JPS5939508B2 - dry etching equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an etching apparatus using plasma.

Recently, there has been an increasing demand for microfabrication technology in many fields, and microfabrication technology on the micrometer order and even below micrometers is desired, particularly in semiconductor device manufacturing.

Requirements for processing technology vary depending on the purpose, but when considering application to the semiconductor industry, for example, the following points are essential.
(c) Etching of fine patterns (several μm or less) is possible.

(2) Practical etching speed (~100Λ/min or more)
is obtained.

(3) Less damage to the substrate.

(4) Less contamination (especially metal contamination) on the substrate.

The substrate damage (3) includes defects in the substrate lattice due to the impact of accelerated ions, distortion due to temperature rise, and the like. In the current semiconductor industry, a mask pattern is formed on a substrate, and the substrate material is etched from above through the openings in the mask. Generally, photoresist is used as a mask material, and in order to form a mask with a fine pattern, it is necessary to reduce the thickness of the mask. Therefore, in order to enable the etching of the fine pattern described in 1 above, the following points are required. (a) Only the substrate material is etched, and the mask material is not etched.

That is, the selectivity is good. (b) There is no undercut. (c
) Good adhesion between the mask and the substrate and no peeling of the mask.

Item a above is a requirement to prevent the thin mask from being etched away together with the substrate material.

The undercut means that the etching progresses to the bottom of the mask 2, as shown in FIG. 1, and the cross section of the substrate 1 under the mask 2 is etched into a trapezoidal or triangular shape. The above condition c is a condition that prevents the mask from peeling off from the substrate and etching from proceeding through the gap. Etching techniques are broadly divided into etching techniques using aqueous solutions, that is, wet etching, and dry etching using ion beams or plasma.

Since wet etching causes large mask peeling and undercuts, dry etching is superior as a microfabrication technology. Dry etching is classified into sputtering using an ion beam (ion beam sputtering), sputtering in plasma (plasma sputtering), and plasma chemical etching using a chemical reaction in plasma. Ion beam sputtering and plasma sputtering require high energy (generally 1ke
The sputtering phenomenon caused by accelerated ions (above) is used. In the sputtering phenomenon, the selectivity of the material to be etched is poor, and high-energy ions collide with the substrate, causing damage to the substrate and heating of the substrate. There is also the problem of contamination, in which ions that have left the sample spatter the sample stage, causing the sample stage material to re-adhere to the sample surface. Considering the above points, plasma chemical etching is suitable for semiconductor applications. Plasma chemical etching has been actively researched recently.

Most conventional plasma chemical etching equipment uses RF (R
adiO Frequency) discharge. FIG. 2 is a schematic diagram of a conventional plasma chemical etching apparatus using RF discharge. R generated in the RF power supply 18
F waves (usually 13.56 MHz) are applied to a counter electrode placed outside the reaction tube 19. For example, when etching Si or SiO2, a mixed gas of CF4 and 02 is introduced into the reaction tube to maintain a gas pressure of ~0.1T0rr. The sample 5 is placed inside the reaction tube 19 in an electrically insulated state, and is etched by active atoms, molecules, and ions generated by the discharge. In plasma chemical etching, since etching is performed purely by chemical reaction, there is little damage to the substrate or heating, and the selectivity of the material to be etched is good. However, conventional plasma chemical etching equipment has large undercuts, making it difficult to etch fine patterns. As shown in FIG. 3, the etching progresses in the lateral force direction to the same extent as the etching in the perpendicular force direction to the substrate, and the base angle θ becomes ~45 force. That is, the etching is isodirectional. The reason why the undercut became larger is as follows. Discharge gas pressure is ~0.1T0
rr, the mean free path of active atoms, molecules, and ions (hereinafter referred to as active particles) that are thought to contribute to etching is approximately 0.1 mm, which is extremely short compared to the distance from the discharge region to the sample (several CWL). . Therefore, active particles are formed in the plasma and collide many times before reaching the sample surface, and are almost uniformly incident on the substrate surface (from all directions of force). As a result, the etching becomes uniform and the undercut becomes large. The current situation of conventional plasma chemical etching technology can be summarized as follows.

