JPS6228574B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ion etching method for selectively etching a silicon oxide layer, which is an object to be etched, 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 (CF ₄ ), usually at a pressure of about 0.1 to 1 torr, is introduced into a cylindrical tube, and this is dissociated by discharge to generate gas plasma, and the object to be etched is placed in the plasma. It is etched by placing it on the surface. 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 simplification of the etching process, improvement in pattern dimensional accuracy, and non-pollution.

On the other hand, since the plasma etching method is originally based on a chemical reaction, undercutting of the object to be etched under the mask, which occurs in the conventional solution method, is an unavoidable drawback even in the plasma etching method.
That is, in plasma etching, the pressure of the reactive gas introduced into the cylindrical tube is as high as 0.1 to 1 torr.
The average path of the reactive species is short (about 10 to 100 μm),
This results in an isotropic etching as shown in FIG. However, in Fig. 1, the object to be etched 1
In this example, a polycrystalline silicon layer 3 is provided on a silicon oxide layer 2. A photo resist serving as a mask is provided on the polycrystalline silicon layer 3.
When plasma etching is performed, polycrystalline silicon layer 3 is etched isotropically, and the lower part of photoresist 4 is scraped off as shown in the figure.
Because of the occurrence of such undercuts, plasma etching methods can only produce patterns of about 4 .mu.m at most, and are unsuitable for ultra-fine integrated circuits. In addition, regardless of the type of reactive gas, a silicon layer is etched faster than a silicon oxide layer, so there is a drawback that it cannot be used in applications where, for example, a silicon oxide layer is etched more quickly. On the other hand, sputter etching involves applying a high frequency voltage to a counter electrode provided in a reaction chamber.
A discharge is caused in a gas to be ionized, such as C ₂ F ₆ or C ₃ F ₈ , maintained at a pressure of about 10 ^-1 to ^{10 -3} torr,
Etching is performed by accelerating and colliding ions in the generated plasma with the object to be etched, which is used as a cathode. 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 the silicon oxide layer 7, and the silicon oxide layer 7 is vertically cut by the collision of the ions. Therefore, undercuts under the mask do 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, e.g., about 200°C, due to the radiant heat from the plasma. The photo resist 8 is etched too quickly, causing the photo resist 8 to retreat. 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. Furthermore, in this method, when the reactive gas used is CF ₄ , CF ₂ Cl _{2 ,} etc., _the silicon layer is etched _faster than the silicon oxide layer;
In the case of C ₂ F ₆ , the silicon oxide layer is etched faster than the silicon layer. For example, if the object to be etched is composed of three layers of polycrystalline silicon, silicon oxide, and single crystal silicon, a photoresist as a mask is provided on the polycrystalline silicon layer.
Consider the case where polycrystalline silicon and silicon oxide are sequentially etched. In this case, first CF ₄ ,
Etching of the polycrystalline silicon layer must be reduced by using CF ₂ Cl ₂ or the like, and then by using C ₃ F ₈ , C ₂ F ₆ or the like after the silicon oxide layer appears. In this way, the type of gas to be ionized must be changed in order to etch the same object to be etched, which has the drawback of making the process more frequent. Next, in ion etching, a gas to be ionized, usually argon, at a pressure of 10 ^-4 torr or less is dissociated by discharge in a discharge chamber.
Etching is performed by accelerating the ions in the generated plasma using a grid and causing them to collide with the object to be etched, which is placed in a sample chamber separate from the discharge chamber. According to such a method, unlike plasma etching, the object to be etched can be vertically cut by the collision of ions. Furthermore, since the object to be etched is not placed in the discharge chamber, it is less likely to receive radiant heat from plasma unlike in sputter etching, and the photoresist will not be heated and etched quickly. Therefore, ultra-fine processing of integrated circuits becomes possible.

However, when argon is used as the ionized gas, the difference in etching rate for polycrystalline silicon and etching rate for silicon oxide, for example, is small and the selectivity of the etching target is low. Therefore, when the object to be etched is composed of several layers, it is difficult to use it for selectively etching only one layer or several layers among them.

The present invention was developed in view of these circumstances, and uses a reactive gas, such as a gas containing halogen, to dissociate this gas by discharge in a discharge chamber, and accelerate the generated ions at a low voltage of 900 volts or less to generate a large number of ions. The object of the present invention is to provide an ion etching method capable of selectively etching a silicon oxide layer, which is an object to be etched, with high processing accuracy by applying the ion to a sample having a laminated structure of a crystalline silicon layer and a silicon oxide layer.

