Plasma etching using polycarbonate mask and low-pressure...

Details Plasma etching using polycarbonate mask and low-pressure... Plasma etching using polycarbonate mask and low-pressure...

1999-04-29

2003-06-24

Goudreau, George (Department: 1763)

Etching a substrate: processes

Masking of a substrate using material resistant to an etchant

Mask resist contains organic compound

C216S005000, C216S055000, C216S062000, C216S072000, C216S075000, C216S079000

Reexamination Certificate

active

06582617

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to plasma etching. More particularly, this invention relates to plasma etching of large panel etch substrates for large field emission display devices.
BACKGROUND OF THE INVENTION
Thin film etching processes fall into two broad categories. One category is conventional liquid phase chemical etching or “wet etching”. The other is gas phase plasma-assisted etching or “dry etching”.
There are two primary types of dry etch mechanisms: (a) a physical mechanism and (b) a chemical reaction mechanism. In the physical etch mechanism, ions are extracted from a glow discharge and accelerated towards an etch structure whose surface is eroded by momentum transfer upon being hit by the ions. The etch structure typically includes a layer to be etched (“etch layer”), an overlying patterned layer that serves as an etch mask, and an underlying supporting substructure. In the chemical reaction etch mechanism, a glow discharge is employed to generate chemically active ions which diffuse to the etch structure where they react with the surface of the etch structure to produce volatile products.
An important factor related to the type of mechanism used in the etch process is selectivity. Selectivity is a measure of etch rates of different materials. Dry etch processes in which different materials are etched at approximately the same rate are referred to as nonselective. These processes typically use physical etch mechanisms. Dry etch processes in which different materials are etched at substantially different rates are referred to as selective. Some selective etch processes use chemical reaction mechanisms in which chemically reactive ions preferentially react with one material over another. In other selective etch processes, etched material is preferentially redeposited on one material over another.
In some dry etch processes, both types of etch mechanisms are present. In these processes, chemically active ions are extracted from a glow discharge and accelerated toward the etch structure. As a result, the surface of the etch structure is etched by momentum transfer and by chemical reaction.
Reactive ion etching (RIE) is an example of a dry etch process in which both types of etch mechanisms are present. Chemically active ions (reactive ions) are accelerated towards an etch structure which is etched by momentum transfer upon being hit by the reactive ions and by chemical reaction with the reactive ions.
A conventional RIE reactor is schematically shown in prior art FIG.
1
. The RIE reactor in prior art
FIG. 1
includes a reaction chamber
10
and an electrode
12
capacitively coupled to a high frequency power generator
13
. An etch structure
14
is placed on electrode
12
. In operation, a suitable feed gas is introduced into reaction chamber
10
, and a glow discharge, shown as region
16
, is formed. Since electrons are more mobile than ions, electrode
12
acquires a negative self-bias voltage. Positively charged ions are attracted to electrode
12
and etch structure
14
, and reactive ion etching occurs.
A cross-sectional view of a typical etch structure in which a mask
20
overlies an etch layer
22
is shown in prior art
FIG. 2
a
for an RIE process. Mask
20
defines apertures
24
through which etch layer
22
is etched. Prior art
FIG. 2
b
is an expanded cross-sectional view illustrating the reactive ion etch of a single aperture
24
in prior art
FIG. 2
a
. R
+
represents reactive ions in prior art
FIG. 2
b
. As reactive ions R
+
travel through mask aperture
24
, they collide with the aperture side walls and with other gas molecules. The collisions result in physical and reactive etching as well as recombination with free electrons. As shown, item
26
identifies a region of physical etching, item
28
identifies a region of reactive etching, and item
30
identifies a region of ion-electron recombination.
A deficiency in the reactive ion concentration occurs near the surface of etch layer
22
in apertures with high aspect (depth/width) ratios. Since etch rates are dependent upon reactive ion concentration, the deficiency results in a relatively low etch rate of etch layer
22
. This low etch rate, in combination with the relatively high etch rate for mask
20
due to the substantial reactive ion concentration near the aperture opening, results in a loss of selectivity in the etch between etch layer
22
and mask
20
in apertures with high aspect ratios.
Recently there is a trend towards use of low-pressure high density plasmas in dry etch processes. As the name suggests, low-pressure high density plasmas are characterized by high densities of charged and excited species at low-pressures. This trend is fueled as minimum feature sizes are reduced to submicrometer dimensions, and aspect ratios increase. Horiike, “Issues and future trends for advanced dry etching,” ESC Conference, May 1999 (19 pages) discusses present issues and future trends of dry etching processes employing inductively coupled plasma (ICP), electron cyclotron resonance (ECR), and helicon wave technologies.
In ECR technology, microwave energy is coupled to the natural resonant frequency of electron gas in the presence of a static magnetic field. A conventional ECR waveguide apparatus is schematically shown in prior art FIG.
3
. The apparatus includes a waveguide
40
that directs microwave energy
42
into a reaction chamber
50
. Process gases are fed into reaction chamber
50
. Reaction chamber
50
is surrounded by one or more coils
46
that produce an axial magnetic field. An etch structure
48
is located within reaction chamber
50
. Intense electron acceleration is experienced in an ECR layer
52
that sustains the plasma.
In helicon wave technology, a plasma is magnetized longitudinally, and coupling is achieved by a radio frequency (RF) transverse electromagnetic helicon wave. A conventional helicon wave plasma apparatus is schematically shown in prior art FIG.
4
. An antenna
60
is used to couple power into a reaction chamber
62
. Reaction chamber
62
is surrounded by one or more coils
64
that produce an axial magnetic field. Electrons which resonate with the phase velocity of the helicon wave are accelerated and sustain the plasma. An etch structure
66
is located within reaction chamber
62
.
In ICP technology, an inductive element is used to couple energy from an RF power source to ionize gas. A conventional ICP apparatus using a spiral coupler is schematically shown in prior art FIG.
5
. The apparatus includes an inductive element
70
to which an RF power source is connected. Inductive element
70
is separated from a reaction chamber
72
by a quartz vacuum window
74
. An etch structure
76
is located within reaction chamber
72
. As RF current flows through inductive element
70
, a time varying RF magnetic flux induces a solenoidal RF electric field within reaction chamber
72
. This inductive electric field accelerates free electrons and sustains the plasma. The ICP apparatus shown in prior art
FIG. 5
is also referred to as a transformer coupled plasma (TCP) apparatus.
Bassiere et al, PCT Patent Publication WO 94/28569, discloses a method of manufacturing microtips display devices using heavy ion lithography. The method uses a mask that typically consists of polycarbonate for etching a metal gate layer. Bassiere et al cites RIE as an example of an etch process which can be used to etch the gate layer metal. However, use of RIE to etch high aspect ratio apertures inevitably results in substantial degradation of the mask layer due to the dominant nonselective physical etch mechanism. It is desirable to have a process for selectively etching an etch layer, and in particular a metal gate layer, using a polycarbonate mask without significantly eroding the polycarbonate mask.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method is provided for etching an etch layer using a polycarbonate layer as an etch mask. The etch method of the present invention may be used to fabricate gated electron emitters for la

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