JP4470717B2 - Plasma etching method - Google Patents

Description

  The present invention relates to a dry etching method, and more particularly to a plasma etching method for etching an object by converting an etching gas into plasma.

Conventionally, for an oxide material typified by lithium niobate (LiNbO ₃ : hereinafter referred to as LN) or lithium tantalate (hereinafter referred to as LiTaO ₃ or LT), an inert gas, a halogen gas, or a gas obtained by mixing them. An etching method using plasma is used. In this etching method, a part of the inert gas collides with electrons in the plasma to become positive ions, and these positive ions collide with the material to be etched placed on the negative electrode, so that the object is physically obtained. It uses the removal of surface material (ion bombard effect). On the other hand, the halogen gas undergoes dissociation or ionization in the plasma to generate chemically active radicals and ions. Some of these physically remove the material on the surface of the object, like the positive ions of the inert gas.

  Furthermore, the plasma etching method forms a reaction product with a high vapor pressure on the surface of the material to be etched, chemically removes the surface material, and chemically weakens the binding energy on the surface of the material to be etched. There is also an effect of reactive ion etching to be removed.

In the above-described conventional plasma etching method, a plasma in which these gas types are used alone or in combination is generated to optimize various conditions such as plasma density, process gas pressure, gas flow rate, material temperature to be etched, and etching rate. The selection ratio and etching shape are controlled.
JP 2001-60587 A T.IEE Japan, Vol.118-E, NO.9, '98 LiNbO3 plasma dry etching characteristics

  The conventional plasma etching method has a problem that the selection ratio, which is the ratio of the etching rate of the object to be etched and the removal rate of the mask material by etching, does not increase as compared with the semiconductor material or the like. In addition, among the elements contained in oxide materials such as LN and LT, a chemical reaction with a halogen radical or ion often occurs to form a reaction product having a low vapor pressure. The only way to remove this and increase the etching rate is to increase the kinetic energy of incoming ions, promote chemical reaction, or remove by physical effects.

  However, this method increases the etching rate of the photoresist or metal film used as an etching mask, and thus has a problem that the selectivity does not increase. That the selectivity does not increase means that processing with a high aspect ratio (ratio of depth to opening width) such as deep grooves is impossible.

  Therefore, as a method for increasing the selection ratio, there is a Bosch process applied to deep groove processing of silicon. In this process, an etching gas and a deposition gas for forming a protective film for the side wall and the mask are alternately used, so that a high selection ratio can be obtained and a good etching shape is realized.

  However, since this method uses two types of process gases, a variety of secondary reaction products that chemically react with the material to be etched are generated, and the reproducibility and stability of etching characteristics are poor. . In addition, there are various demerits such as a high manufacturing cost of the apparatus and a disadvantage of maintaining the apparatus due to contamination of the vacuum chamber.

  Further, when the kinetic energy of ions is increased as described above, the temperature of the upper surface of the material to be etched (contact surface with the plasma) rises, and the material to be etched is etched halfway due to the difference in thermal expansion from the lower surface of the material to be etched. In many cases, cracks occurred. Furthermore, since the oxide is a non-conductive material, the charge on the surface of the material to be etched coming from the plasma is not discharged, and the excessively accumulated charge is caused by the charge distribution state of the material to be etched and the difference in stage potential. In some cases, a large current was instantaneously generated, and the material to be etched was destroyed. Further, in the pyroelectric materials such as LN and LT, this phenomenon is particularly remarkable as compared with other materials due to the temperature gradient of the surface due to plasma and the accumulation of electrons.

  The present invention has been made in view of the above-described conventional technology, and is capable of increasing the etching selectivity while suppressing cracks and breakage of the substrate to be etched and the like. It is an object of the present invention to provide a plasma etching method that enables processing.

The present invention relates to a plasma etching method for etching a surface of a material to be etched, which is an oxide material, with reactive ions or radicals using an etching gas, the main component being a gas containing at least fluorine atoms, and an inert gas or Using fluorine-containing plasma mixed with a halogen gas, an etching step for applying an etchable self-bias voltage to the plasma, and applying a self-bias voltage to the plasma so that no etchable voltage is applied or etching does not proceed. In this plasma etching method, etching is performed by alternately repeating the step of modifying the surface of the metal etching mask covering the surface of the material to be etched with the plasma. The switching between etching and surface modification is controlled by increasing or decreasing the high-frequency power applied to the negative electrode.

The plasma is generated using electron cyclotron resonance. A metal material such as nickel is used as an etching mask material for reactive ion etching. Further, the etching, which performs by applying the following frequency power 3MHz negative electrodes, switching of the etching and the surface modification is to control by increasing or decreasing the high frequency power.

  The material to be etched is an oxide material such as lithium niobate or lithium tantalate. The etching gas contains sulfur hexafluoride gas or Freon 14 or the like. Further, the fluorine-containing plasma of the etching gas is a mixture of argon or xenon.

  According to the plasma etching method of the present invention, there is no cracking or destruction of the oxide material to be etched without using a complicated apparatus or process, the etching selectivity is large, and the precision and high aspect ratio by etching are fine. Processing is possible.

  Embodiments of the present invention will be described below. The plasma etching method of this embodiment generates plasma of an etching gas mainly composed of sulfur hexafluoride gas or Freon 14, and etches the surface of the material to be etched such as LN and LT. The etching process and the surface modification process are alternately performed while reacting with the gas. The etching gas is not particularly limited as long as it contains at least fluorine atoms.

