JPS5813624B2 - Ion etching method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ion etching method for etching a silicon oxide film on a thin film of silicon or other high melting point metal by ionizing and accelerating a reactive gas.

In recent years, dry etching using reactive gas plasma has been used in place of conventional wet etching using chemicals in the etching process for manufacturing integrated circuits and the like.

Dry etching using gas plasma includes various types such as so-called plasma etching, sputter etching, and ion etching.

In plasma etching, a reactive gas such as Freon CF4, usually at a pressure of about 0.1 to 1 torr, is introduced into a cylindrical tube, and the gas is dissociated by discharge to generate gas plasma, and the object to be etched is placed in the plasma. It is then etched.

This etching is thought to be caused by a chemical reaction between active neutral atoms in the plasma, such as fluorine radicals, and the object to be etched.

This method is superior to conventional wet etching in terms of simplifying the etching process, improving pattern dimensional accuracy, and being pollution-free.

On the other hand, since the plasma etching method is originally based on a chemical reaction, there is an unavoidable drawback in the plasma etching method, such as undercutting under the mask of the object to be etched, which occurs in the conventional solution method.

That is, in plasma etching, the pressure of the reactive gas introduced into the cylindrical tube is high, about 0.1 to 1 torr, and the mean free path of the reactive species is short (about 10 to 100 μm).
This results in isotropic etching as shown in FIG.

However, in FIG. 1, the object to be etched 1 is made of polycrystalline silicon (poly-Si) on the silicon oxide layer 2.
Layer 3 is provided.

A photoresist serving as a mask is provided on the polycrystalline silicon layer 3, and when plasma etching is performed, the polycrystalline silicon layer 3 is etched isotropically, and the lower part of the photoresist 4 is etched as shown in the figure. It gets scraped off.

Due to the occurrence of such undercuts, plasma etching methods can only form patterns of about 4 μm at most, and are unsuitable for ultra-fine integrated circuits.

Also, regardless of the type of reactive gas, a silicon layer is etched faster than a silicon oxide layer (SiO). There is a drawback that it cannot be used for any purpose.

On the other hand, in sputter etching, a high frequency voltage is applied to a counter electrode provided in a reaction chamber, and a gas to be ionized, such as C2F6,
Etching is performed by causing a discharge in C3F8 or the like, and causing ions in the generated plasma to accelerate and collide with the object to be etched, which is used as a cathode electrode.

FIG. 2 shows this state, and the object to be etched 5 has a silicon oxide layer 7 provided on a silicon layer 6.

A photoresist 8 serving as a mask is further provided on this silicon oxide layer 7, and the silicon oxide layer 7 is vertically cut by the collision of the ions.

Therefore, undercutting under the mask does not occur as in the plasma etching method described above.

However, in this method, the temperature of the object to be etched 5 rises to a considerably high temperature, for example, about 200° C., due to radiant heat from the plasma, and as a result, the photoresist 8, for example, OFP
Even R, OMR83 (trade name, manufactured by Tokyo Ohka Co., Ltd.), etc., are etched quite quickly, and the photoresist 8 is retreated.

In the end, even if undercuts under the mask can be avoided, the dimensional accuracy of the pattern will not be much different from plasma etching.

Next, in the ion etching system, an ionized gas, usually argon, is dissociated in a discharge chamber at a pressure of 10 torr or less, and the ions in the generated plasma are accelerated with a grid and placed in a sample chamber separate from the discharge chamber. Etching is performed by colliding with the object to be etched.

According to this method, since the sample is away from the plasma, the temperature rise is small and the etching rate of the resist is low, which is convenient, but the etching rate of the material to be etched does not change much depending on the material, and the selectivity is extremely low. It is unsuitable for manufacturing integrated circuits.

The present invention was developed in view of these circumstances, and uses a gas containing halogen and polyvalent carbon atoms as a reactive gas, dissociates this gas by discharge in a discharge chamber, and directs the generated ions to the object to be etched. The object of the present invention is to provide an ion etching method that enables selective etching of an object to be etched with high processing accuracy by applying ion ions to the object.

The present invention will be specifically described below with reference to the drawings.

First, as an example of the apparatus used in the method of the present invention, the Kauf
A Mann type (electron impact type) ion source device will be explained with reference to FIG.