The present invention will be specifically described below with reference to the drawings. First, a Kaufmann type (electron impact type) ion source device will be explained with reference to FIG. 3 as an example of the device used in the method of the present invention. 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,
The etching chamber 12 is provided with a vacuum exhaust port 14 . A filament cathode 15 is provided in the discharge chamber 11.
is arranged, and by applying a voltage from an AC power supply 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 etching chamber 12
Three electrodes 20 to 22 having a large number of through holes are provided between the openings 18 of the electrodes 20 to 22, and annular insulating members 23 to 25 are interposed between the electrodes 20 to 22 to maintain airtightness. It is structured as follows. In this way, the discharge chamber 11 and the etching 12 are communicated with each other. Among the three electrodes 20 to 22, electrode 2
0 is a grid held at a positive high potential, electrode 21
is an electron suppressor that holds electrons at a negative potential in order to reverse the electrons trying to exit from the discharge chamber 11;
Further, the electrode 22 is a grid held at zero potential.

On the other hand, a holding mechanism 28 for holding an object 27 to be etched is provided within 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). Holding mechanism 28 and grid electrode 2
A neutralizing filament 30 and an ion shutter 31 are arranged in the space between the two. The neutralizing filament 30 is heated by passing an electric current through it, and 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 10 ^-7 torr through the exhaust port 14 using a diffusion pump, etc., and then the reactive gas 4 is injected through the inlet 13.
0 is introduced into the discharge chamber 11, and the pressure is set to 1×10 ^-4 torr.
increase to a certain degree. Then, an cross-grain voltage is applied between the reactive gas 11 and the cathode 15, and the magnet 35
When an alternating current is applied to the discharge chamber 11, discharge plasma is generated within the discharge chamber 11. Ions in the 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. Then, the ion flow collides with the object to be etched 27 to perform etching.

Here, Freon (CF ₄ ) was used as a reactive gas, and for comparison, argon (Ar), which is conventionally used, was used to etch silicon oxide (SiO ₂ ) and polycrystalline silicon (poly-Si). In this case, the etching rate was measured by changing the voltage between the grid electrodes 20 and 22, that is, the ion acceleration voltage. As a result, the effects shown in Figure 4 were obtained. The curve shown by the solid line in the figure is the case when freon is used, and the curve shown by the broken line is the case when argon is used.

When Freon gas is used, the etching rate of polycrystalline silicon exceeds the etching rate of silicon oxide when the ion accelerating voltage is approximately 900 volts or more, and conversely, the etching rate of silicon oxide exceeds the etching rate of silicon oxide when the ion accelerating voltage is approximately 900 volts or more. Exceeds the etching speed of crystalline silicon. This is because the etching characteristics of silicon oxide using Freon are linear, whereas the etching characteristics of polycrystalline silicon are quite low at accelerating voltages of about 600 volts or less, but increase sharply at accelerating voltages of about 600 volts or higher. It is.

In particular, at an accelerating voltage of about 600 volts, the etching rate of silicon oxide reached about 3.5 times that of polycrystalline silicon.

On the other hand, in the case of argon, which is conventionally used as an ionization gas, the etching rate of both silicon oxide and polycrystalline silicon increases monotonically with respect to the ion acceleration voltage, and there is a large difference in the etching rate of each at the same acceleration voltage. Nakatsuta.

From the above measurement results, when Freon gas is used, it is possible to selectively etch one layer of a material having a silicon oxide layer and a polycrystalline silicon layer by appropriately selecting the ion accelerating voltage. When argon gas is used, changing the ion acceleration voltage only changes the etching rate of both silicon oxide and polycrystalline silicon, and it is understood that it is difficult to selectively etch both.

In particular, an accelerating voltage of about 500 volts is optimal for etching silicon oxide, and an accelerating voltage of about 1100 to 1200 volts is optimal for etching polycrystalline silicon.

Next, FIG. 5 shows the etching characteristics of a photoresist serving as a mask when Freon gas and Argon gas are used. The photoresist used was positive type OFPR (product name manufactured by Tokyo Ohka).
It is. The solid line shows the characteristics when argon (Ar) is used, and the broken line shows the characteristics when Freon (CF ₄ ) is used. When using argon,
It can be seen that while the etching rate increases considerably as the ion accelerating voltage increases, the etching rate does not increase so much when Freon is used.