  In addition, by controlling the high frequency power of this etching plasma, it is possible to perform anisotropic processing with high selectivity on oxides such as LN and LT. Furthermore, transfer of a fine sharp shape is also possible.

For example, in the case of an ECR (electron cyclotron resonance) type plasma etching apparatus, it is desirable to set the process pressure to 1 Pa or less and the microwave supply power to a high density of about 1 × 10 ¹¹ / cm ³ . The bias RF power is desirably about 3 W / cm ² , and the switching of the bias power is desirably about 30 seconds to 2 minutes for etching and about 15 seconds to 1 minute for surface modification.

  Even with a capacitively coupled or inductively coupled plasma (ICP) type plasma etching apparatus, the same plasma density is generated and the bias RF power is controlled in a time-sharing manner as described above, as described above. Etching with a high selectivity is possible.

  In order to realize an etching rate increase or a high aspect ratio etching shape, an ECR type or ICP type etching apparatus capable of high plasma density and capable of independently controlling plasma generation and ion kinetic energy is advantageous. .

Further, in order to increase the kinetic energy of ions in the vicinity of the sheath of the etching apparatus, it is desirable to supply a high frequency bias RF power of 400 kHz to 3 MHz, preferably about 1 MHz. A metal material such as nickel is used as an etching mask material for reactive ion etching.

Here, the reason why a high selectivity can be achieved by the present invention is as follows. First, in the case of high power, the kinetic energy of positive ions flying to the negative electrode increases, and the etching action by SF _X ⁺ ions becomes dominant. On the other hand, in the case of low-power, kinetic energy of the ions from the reducing, surface modification by SF _X and F radicals becomes dominant. Specifically, during the surface modification process with low power, the mask surface is modified to nickel fluoride (NiF ₂ ) or the like that is highly resistant to plasma, so that the etching rate of the mask material decreases. To do. For this reason, the selectivity of the mask material and the material to be etched is relatively increased.

  Next, the reason for the destruction of the material to be etched due to the surface charge and thermal stress is that when the low power is supplied (during surface modification), the flow rate of ions flying onto the surface of the material to be etched and individual ions are retained. The kinetic energy is reduced. Accordingly, the amount of heat and charge accumulated in the material to be etched is reduced. As a result, the temperature difference between the front and back surfaces of the material to be etched and the potential difference between the surface of the material to be etched and the stage on which the material is placed are reduced, and the destruction of the material to be etched is suppressed.

  According to the plasma etching method of this embodiment, the etching selectivity is large with a simple process using an ECR etching apparatus or the like, and fine processing with high precision and high aspect ratio is possible by etching. Furthermore, there is no crack or destruction of the oxide material that is the material to be etched, and the etching can be performed stably, and complicated fine processing, surface processing of the thin material to be etched, and the like can be performed precisely.

Next, an embodiment of the plasma etching method according to the present invention will be described. In this example, reactive ion etching of LT using SF ₆ plasma was performed using an ECR type reactive ion etching apparatus. The conditions at this time are the conditions of the above embodiment, the microwave power is 170 W, the process pressure is 0.4 Pa, the bias RF power at the time of etching is 300 W, the bias RF power at the surface modification is 0 W, and the process time is 60 minutes. (Etching time is 2 minutes, modification time is 1 minute and one cycle is 20 cycles). As a result, the etching shape of FIG. 1 was obtained. The etching depth was 4 μm and the selection ratio was 7.

  When continuous etching is performed under the same conditions (microwave power 170 W, process pressure 0.4 Pa, bias RF power 300 W during etching) without performing the surface modification process, the mask is consumed very much and is not less than 5 minutes. Etching is not possible, and many phenomena such as charge-up and substrate cracking due to temperature rise were observed. The etching depth at this time was about 1 μm and the selectivity was 2.

For plasma etching of LN SF ₆ using an ECR type etching apparatus, the composition ratio of nickel and fluorine on the LN surface was measured while changing the surface modification time. The results are shown in Table 1. Here, in continuous etching, Ni: F = 1: 1.56. However, by providing a surface modification time, the modification (fluorination) of the mask material proceeds, and the chemistry of nickel fluoride is increased. The value is close to the stoichiometric composition ratio (Ni: F = 1: 2). That is, it can be seen that the mask surface is modified to nickel fluoride (NiF ₂ ) or the like that is highly resistant to plasma.

It is an electron micrograph which shows the etching result of one Example of this invention.

Claims (4)

In the plasma etching method of etching the surface of the material to be etched, which is an oxide material, with reactive ions by an etching gas,
An etching step of applying a self-bias voltage capable of etching to this plasma using fluorine-containing plasma in which a gas containing at least fluorine atoms is a main component and an inert gas or a halogen gas is mixed with the gas,
Applying a surface modification of a metal etching mask covering the surface of the material to be etched by applying a self-bias voltage that does not allow etching to be applied to the plasma or applying a self-bias voltage that does not allow etching to proceed,
A plasma etching method, wherein etching is performed alternately and repeatedly. The plasma etching method according to claim 1 , wherein the plasma is generated using electron cyclotron resonance . The etch is to perform by applying the following frequency power 3MHz negative electrodes, switching of the etching and the surface modification, according to claim 1 or 2 wherein those controlled by increasing or decreasing the high-frequency power Plasma etching method.   The material to be etched is lithium niobate or lithium tantalate which is an oxide material, the etching gas contains sulfur hexafluoride gas or Freon 14, nickel is used as the material for the etching mask, and the fluorine-containing plasma 4. The plasma etching method according to claim 3, wherein argon is mixed with xenon.