In other words, by using a gas that produces active particles by discharging a mixed gas such as (CF4 or C2Cl2+02), it has advantages not found in other dry etching methods, such as less substrate damage, heating, and contamination, and good selectivity of the etched material. I have it. However, since the etching is uniform and the undercut is large, the main purpose of dry etching, which is etching fine patterns, cannot be achieved. That is, the effect of using a chemical reaction in plasma is not utilized for the ultimate goal of etching a fine pattern. The main reason for this result is that a discharge method (RF discharge) that discharges only at high gas pressure was used. By using a discharge force method that produces high-density plasma at low gas pressure, etching with fewer undercuts can be achieved while taking advantage of the above-mentioned advantages of plasma chemical etching, resulting in an unprecedented and revolutionary etching technology. It will be done. The present invention advocates plasma chemical etching using low gas pressure, high density plasma, and provides a specific force formula for this. SUMMARY OF THE INVENTION An object of the present invention is to provide an etching apparatus that can perform etching with less undercut.

In order to achieve the above object, the present invention is characterized by a means for generating microwaves, a means for transmitting the microwaves, and a discharge portion that absorbs the microwaves and generates a discharge. a means for generating a magnetic field, and a Ganu of 5×10-3T0rr or less, in which active atoms, molecules or ions are formed by the discharge, is introduced into the discharge part to generate a discharge, and the active atoms, Located in an etching device that etches a sample with molecules or ions.

The present invention will be explained below using specific examples.

FIG. 4 shows one embodiment of the invention. Microwaves generated by the magnetron 7 are introduced into a discharge chamber 10 covered with a quartz tube through a rectangular waveguide 8 and a circular waveguide 9.
There are two coils 11,1V and one permanent magnet 1 in the discharge chamber.
2 forms a mirror magnetic field. The maximum and minimum values of magnetic field strength (magnetic flux density) are ~2kGauss, ~0.5
kGauss was suitable. The magnetic field strength and shape are
There is no exact optimum value, and it is variable over a wide range. The magnetic flux density Bec for causing electron cyclotron resonance with a microwave of frequency f is given by f: microwave frequency [Hz] m: mass of electron e: charge of electron.

For example, when using a magnetron that is mass-produced for home appliances, the frequency is 2.45 GHz, and the Be
c becomes 875 Gauss. Generally, when the magnetic flux density is made larger than this Bec, microwaves are efficiently absorbed by the plasma, and a stable high-density discharge can be obtained. The magnetic field shape is made into a mirror type because this force stabilizes the discharge and increases the plasma density, and discharge is possible even if it is not necessarily a mirror type. The inside of the circular waveguide 9 is at atmospheric pressure, and the discharge chamber and sample chamber 13 are evacuated. A discharge gas is introduced to a predetermined pressure through the leak valve 4 to generate microwave discharge. Note that the walls of the discharge chamber are air-cooled by air holes 16, 16'. A sample stand 15 is placed on top of the permanent molten stone via an insulating stand 14, and a sample 5 is placed thereon and etched by active particles generated by electric discharge. For example, when etching Si or SiO2, it is suitable to use a mixed gas of CF4 and 02. However, the type of discharge gas is not uniquely determined. For example, the above mixed gas. In the case of CF4, C2F6 may be used instead of CF4, and in general, it can be used as long as it has chemical properties similar to CF4, that is, the property that it can become active particles by electric discharge. When this device is used, discharge is possible at a gas pressure of 5×10 −1 T0rr or higher, and stable discharge can be obtained at a Ganu pressure of 1×10 −4 T0rr or higher.

Therefore, etching is possible at 5×10 −5 T0rr or more. The plasma density Np of the next generated plasma,
When the electron temperature Te is measured by the probe method, ・Ne=1,
7~7x1011c7rL-3, kTe-3~5e. However, k is Boltzmann's constant. The RF discharge used in conventional plasma chemical etching is
Although it is possible to discharge at a gas pressure of 3T0rr or more, in order to obtain a practical etching speed, it is actually 0.1T0rr.
This is done at a gas pressure of rr or more. In addition, the plasma parameters are Ne-2~7x109? -3,Te-8
~12e (C.C.GOOdearandA.
VOnEngel: PrOc. Phys. SOc. 79
(1962) 732-740). Therefore, this apparatus can perform etching at a gas pressure 103 times lower than that of the conventional apparatus, and the plasma density at this time is about 102 times higher. Figure 5 shows a sample in which Al manuk (8008) was formed on a Si substrate using this device.
A reproduction of a photograph observed with nMicrO-ScOpe is shown.

