JP4568445B2 - Dry etching - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a dry etching method that can be used for microfabrication of a glass substrate used for optical waveguides, bio-MEMS, and the like.
[0002]
[Prior art]
In recent years, application of semiconductor microfabrication technology to the production of optical waveguides, bio MEMS (Micro Electro Mechanical System), etc. has attracted attention.
The optical waveguide is usually formed in a quartz glass substrate to a depth of 5 μm to 50 μm and a width of 5 μm to 15 μm, and is applied to, for example, a photosynthesis / demultiplexer as a component of optical fiber communication.
Bio MEMS is a measuring / analyzing instrument used to form a microcapillary with a depth of 10 μm to 20 μm and a width of several μm to 10 μm on a quartz glass substrate, and to evaluate the health status of the patient from a minute amount of blood of the patient. It is applied as a component.
[0003]
[Problems to be solved by the invention]
By the way, in the semiconductor process, the etching depth is about 1 μm at most, but in the optical waveguide and bio MEMS, the etching is performed to a depth of 20 μm or more. It was found that an abnormal hole shape that was not observed by etching with a depth of about 1 μm occurred on the surface of the glass substrate.
[0004]
FIG. 1 of the accompanying drawings schematically shows a case where the quartz substrate 1 is covered with a Cr mask 2 and etched to a depth of 2 to 3 μm and a case where the quartz substrate 1 is etched to a depth of 10 to 20 μm. As can be seen from this figure, when deep etching is performed, holes 3 are formed on the surface. Actually, it is not so difficult to etch to a depth of 2 to 5 μm, but irregularities appear when the depth is deeper than that.
[0005]
Such an abnormally shaped hole in the surface of the etched portion of the substrate causes the smoothness of the wall surface to be disturbed in the optical waveguide and causes light scattering. On the other hand, in bioMEMS, for example, lymph in the sample solution When separating the spheres by electrophoresis, the flow path is disturbed, both of which are problematic.
[0006]
Considering the formation of such an abnormally shaped hole, as shown in FIG. 2, when deep etching is performed with a mask 2 of μm on the quartz substrate 1, the initial stage of etching is shown in FIG. Proceeding as shown in B, C, the μm mask 2 gradually recedes, that is, a facet with a mask pattern shoulder dipper is produced. Then, as shown in FIG. 2D, grooves (trench) 4 are formed around the pattern due to accumulation of negative charges due to ions and electrons reflected on the side surfaces of the pattern during etching. Then, as shown in FIG. 2E, the etching proceeds in the lateral direction while maintaining the facet angle, a cut 5 due to reverse microloading occurs, and finally a hole 6 is formed as shown in FIG. 2F. all right.
[0007]
In order to elucidate the mechanism by which such holes are formed, the present inventors have conducted experimental research on various factors in the etching process of a quartz substrate using an etching method using a magnetic neutral line discharge (NLD) plasma. It was.
[0008]
In FIG. 3, an etching apparatus using magnetic neutral line discharge (NLD) plasma is used, the magnetic neutral line discharge output is 800 W, the gas is C ₃ F ₈ / CF _4, and it is applied to the substrate electrode. When the bias voltage (Vdc) is −500 V, the gas flow rate is C ₃ F ₈ / CF ₄ = 30/90 sccm, and the pressure in the etching chamber is changed to 1.5 mTorr, 2.0 mTorr, and 3.0 mTorr It shows about the pressure dependence of formation of a hole. From this figure, it can be seen that changing the pressure does not directly affect the formation of the hole.
[0009]
In FIG. 4, an etching apparatus using magnetic neutral line discharge (NLD) plasma is used, the magnetic neutral line discharge output is 800 W, the gas is C ₃ F ₈ / CF _4, and it is applied to the substrate electrode. Hole formation when the etching depth is changed by setting the bias voltage (Vdc) to be −310 V, the gas flow rate to C ₃ F ₈ / CF ₄ = 30/90 sccm, and the pressure in the etching chamber to 3.0 mTorr. Indicates the state. As the etching depth increases from 8.4 μm to 9.1 μm, it is recognized that the hole diameter is enlarged. The measurement result of the relationship between the generation of holes and the etching depth is shown in the graph of FIG.
[0010]
In FIG. 6, an etching apparatus using magnetic neutral line discharge (NLD) plasma is used, the magnetic neutral line discharge output is 800 W, the gas is C ₃ F ₈ / CF _4, and it is applied to the substrate electrode. The bias voltage (Vdc) to be set is −500 V, the gas flow rate is C ₃ F ₈ / CF ₄ = 30/90 sccm, the etching is performed continuously for 30 minutes, and the etching is repeated for 30 minutes after repeating the etching for 3 minutes. The state of hole formation in the case of intermittent etching for 30 minutes is shown.