In the figure, a container 10 of an ion etching apparatus is roughly divided into a discharge chamber 11 and an etching chamber 12.

The discharge chamber 11 is provided with a reactive gas inlet 13, and the etching chamber 12 is provided with a vacuum exhaust port 14.

A filament cathode 15 is disposed within the discharge chamber 11, and by applying a voltage from an AC power source 16 between the anode and the wall of the discharge chamber 11, a discharge is caused within the discharge chamber 11. .

Opening 17 of discharge chamber 11 and opening 1 of etching chamber 12
Three electrodes 20 to 22 with a large number of holes between them.
Further, annular insulating members 23 to 25 are interposed between the electrodes 20 to 22 to maintain airtightness.

In this way, the discharge chamber 11 and the etching chamber 12 are communicated with each other.

Of the three electrodes 23 to 25, electrode 23 is a grid held at a positive high potential, electrode 24 is an electron suppressor held at a negative potential in order to reverse the electrons trying to exit from the discharge chamber 11, and Electrode 25 is a grid held at zero potential.

On the other hand, a holding mechanism 28 for holding the object 27 to be etched is provided in the etching chamber 12 .

The holding mechanism 28 is fixed to the inner wall of the etching chamber 12, and has a cooling water passage provided therein (not shown).

A neutralizing filament 30 and an ion shutter 31 are arranged in the space between the holding mechanism 28 and the grid electrode 25.

The neutralizing filament 30 is heated by passing an electric current through it,
It emits thermoelectrons.

The ion shutter 31 is a bent rod-shaped tungsten member, and is led out from the inside of the etching chamber 12 to the outside of the container 10.

It has the effect of blocking ions and, at the same time, plays the role of a monitor that measures the current density of ions.

Note that 35 is an ion focusing magnet.

In such an apparatus, first, the entire inside of the container 10 is evacuated to a pressure of about 10 torr through the exhaust port 14 using a diffusion pump or the like, and then the reactive gas 40 is introduced into the discharge chamber 11 through the inlet 13. Increase the pressure to approximately I×IO torr.

Then, an AC voltage is applied between the discharge chamber 11 and the cathode 15,
When an alternating current is passed through the magnet 35, discharge plasma is generated within the discharge chamber 11.

Ions in this plasma are accelerated by the voltage between the high voltage grid electrode 20 and the earth grid electrode 22 and drawn into the etching chamber 12.

Electrons in this ion flow are reflected into the discharge chamber 11 by the electron suppressor electrode 21.

A part of the ions accelerated in this way are neutralized by thermionic electrons from the neutralizing filament 30 and become neutral atoms.

The ion flow then collides with the object to be etched 27 to perform etching.

Incidentally, even if the electronic subpressor electrode 21 is placed between the earth grid electrode 22 and the neutralizing filament 30, it is possible to perform the same operation as the above-described device.

Here, CF4 gas and C3F8 gas are used as reactive gases, and for comparison, conventionally used argon (Ar) is used to etch silicon oxide SiO2 and polycrystalline silicon (poly-Si). The etching rate was measured by changing the voltage between the electrodes 20 and 21, that is, the ion acceleration voltage.

As a result, effects as shown in FIG. 4 were obtained.

The curve shown by the solid line in the figure is the case when silicon is etched, and the curve shown by the broken line is the case when SiO2 is etched.

As is clear from this result, in the case of argon gas, the etching rate of silicon oxide and polycrystalline silicon gradually increases as the accelerating voltage increases, but the etching selectivity of silicon oxide and polycrystalline silicon is also low.

However, in the case of CF4 gas, the etching rate of silicon oxide increases linearly with increasing acceleration voltage, and is higher than in the case of argon gas.

On the other hand, for polycrystalline silicon, the etching rate change is extremely low up to an acceleration voltage of 600 mm, but increases rapidly above that, and exceeds the etching rate of silicon oxide at an accelerating voltage of 900 mm or higher.