Therefore, the use of Freon gas is more advantageous in improving the precision of the pattern because etching of the resist does not proceed as much as when using argon gas.

In the case of Freon gas, the ion acceleration voltage is 1000
Even in the case of bolts, the etching rate is at most about 60 Å/min, and below that it is extremely slow at 30 to 50 Å/min. When the ion accelerating voltage is 500 volts, the etching rate of silicon oxide is about 9 to 10 times that of photoresist, making it suitable for etching using a mask.

As a result of experiments, it was possible to perform sharp silicon oxide etching with a width of 1 μm and a height of 1 μm without undercuts under the mask.

In the above explanation, the etching characteristics using Freon gas were described, but other methods such as reactive gas containing halogen in the form of a single substance or a compound,
For example, similar characteristics were obtained when C ₂ F ₆ , C ₃ F ₈ , CF ₂ Cl _{2 ,} etc. were used. In this case, excellent etching properties were obtained, especially when the gas contained carbon. When using reactive gases containing these halogens, the practical ion acceleration voltage is approximately 900 volts (900 V for CF ₄ , approximately 1100 V for C ₃ F ₈ ).
Under the following low acceleration conditions, the silicon oxide layer on the silicon layer is selectively etched, and at approximately 900 volts (about 900 V for CF ₄ and 1100 V for C ₃ F ₈ ), the silicon layer or tungsten on the silicon oxide layer is etched. It has been confirmed that layers of high melting point metals such as molybdenum, niobium, titanium, and vanadium can be selectively etched. Furthermore, when the reactive gas was a gas containing chlorine, selective etching of chromium, aluminum, etc. was possible.

As described below, in the method of the present invention, a reactive gas, such as a gas containing halogen gas, is introduced into the discharge chamber 11 from the inlet 13 in the apparatus shown in FIG.
For example, the voltage between 0.
Around 900 volts (900± depending on the type of reactive gas)
The etching process is performed by applying a voltage lower than 200 volts (with a range of about 200 volts) and accelerating ions to collide with the object to be etched 27 in the etching chamber 12 located at a distance from the discharge chamber 11. .

In the above description, the case where the holding mechanism 28 that holds the object to be etched 27 is in a stationary state is described, but the holding mechanism 28 rotates the disc-shaped portion 50 on which the object to be etched 27 is placed, and further rotates the portion. The parts 51 other than 50 are driven circumferentially to make the disc-shaped part 50 revolve, and the object to be etched 2 is
7 can also be etched evenly. Further, the disk-shaped portion 50 is tilted with respect to the direction of ion irradiation, so that the etching steps of the selectively etched layer of the object 27 are tapered, thereby completing the final stage of the integrated circuit manufacturing process. It is also possible to effectively prevent the breakage of aluminum wiring that occurs in the process.

According to the present invention described in detail above, for example, a reactive gas containing halogen is dissociated by discharge in a discharge chamber, and the generated ions are accelerated at a low voltage depending on the type of the object to be etched and irradiated onto the object to be etched. This allows the object to be etched to be selectively etched with high processing accuracy, and has high practical value.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views showing the state of the object to be etched after being etched using the plasma etching method and the sputter etching method, respectively;
FIG. 3 is a longitudinal sectional view showing an example of the apparatus used in the method of the present invention, and FIG. 4 shows the etching of silicon oxide and polycrystalline silicon using freon and argon as reactive gases, respectively.
Figure 5 is a characteristic diagram showing the relationship between ion acceleration voltage and etching rate. Fig. 5 is a characteristic diagram showing the relationship between ion acceleration voltage and etching rate when photoresist is etched using Freon and argon as reactive gases. . In the figure, 10... Container, 11... Discharge chamber, 122... Etching chamber, 15... Filament cathode, 16... AC power source, 20-22... Grid electrode, 27... Object to be etched.

Claims (1)

[Claims] 1 A reactive gas containing a halogen is dissociated by discharge in a discharge chamber, and the dissociated ions are accelerated at a voltage of approximately 900 volts or less. An ion etching method characterized in that the silicon oxide layer is selectively etched by irradiating a sample having a stacked layer structure.