[0011]
In FIG. 7, an etching apparatus using magnetic neutral line discharge (NLD) plasma is used, the magnetic neutral line discharge output is 800 W, the gas is C ₃ F ₈ / CF ₄ , and the gas flow rate is The hole formation state is shown when C ₃ F ₈ / CF ₄ = 30/90 sccm, the pressure in the etching chamber is 3.0 mTorr, and the bias voltage (Vdc) applied to the substrate electrode is changed. It can be seen that the generation of holes decreases as the bias voltage (Vdc) increases to −500 V and −940 V compared to −310 V.
[0012]
FIG. 8 shows, as an average value, the result of measuring the number of generated holes by changing the CF ₄ concentration from 0% to 100% while keeping the total gas flow rate constant. FIG. 9 shows the dependency of the etching shape on the CF ₄ gas concentration.
[0013]
In FIGS. 10 and 11, samples having various steps are prepared by changing the etching time for the quartz substrate, and the size (width) of the 0.2 μm thick Cr mask is changed for 40 minutes. The etching result when it etches is shown. When the height of the step portion is 55 nm, all of the Cr mask sizes (widths) 0.11 μm to 4.00 μm are etched flat, but when the height of the step portion is 167 nm and 333 nm, the etching is flat. If the bottom becomes a flat dent and the size (width) of the Cr mask increases, a triangular pyramid remains on the bottom of the concave shape. It has been found that it remains in the shape of a hole.
[0014]
From these experimental results, it was found that the high-frequency self-bias voltage (Vdc) of the etching characteristics is greatly related to the formation of abnormal holes, and no abnormal holes are formed by devising the initial etching. In addition, the inventors have found that the etching selectivity can be controlled and the vertical shape can be improved by setting the concentration of the fluorocarbon gas relative to the total flow rate of the chemically active gas.
[0015]
Accordingly, an object of the present invention is to provide a plasma etching method that can prevent formation of an abnormally shaped hole by controlling a high-frequency self-bias voltage (Vdc) in initial etching.
[0016]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, a fluorocarbon gas is brought into a plasma state by discharge under reduced pressure, and a substrate is formed by a reaction between generated species such as ions and neutral radicals and a glass substrate material. In the method of etching
A first bias voltage in the range of −500 V to −940 V is applied to the substrate at the beginning of etching, and then a bias voltage lower than the first bias voltage is applied for a time longer than the application time of the first bias voltage to the substrate. It is characterized by applying.
[0017]
In the plasma etching method according to the present invention, another fluorocarbon gas 2 (for example, CF ₄ ) can be preferably added to the fluorocarbon gas 1 (for example, C ₃ F ₈ ). In this case, the concentration of the fluorocarbon gas 2 added to the fluorocarbon gas 1 can be 69% or more in order to maximize the etching characteristic selection ratio and improve the vertical shape.
[0018]
Also preferably, the flow rate of the fluorocarbon gas can be controlled to suppress reverse microloading.
[0019]
In another feature of the plasma etching method according to the present invention, the foreign material and the altered layer on the surface of the substrate are removed by sputtering the substrate with a rare gas prior to the etching.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to FIGS. 12 and 13 of the accompanying drawings.
[0021]
FIG. 12 shows the surface state of the substrate when the quartz substrate is etched according to one embodiment of the present invention. In this embodiment, an etching apparatus using magnetic neutral line discharge (NLD) plasma is used, the magnetic neutral line discharge output is 800 W, the gas is C ₃ F ₈ / CF ₄ , The flow rate was C ₃ F ₈ / CF ₄ = 30/90 sccm, and the pressure in the etching chamber was 2.0 mTorr. The bias voltage (Vdc) applied to the substrate electrode was set to −850 V for 2 minutes at the beginning of etching, and then set to −500 V for 28 minutes. As a result, the etching can be performed without generating holes, and the surface of the etched portion is smooth.
[0022]
FIG. 13 shows the surface state of the substrate when the quartz substrate is etched according to another embodiment of the present invention. In this embodiment, an etching apparatus using magnetic neutral line discharge (NLD) plasma is used as in the embodiment shown in FIG. 11, the magnetic neutral line discharge output is 800 W, and the gas is C ₃ F ₈ / CF ₄ was used, and the gas flow rate was C ₃ F ₈ / CF ₄ = 30/90 sccm. The pressure in the etching chamber was 20.0 mTorr, and the bias voltage (Vdc) applied to the substrate electrode was set to −900 V for 2 minutes at the beginning of etching, and then set to −500 V for 30 minutes. As a result, also in this case, the etching can be performed without generating holes, and the surface of the etched portion is smooth.
[0023]
In addition, the generation of holes on the surface of the substrate was found to be related to the presence of foreign substances and altered layers adhering to the surface of the substrate as a result of experiments. Therefore, before etching, the substrate such as Ar or He was removed. It is effective to perform a pretreatment of sputtering with a rare gas.