The magnetron output during etching is 180 W. The discharge gun is a mixture of CF4 and 02, and the partial pressure of 02 is 20%. The partial pressure of 02 is not strict, and etching can be performed even at O(:f).

However, the etching speed is maximum when the partial pressure is 20%.
In addition, the gas pressure is 5×10-4T0r in the case of Figure 5a.
In the case of r and b, 5×10-3T0rr, in the case of c, 0.
It is 1T0rr. Discharge gas pressure p is 5×104T0r
In case r, that is, in FIG. 5a, a 1 .mu.m pattern is etched to a depth of 1 .mu.m without undercuts.
This is not possible with conventional plasma chemical etching, and is made possible for the first time by the apparatus of the present invention. The etching speed at this time was 200 to 500 people/
minutes, which is a practical etching speed. Furthermore, the mask material (Al) was not completely etched, indicating the good etching selectivity of this apparatus. Figure 5B,
Figure c shows how an undercut appears as the discharge gun pressure increases. gas pressure is 5
For ×10-3T0rr, there is a slight undercut, and the upper part of the pattern is etched with a slight delay in rounding.
When the gas pressure is 0.1T0rr, the pattern is etched in a triangular shape. The angle θ at the base of the triangle is ~
50 inches, indicating that the etching is performed almost isokinetically. Figure 6 shows the etching speed RSi, r8lO2 of Si and SiO2 and their ratio η-R.
The relationship between 8lO2/R8l and gas pressure P is shown.

The etching gas was CF4 mixed with 02 at a partial pressure of 20%, and the magnetron output was 180W. P is 4×1
Below 0-3T0rr, R8l+300 to 500 people/min, R5lO2+100 to 300λ/min, and η-0.
5-1. In other words, when P becomes larger than 5XI0-3T0rr, R8lO2 increases even though R8l increases slightly.
is rapidly decreasing, and as a result, η is rapidly decreasing to 0.
．． Becomes 03 or less. Therefore, the Ganu pressure P is Pcr-
The reaction mechanism changes rapidly at 4 to 5×10 −3 T0rr. Conventional plasma chemical etching using RF discharge is performed at a gas pressure of P+0.1T0rr, which corresponds to the region P>POr. In fact, in a device using conventional RF discharge, η+0.03(Y.HOr
iikeandShibagaki: PrOceedi
ngsOfthe7thConferenceOnSO
lidStateDevices. TOkyOl975
．． pPl3-18). Figure 7 shows the angle δ between the direction of force connecting the discharge chamber and the sample and the normal line 20 to the sample (see Figure 8).
A reproduction of a SEM photograph of the cross-sectional shape when etching is performed by changing the is shown.

The etching gas is a mixed gas of (CF4+02), the partial pressure of 02 is 20 (f), and the magnetron output is 180! W, gas pressure is 5×10 −4 T0rr.

Looking at this, as the angle δ increases, the angle β between the side surface of the pattern and the normal to the sample surface becomes slightly larger than O, but the change is considerably smaller than δ. If etching is performed by neutral radicals 21, then
β should be the same angle as δ (see Figure 9). Assuming that etching is performed by ions, the ions are accelerated along the lines of electric force within the ion sheath formed in front of the sample, as will be described later. Force direction of electric lines of force. is perpendicular to the sample surface regardless of δ, so etching is also performed almost perpendicularly regardless of δ. However, if the effect of the magnetic field is taken into account, when δ is other than O, β will also deviate slightly from O, but this deviation is quite small compared to δ. Results in Figure 7 and discussion above. From P-5×10-4T
When this apparatus is used at 0rr, it can be concluded that etching is performed by ions. First, with conventional etching equipment using RF discharge, it is possible to etch even regions without ions, and it is clear that etching is performed by neutral radicals (NIKKEI ELECTRO).
NICS (1976) 8.23P96). P=Pc,-
.eta. changes rapidly between 4 and 5.times.10@-3T0rr, and after this pressure, when P<PO, etching is performed by ions, and when P>POr, etching is performed by neutral radicals.

With conventional equipment using RF discharge, etching could only be performed in the region where P>Pcr, but with the equipment of the present invention, etching where P<POr was possible for the first time.