Furthermore, in the case of C3F8 gas, the etching rate of silicon oxide increases little by little, and the rise in etching of polycrystalline silicon is approximately 1100 V, which is higher than in the case of CF4 gas.
occurs at an accelerating voltage of

At an accelerating voltage of around 600 cm, the ratio of silicon oxide etching rate to polycrystalline silicon etching rate with CF4 gas is about 3 to 4 times, whereas in the case of 03F8 gas, it reaches about 7 to 8 times.

The reason why the selectivity is excellent when using a gas having such a C-F bond is that carbon is adsorbed on silicon due to the discharge of the gas having a C-F bond, and at low energy, this carbon layer is ionized. It is presumed that this is because the etching rate of silicon oxide exceeds the etching rate of polycrystalline silicon because it is difficult to penetrate, but on silicon oxide, oxygen is generated and CO is generated during etching, and the silicon oxide surface is constantly exposed.

From the above measurement results, when a gas containing polyvalent carbon atoms and polyvalent fluorine atoms is used, not only the etching rate of silicon oxide increases, but also the selectivity with respect to polycrystalline silicon improves.

In addition to C3F8, gases such as C2F6 and C4F8 have similarly excellent selectivity; for example, in the case of C4F8, it has been confirmed that a selectivity of 12 to 15 times can be reached at an accelerating voltage of around 600 cm.

In these experiments, the etching rates of polycrystalline silicon and single crystal silicon were about the same.

Next, FIG. 5 shows the etching characteristics of a photoresist serving as a mask when C3F8 gas and argon gas are used.

The photoresist used was negative type OMR-83 (manufactured by Tokyo Ohka).

The broken line is the etching rate of the resist on silicon oxide, and the solid line is the etching rate of the resist on poly-Si.

The reason why the etching rate of resist on SiO 2 is generally high is considered to be that oxygen is released during etching of SiO 2 and the oxygen multiplies the etching of the resist.

Even in that case, the resist etching rate is lower when C3F8 gas is used than when Ar gas is used, and although not shown, when CF4 gas is used, it is just as good as when Ar gas is used and when C3F8 gas is used. It will be an intermediate value.

Therefore, it can be understood that using a gas containing a large amount of carbon atoms and fluorine atoms reduces etching of the resist compared to using argon or CF4 gas, and is thus more advantageous in improving pattern accuracy.

For example, at an accelerating voltage of 300μ, a sharp silicon oxide film with a width of 0.25 μm and a height of 1 μm without undercuts under the mask can be etched using positive resist using C2F6 gas. Ta.

By the way, as shown in Fig. 4, the etching rate of silicon oxide is approximately 400 to 500 volts at an accelerating voltage of about 600 volts.
It's not too big with 00 people.

In the manufacturing process of integrated circuits, a SiO2 film with a thickness of 1 μm is generally used, and at this level of etching rate, it is difficult to use it in an actual manufacturing line.

On the other hand, at an accelerating voltage of 600 volts or more, the etching rate of silicon oxide increases, but at the same time, the etching of the resist also increases rapidly, making it difficult to use such an accelerating voltage in practice.

Therefore, in order to effectively etch silicon oxide, it is necessary to increase the ion current and lower the acceleration voltage.

For this purpose, an etching apparatus equipped with a speed reduction mechanism as shown in FIG. 7 may be used.

The basic principle of this device is to extract the ions produced by the ion gun and apply a voltage to the ions with approximately the same polarity and opposite polarity as the voltage required to extract them.In other words, the ions are accelerated once and then decelerated. It is something.

In the figure, the main body 100 of the apparatus is roughly divided into two chambers 101 and 102.

One chamber 101 is an etching chamber, and an exhaust port 104 connected to a diffusion pump or the like is provided at the lower end of a cylindrical container 103 forming this chamber.

Further, a viewing window 105 is provided on the side wall of the container 103 so that the etching state of the object to be etched 106 placed in the chamber 101 can be observed.

A sample stage 107 on which the object to be etched 106 is placed is provided with a water channel (not shown) inside and is filled with water.

In addition, the sample and stand 107 are connected to a drive mechanism 10 outside the chamber 101.
8, it is rotated in the horizontal direction and also revolved.

On the other hand, the chamber 102 has annular electrodes 109 to 113 and a lid electrode 114 and cylindrical spacers 115 to 119 alternately stacked on the upper open end of the cylindrical container 103.
The support rods 120 and 121 are inserted into holes provided in the support rods 4 and integrated.