[0024]
In addition, it is recognized that if the reverse loading is large in relation to the pattern width and the etching rate, it will lead to the generation of holes on the substrate surface. It is also effective to control.
[0025]
By the way, in the illustrated embodiment, the case where the quartz substrate is etched has been described, but it is naturally applicable to other glass substrates. Moreover, although the example using the magnetic neutral line discharge plasma has been described for carrying out the method of the present invention, the present invention can be implemented using other dry etching techniques.
[0026]
【The invention's effect】
As described above, according to the present invention, a fluorocarbon gas is brought into a plasma state by discharge under a low pressure, and the substrate is etched by a reaction between the generated active species such as ions and neutral radicals and the glass substrate material. In this method, by applying a high bias voltage of −500 V to −900 V to the substrate in the initial stage of etching, even when etching with a depth of 20 μm or more is performed, etching is performed smoothly without generating holes in the substrate surface. be able to. As a result, it is possible to provide a useful method that can sufficiently cope with fine processing of a glass substrate used for an optical waveguide, bio-MEMS, or the like that requires smoothness of a wall surface.
[0027]
Also, by mixing two kinds of fluorocarbon gas 1 and fluorocarbon gas 2 (for example, C ₃ F ₈ and CF ₄ ), a high etching selectivity can be obtained. In particular, the concentration of fluorocarbon gas 2 When the thickness is 69% or more, vertical etching is obtained.
[0028]
Further, according to another feature of the present invention, one of the factors for generating holes on the substrate surface is eliminated by sputtering the substrate with a rare gas prior to etching the substrate to remove foreign substances and altered layers on the substrate surface. Therefore, etching with a depth of 20 μm or more can be performed more reliably without generating holes.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing whether or not holes are generated depending on the etching depth of a quartz substrate.
FIG. 2 is a diagram for explaining a generation mechanism of abnormally shaped holes in deep etching of a quartz substrate.
FIG. 3 is a diagram showing the pressure dependence of generation of abnormally shaped holes in deep etching of a quartz substrate.
FIG. 4 is a diagram showing the pressure dependence of the generation state of abnormally shaped holes in etching a quartz substrate.
FIG. 5 is a graph showing a measurement result of a relationship between hole generation and etching depth.
FIGS. 6A and 6B are diagrams showing a state where holes are generated when a quartz substrate is continuously etched and intermittently etched. FIGS.
FIG. 7 is a graph showing the number of abnormally formed holes and bias voltage dependence in deep etching of a quartz substrate.
FIG. 8 is a graph showing the concentration dependence of CF _{4 on} the number of abnormally formed holes in deep etching of a quartz substrate.
FIG. 9 is a graph showing the dependency of etching shape on CF ₄ gas concentration.
FIG. 10 is a table showing a relationship between a step and an etching shape with respect to a quartz substrate.
11 is a diagram showing various measurement results of FIG.
FIG. 12 is a diagram showing a surface state of a substrate when the quartz substrate is etched according to one embodiment of the present invention.
FIG. 13 is a view showing a surface state of a substrate when a quartz substrate is etched according to another embodiment of the present invention.

Claims (8)

In a method of etching a substrate by a reaction between a glass substrate material and activated species such as generated ions and neutral radicals in a plasma state by discharge of fluorocarbon gas under reduced pressure,
A first bias voltage in the range of −500 V to −940 V is applied to the substrate at the beginning of etching, and then a bias voltage lower than the first bias voltage is applied for a time longer than the application time of the first bias voltage to the substrate. A plasma etching method characterized by applying.   The plasma etching method according to claim 1, wherein the first bias voltage is -850V to -900V.   The plasma etching method according to claim 1, wherein the bias voltage lower than the first bias voltage is -500V. The plasma etching method according to any one of claims 1 to 3, wherein the fluorocarbon gas is a mixture of two or more fluorocarbon gases.   The plasma etching method according to claim 4, wherein the concentration of the fluorocarbon gas 2 added to the fluorocarbon gas 1 is 69% or more.   The plasma etching method according to claim 1, wherein the flow rate of the fluorocarbon gas is controlled so as to suppress reverse microloading. In a method of etching a substrate by making a fluorocarbon gas into a plasma state by discharge under a low pressure and reacting generated species such as ions and neutral radicals with a glass substrate material,
Sputtering the substrate with a rare gas to remove foreign matter and altered layers on the substrate surface,
A first bias voltage in the range of −500 V to −940 V is applied to the substrate at the beginning of etching, and then a bias voltage lower than the first bias voltage is applied for a time longer than the application time of the first bias voltage to the substrate. A plasma etching method characterized by applying.   The plasma etching method according to claim 7, wherein the fluorocarbon gas is a mixture of two or more types of fluorocarbon gases.

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