Etching with P<PO has become possible because this apparatus can provide high-density plasma at low gas pressure. As a result, etching without undercuts became possible for the first time for reasons described later. The reason why the particles contributing to etching change from ions to neutral particles at Pcr is as follows.

That is, the plasma density (nion density) of the discharge does not change much depending on the discharge gas pressure, but the neutral particle density increases in proportion to the discharge gas pressure. As a result, as the gas pressure increases, the contribution of neutral particles to etching increases. Furthermore, as the discharge pressure increases, the mean free path of the electrons becomes shorter, and as a result, the average kinetic energy of the electrons becomes smaller. For this reason, although electrons can decompose gases such as CF4 to create neutral radicals, they are no longer able to ionize them, and etching is performed primarily by neutral particles rather than ions. Since the plasma density of the discharge and the ionization energy of active particles do not change much depending on the type of discharge gas, the Ganu pressure P at which the particles to be etched change from neutral particles to ions. r does not change much depending on the type of discharge gas (for example, C2F6, C3F8, CCl4, etc.), and is 4 to 5 as determined in experiment 1 shown in Figure 6.
×10-3T0rr. However, if the distance D between the discharge chamber (the edge on the sample side) and the sample is too long, the ions will collide with neutral particles many times from the time they are generated in the discharge chamber until they reach the sample. However, the ion density decreases due to diffusion and neutralization. Therefore, in order to perform etching mainly using ions, it is desirable that D≦λi, where λi is the mean free path of the ions. λ when the discharge gas pressure is Pcr-4 to 5×10-3T0rr
Is i a number? Therefore, D≦number? It is desirable that However, since λi is inversely proportional to the discharge gas pressure P, it does not matter if P is lowered or D is increased. For example, when P=5X10-4T0rr, D≦numerical 10 (1-70 is acceptable.In order to perform etching without undercuts using the apparatus of the present invention, etching is mainly performed with ions, and for the reasons described later, ion etching is required. When the width of the sheath is Di8, if it is necessary that λi》Di8, it is generally about Di8 + 0.1 mm, so λi must have a length of ?, and for this, the discharge gas pressure P must be less than a few times 10-3T0rr.At this time, the condition of P<POr is also satisfied, and the condition that etching is mainly performed with ions is also satisfied.This device allows etching without undercuts. The reason why etching is possible is because of ionized radicals (
Theoretical explanation of what is done by active atoms, molecules). Since the sample is electrically insulated, the floating potential V, (f<
O), and an ion sheath is formed in front of the sample.

The floating potential f is calculated from the deep needle theory and is expressed as:

Using the results of the probe measurement of this device, we set KTe to -3.8eV, assuming P+ ions and set Al9 to f = -18V.
becomes. The width Di8 of the ion sheath is calculated from the Lamming-Miller-Child formula as follows.

N, -3.3X1011cfn-3, 0.08
m77! +0.1m77! becomes. Plasma ionization degree α
When Dis is defined as Dis, the degree of ionization of this device is approximately 2 according to the probe measurement.
0/).

Therefore, the mean free path λi of an ion is determined by collisions with neutral particles and is inversely proportional to the gas pressure P and is P-5X
When 10-4T0rr, λi is 2CT! It becomes L. Ions are in thermal motion within the plasma.

When the pressure is 5×10 −4 T0rr, Di8<λi. Also, if the ion temperature is T1, generally KTl<1V
It is fI. Therefore, when ions cross the boundary between the plasma and the ion scene, they are accelerated by the electric field without collision and are incident perpendicularly to the substrate (see FIG. 10a). As a result,
Etching proceeds only in the direction of vertical force, eliminating undercuts. When the gas pressure increases, Di8≧λi, and the ions repeatedly collide within the ion sheath and are equiforced to the substrate (see FIG. 10b). Gas pressure P is 0.1T0
When rr, λ1 is about 0.1u. As a result, the undercut becomes larger. The sample has a floating potential f (z-18V) with respect to the plasma.