A cylindrical electrode 123 is fixed to the inner wall of the lid-shaped electrode 114 via an insulating member 122, and these electrodes 114, 12
The space surrounded by 3 is the discharge chamber 124, and the PIG
It constitutes a type electron gun.

A reactive gas, for example, 03F8 gas, is introduced into the discharge chamber 124 through a gas introduction pipe 125.

When a high voltage is applied using the electrode 114 as an anode and the electrode 123 as a cathode, the reactive gas is dissociated by discharge and ionized.

Note that 126 is a stainless steel cold trap. The generated ions are extracted and accelerated by an extraction electrode 130.

The extraction voltage was approximately 25 KV.

The electrodes 131 and 132 are called Mainzel lenses, and function to converge the ion flow.

Electrodes 133 to 135 are deceleration electrodes and decelerate ions.

In this way, the ions are finally accelerated to about 200V and irradiated onto the object to be etched 106.

At this time, the degree of vacuum inside the device 100 is I× 10 to
It was about rr.

When etched in this manner, the etching rate of the resist was as low as about 100 Å/min, and the etching rate of silicon oxide reached about 1500 Å/min, which was sufficient for practical use.

The etching rate of the silicon oxide layer was about 7 to 8 times the etching rate of the polycrystalline silicon layer.

Next, instead of the 03F8 gas, CF is added from the gas introduction pipe 125.
Etching was performed by introducing a mixed gas of 4 gas and a gas containing carbon such as C2H2 or C2H4.

In this case, the etching rate of the silicon oxide layer reached 30 times the etching rate of the polycrystalline silicon layer.

The ion source of the ion etching apparatus described above is the Kaufman type and the PIG type, but is not limited to these, and may be a duo plasma type, a Freeman type, or a mixed type thereof.
Furthermore, a microwave ion source using electron cyclotron resonance may be used.

Similar results were also obtained when the polycrystalline silicon film that served as the base of the silicon oxide film used in the above experiment was replaced with single crystal silicon or other heat-resistant high-melting point metals such as tungsten, molybdenum, and niobium.

And, as a reactive gas, not only C3F8 but also C4F8
It was possible to obtain sufficient etching selectivity even when using .

Furthermore, even when the silicon oxide film is replaced with an oxide such as alumina (M203), it is now possible to selectively etch the alumina layer by adding c2H2 gas to a gas containing chlorine, such as chlorine gas. .

As explained in detail above, according to the present invention, it is possible to selectively etch oxide films on single crystal silicon, polycrystalline silicon, or heat-resistant metals such as tungsten, molybdenum, and niobium at a much higher rate than the former. It is possible.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing how a polycrystalline silicon film is etched using a plasma etching device, and Figure 2 is a cross-sectional view showing how a silicon oxide film is etched using a parallel plate sputter etching device. Figures 3 and 6 are vertical cross-sectional views showing an example of the apparatus used in the method of the present invention, and Figure 4 shows reactive gases such as argon, CF4, and 03F.
Figure 5 is a characteristic diagram showing the relationship between ion acceleration voltage and etching rate when silicon oxide and polycrystalline silicon are etched using argon and C3F8 as reactive gases. FIG. 4 is a characteristic diagram showing the relationship between the ion acceleration voltage and the etching rate of a photoresist serving as a mask when etching the photoresist. 11,124...Discharge chamber, 12,101...Etching chamber, 27,106...Object to be etched.

Claims (1)

[Claims] 1. A reactive gas containing at least halogen atoms and polyvalent element atoms is ionized, and the ionized ions are formed with a first layer formed of an oxide and silicon or a high melting point metal. An ion etching method characterized in that the object to be etched is etched by irradiating the object to be etched, which is a laminate made up of a second layer, with an accelerating voltage of 600 volts or less. 2. The ion etching method according to claim 1, wherein a gas containing polyvalent carbon bonded to polyvalent halogen atoms is used as the reactive gas. 3. The ion etching method according to claim 1, wherein the oxide is silicon oxide. 4. The ion etching method according to claim 1, characterized in that the extracted and accelerated ions are decelerated before being irradiated onto the object to be etched.