (Department: 1763)

Coating apparatus

Gas or vapor deposition

C118S730000

Reexamination Certificate

active

06475284

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to semiconductor process equipment, and more particularly, to a method and systems for dispersing gas flow in a semiconductor reactor.
BACKGROUND OF THE INVENTION
Semiconductor processing typically involves the formation of one or more layers on a semiconductor substrate. For example, silicon epitaxy, sometimes called epi, is a process in which one or more layers of single-crystal (monocrystalline) silicon are deposited on a monocrystalline silicon wafer.
FIG. 1
is a schematic representation of a semiconductor processing system
10
in accordance with the prior art. As shown in
FIG. 1
, system
10
included a susceptor
12
enclosed within a barrel reactor
14
. Susceptor
12
was typically suspended from an assembly (not shown), which rotated susceptor
12
during processing. Susceptor
12
supported a plurality of substrates
16
, typically monocrystalline silicon wafers.
During processing, substrates
16
were heated with an external radiation source such as tungsten halogen lamps, resistive heating elements and/or RF heaters (not shown). A process gas was introduced into reactor
14
through two injectors
18
A,
18
B mounted on a gas ring
20
. The flow rate of the flow of process gas to injectors
18
A,
18
B was controlled by a mass flow controller
22
(MFC
22
). Injectors
18
A,
18
B were coupled in parallel to MFC
22
. The process gas reacted with heated substrates
16
resulting in the deposition of layers on substrates
16
as those skilled in the art understand. The spent process gas was then exhausted to exhaust
23
.
As the art moves towards reduced feature size integrated circuits, it has become increasingly importance that the deposited layers on substrates have uniform thickness. One primary parameter which affects the thickness uniformity is the flow characteristics of the process gas into and through the reactor.
Referring again to
FIG. 1
, these flow characteristics were controlled to a large extent by injectors
18
A,
18
B through which the process gas was introduced in reactor
14
. More particularly, injectors
18
A,
18
B aimed the jets of process gas so that the jets collided with each other at a point between susceptor
12
and reactor
14
. The goal in aiming the jets was to eliminate any circumferential velocity components of the jets. The mixed jets flowed generally downwards over substrates
16
to the bottom of reactor
14
and to exhaust
23
.
To obtained the desired thickness uniformity, injectors
18
A,
18
B were calibrated. Calibration was typically an iterative process in which a first layer was deposited on a first test substrate, the thickness uniformity of the first layer was measured, and injectors
18
A,
18
B were adjusted in an attempt to improve the thickness uniformity. A second layer was then deposited on a second test substrate, the thickness uniformity of the second layer was measured, and injectors
18
A,
18
B were again adjusted. This trial and error procedure was repeated until the desired thickness uniformity was obtained. Unavoidably, the iterative process used to calibrate injectors
18
A,
18
B was time consuming, labor intensive and generally unpredictable.
In addition to obtaining the desired thickness uniformity, it is also important to have abrupt transitions between layers.
FIG. 2
is a graph of dopant concentration versus depth in a substrate
16
in accordance with the prior art. Referring to
FIG. 2
, formed on substrate
16
was a heavily doped layer L
1
(hereinafter referred to as HD layer L
1
), a transition layer TL on top of HD layer L
1
, and a lightly doped layer L
2
(hereinafter referred to as LD layer L
2
) on top of transition layer TL.
By way of example, HD layer L
1
was a heavily doped P type silicon layer formed by supplying a process gas having a high P type dopant concentration. Conversely, LD layer L
2
was lightly doped P type silicon layer formed by supplying a process gas having a low P type dopant concentration. Transition layer TL was formed as a result of the change from high to low of the P type dopant concentration of the process gas. As shown in
FIG. 2
, the dopant concentration of transition layer TL gradually changed from heavily doped HD at the bottom of transition layer TL to lightly doped LD at the top of transition layer TL.
As the art moves towards smaller high speed devices, it is important that the transition between layers be abrupt. In particular, referring to
FIG. 2
, it is important to reduce or eliminate transition layer TL between the top of HD layer L
1
and the bottom of LD layer L
2
. However, use of system
10
(
FIG. 1
) inherently resulted in the formation of transition layer TL. This limitation of system
10
essentially eliminates the possibility of the use of barrel reactors for the next generation of integrated circuits. Yet, barrel reactors are relatively simple, reliable and cost effective to operate. Accordingly, the art needs a method and apparatus which allows realization of abrupt transitions between layers formed in a barrel reactor.
SUMMARY OF THE INVENTION
In accordance with the present invention, a semiconductor processing system includes a barrel reactor and a dispersion head within the barrel reactor. The dispersion head includes at least one distributor. During use, process gas is supplied to the dispersion head. The dispersion head is hollow (has an internal channel) so that the process gas flows through the channel of the dispersion head to the at least one distributor. The process gas flows through the at least one distributor and into the reactor. The process gas contacts substrates within the reactor thus forming a layer on the substrates. The spent process gas is then exhausted from the reactor.
Of importance, the process gas is dispersed by the dispersion head as the process gas enters the reactor. By dispersing the process gas, and supplying the dispersed process gas to the reactor, the flow characteristics of the process gas through the reactor is improved by reducing turbulence compared to the prior art. More particularly, use of the dispersion head reduces and/or eliminates turbulence and recirculation in the flow of the process gas through the reactor. Thus, the process gas travels through the reactor from the dispersion head to the exhaust in a uniform flow, i.e., in a curtain-like flow, without the turbulence and recirculation of the prior art.
Since the process gas flow is uniform through the reactor, the process gas uniformly contacts the substrates. Accordingly, use of the dispersion head results in the formation of layers on the substrates having excellent thickness uniformity. Further, since the dispersion head disperses the process gas in a repeatable and predefined manner, calibration of the dispersion head is avoided. This is in contrast to the prior art where the injectors had to be calibrated for each reactor and also had to be recalibrated when process parameters, e.g., the flow rate of flow of process gas, were changed. Thus, use of a dispersion head in accordance with the present invention is less time consuming, is less labor intensive and is more reliable than use of the injectors of the prior art.
In addition, use of the dispersion head allows realization of an abrupt transition between layers formed on the substrates. This is because when the process gas is changed to have a new composition, e.g., from a high dopant concentration process gas to a low dopant concentration process gas, the new process gas travels in a uniform flow through the reactor similar to a curtain falling. As the bottom of this curtain passes the substrates, the process gas contacting the substrates abruptly changes to have the new dopant concentration. Thus, an abrupt transition occurs between the layer formed from the process gas having the original gas composition and the new layer formed from the process gas having the new gas composition. Accordingly, use of the dispersion head enables formation of substrates having abrupt transitions between layers using a relatively simple

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