The ions are accelerated by EvfI, but this energy is insufficient to etch the sample, and etching is performed purely by chemical reaction. Therefore, the advantages of conventional plasma chemical etching apparatuses, such as less damage, heating, and contamination of the sample substrate and good selectivity of the material to be etched, are maintained in this apparatus. Further, since microwave discharge is a non-polar discharge, the discharge chamber can be covered with a quartz tube. Although a mixed gas of CF4 and 02 was used to etch Si and SiO2, it is also possible to etch other materials by appropriately selecting the type of discharge gas. At this time, etching of fine patterns without undercuts is possible, as in the case of Si and SiO2. The discharge gas pressure of this device can be varied over a wide range, so the degree of undercut can be changed freely. By appropriately selecting the discharge gas pressure, the angle θ in Fig. 3 can be changed to 9.
It can be freely changed from 0 force to 45 force. Angle θ
FIG. 11 shows the relationship between the pressure and the Ganu pressure. An example of an application of this feature is, for example, the etching of a diffraction grating for a spectrometer. Further, in order to improve the degree of integration of semiconductor elements, it is preferable to set the angle θ of the pattern etching shape to 90 degrees.

However, if we consider the wiring process to be performed after etching, the force of etching the head slightly obliquely as shown in FIG. 12 will reduce the number of disconnections in the wiring. With this device, initial etching is performed at a high gas pressure (10-2 to 10-1T0rr), and then at a low gas pressure (10-2 to 10-1T0rr).
By etching to a predetermined depth at a depth of 1 to 10-3T0rr), a pattern having a shape as shown in FIG. 12 is obtained. In this example, rectangular and circular waveguides were used to transmit microwaves, but similar results can be obtained using other force methods, such as a coaxial waveguide, as long as high-density plasma can be obtained at low gas pressure. It is natural that it will be done. FIG. 13 shows another embodiment of the invention.

The difference from the embodiment shown in FIG. 4 is that a voltage can be applied to the sample and the sample stage from the outside using a sample power supply 17. In the embodiment shown in FIG. 4, the ions are accelerated by the flow ink potential Vf (-18V) and are incident on the substrate. This ion collision may become a problem in applications where damage to the substrate is extremely important. Power supply 17 of this embodiment
By making the sample potential equal to the plasma potential, ions will only be incident on the substrate with energy corresponding to the ion temperature Ti (KTi<1Vf1), and damage to the substrate will be reduced. However, in this case the undercut is greater than in the case of FIG. Further, by applying a negative potential to the plasma, sputtering can be performed in parallel with the chemical reaction. As a result, the etching speed can be improved. However, in fields where substrate damage and heating are a problem, it is necessary to equip the sample with a cooling device. Furthermore, this example cannot be used for samples of insulators such as SiO2.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the undercut, Figure 2 is a diagram showing a conventional plasma chemical etching apparatus using RF discharge, Figure 3 is a diagram showing the angle θ of the bottom of the undercut, and Figure 4 is a diagram showing the undercut.
5 is a sectional view of the plasma chemical etching apparatus used in the present invention, FIG. 5 is a sectional view of etching according to the present invention, FIG. 6 is a curve diagram showing the relationship between gas pressure and etching rate of Si and SiO2, and FIG. The figure is an explanatory diagram of δ, Fig. 8 is a cross-sectional view of etching when δ is changed, Fig. 9 is an explanatory diagram of etching performed with neutral radicals, and Fig. 10 is an illustration of the reduction of undercuts. Explanatory diagram, 11th
The figure is a curve diagram showing the relationship between Ganu pressure and the undercut angle, FIG. 12 is a diagram showing an example of the application of this method, and FIG. 13 is a diagram showing another embodiment of the present invention. 1...Substrate, 2...Mask, 7...
...Magnetron, 8...Rectangular waveguide, 9...
...Circular waveguide, 10...Discharge chamber, 11,
11t... Coil, 12... Permanent magnet,
13...Sample chamber, 14...Insulation stand, 1
5... Sample stage, 16, 16/... Air hole, 20... Normal line to sample surface, 21...
・Neutral radical.

Claims (1)

[Claims] 1.Equipped with a means for generating microwaves, a means for transmitting the microwaves, and a means for generating a magnetic field in a discharge part that absorbs the microwaves and generates an electric discharge, the apparatus comprises a means for generating microwaves, a means for transmitting the microwaves, and a means for generating a magnetic field in a discharge part that absorbs the microwaves and generates an electric discharge, and the electric discharge generates active atoms, molecules, or An etching apparatus characterized in that a gas at a pressure of 5×10^-^3 Torr or less in which ions are formed is introduced into the discharge portion to generate a discharge, and a sample is etched by the active atoms, molecules or ions.