Silicon etching liquid, silicon etching method, and microelectromechanical element

Abstract

The present invention is able to provide: a silicon etching liquid which anisotropically dissolves single crystal silicon, and which is characterized by containing (1) potassium hydroxide or sodium hydroxide, (2) a hydroxyl amine and (3) a cyclic compound represented by general formula (I), which has a thiourea group and wherein N and N are linked; and a silicon etching method which uses this silicon etching liquid. ##STR00001## (In general formula (I), Q represents an organic group having a saturated or unsaturated carbon-carbon bond.) By using the above-described silicon etching liquid, high etching rate can be achieved without lowering the etching rate of silicon and stability of the etching liquid is not impaired even in cases where copper is present in the etching liquid and/or where copper ions are dissolved in the etching liquid.

Claims

1. A silicon etching liquid, which anisotropically dissolves single crystal silicon, consisting essentially of (1) potassium hydroxide or sodium hydroxide, (2) a hydroxylamine and (3) a cyclic compound represented by formula (I): ##STR00007## wherein in formula (I), Q represents an organic group having a saturated or unsaturated carbon-carbon bond, and is at least one selected from the group consisting of propylenethiourea, 2-thiouracil, 6-methyl-2-thiouracil, 5-butyl-6-ethyl-2-thiouracil, 5-butyl-6-methyl-2-thiouracil, 5-butyl-6-propyl-2-thiouracil, 5,6-dimethyl-2-thiouracil, 6-methyl-5-propyl-2-thiouracil, 5-propylthiouracil, 5-methyl-6-butyl-2-thiouracil, 5-hexyl-2-thiouracil, 5-ethyl-2-thiouracil, 1-methyl-2-thiouracil, 3-methyl-2-thiouracil, 1,3-dimethyl-2-thiouracil, 1,5-dimethyl-2-thiouracil, thiobarbital, 5-(2-methylpropyl)-5-ethyl-2-thiobarbituric acid, 1,3-diethyl-2-thiobarbituric acid, 1,3,5-trimethyl-2-thiobarbituric acid and 1,3-dimethyl-2-thiobarbituric acid.

2. The silicon etching liquid according to claim 1, wherein said (3) the cyclic compound represented by formula (I), is at least one selected from the group consisting of propylenethiourea, and 2-thiouracil.

3. The silicon etching liquid according to claim 1, wherein the concentration of said (1) potassium hydroxide or sodium hydroxide is 7 to 45% by mass, the concentration of said (2) the hydroxylamine is 5 to 15% by mass and the concentration of said (3) the cyclic compound represented by general formula (I), is 0.00001 to 1.5% by mass.

Description

EMBODIMENTS FOR CARRYING OUT THE INVENTION

(1) Hereinafter, the present invention will be described in detail.

(2) The concentration of the hydroxylamine to be used in the present invention can be suitably determined depending on the desired etching rate of silicon, and is preferably 5 to 15% by mass relative to the total amount of the silicon etching liquid. The concentration is more preferably 7 to 13% by mass, and particularly preferably 9 to 11% by mass. When the concentration is 5 to 15% by mass, silicon can be etched effectively.

(3) As the alkali metal hydroxide to be used in the present invention, potassium hydroxide, sodium hydroxide and calcium hydroxide are preferred, and potassium hydroxide is particularly preferred. The concentration of the alkali metal hydroxide to be used in the present invention may be a conventional alkali compound concentration by which desired etching characteristics are obtained, but can also be suitably determined depending on the solubility of the alkali metal hydroxide in water and the concentration of the hydroxylamine and the concentration of thioureas in the etching liquid. The concentration is preferably 7 to 45% by mass, more preferably 10 to 40% by mass, and particularly preferably 15 to 35% by mass relative to the total amount of the silicon etching liquid. When the concentration is 7 to 45% by mass, silicon can be etched effectively.

(4) In the present invention, a cyclic compound represented by general formula (I), which has a thiourea group and wherein N and N are linked, is used:

(5) ##STR00004##
wherein in general formula (I), Q represents an organic group having a saturated or unsaturated carbon-carbon bond.

(6) As specific examples of the cyclic compound represented by general formula (I), which has a thiourea group and wherein N and N are linked, preferred are ethylenethiourea, propylenethiourea, 2-thiouracil, 2-thiobarbituric acid, 6-methyl-2-thiouracil, 5-butyl-6-ethyl-2-thiouracil, 5-butyl-6-methyl-2-thiouracil, 5-butyl-6-propyl-2-thiouracil, 5,6-dimethyl-2-thiouracil, 6-methyl-5-propyl-2-thiouracil, 5-propylthiouracil, 5-methyl-6-butyl-2-thiouracil, 5-hexyl-2-thiouracil, 5-ethyl-2-thiouracil, 1-methyl-2-thiouracil, 3-methyl-2-thiouracil, 1,3-dimethyl-2-thiouracil, 1,5-dimethyl-2-thiouracil, thiobarbital, 5-(2-methylpropyl)-5-ethyl-2-thiobarbituric acid, 1,3-diethyl-2-thiobarbituric acid, 1,3,5-trimethyl-2-thiobarbituric acid and 1,3-dimethyl-2-thiobarbituric acid, and more preferred are ethylenethiourea, propylenethiourea, 2-thiouracil and 2-thiobarbituric acid, which are respectively represented by the following structural formulae:

(7) ##STR00005##

(8) The concentration of the cyclic compound represented by general formula (I), which has a thiourea group and wherein N and N are linked, to be used in the present invention is preferably 0.00001 to 1.5% by mass, more preferably 0.00005 to 0.5% by mass, and particularly preferably 0.0001 to 0.1% by mass relative to the total amount of the silicon etching liquid. When the concentration is 0.00001 to 1.5% by mass, silicon can be etched effectively.

(9) As the silicon etching method of the present invention, usually employed is a method in which: an etching target is immersed in a heated etching liquid and taken out therefrom after a predetermined amount of time; the etching liquid adhering to the target is washed away with water or the like; and then water adhering to the target is dried. The etching temperature is preferably a temperature of 40 C. or higher and lower than the boiling point, more preferably 50 C. to 90 C., and particularly preferably 70 C. to 90 C. When the etching temperature is within this range, silicon can be etched effectively. By increasing the temperature of the etching liquid, the etching rate is increased, but appropriate treatment temperature may be suitably determined in view of suppression of composition change of the etching liquid to a low level and the like.

(10) The etching time can be suitably selected depending on etching conditions and a target for etching. The etching time is usually 1 to 300 minutes, preferably 10 to 180 minutes, more preferably 10 to 120 minutes, and particularly preferably 20 to 90 minutes, but appropriate treatment time may be suitably determined.

(11) The target of the etching treatment in the present invention is a substrate containing single crystal silicon, wherein single crystal silicon exists in the whole area or a partial area of the substrate.

(12) Note that according to the present invention, reduction in the silicon etching rate can be suppressed in both the case where copper as a wiring material is exposed on the surface of the substrate from the start and the case where copper in the inner portion of the substrate is exposed on the surface by silicon etching. Single crystal silicon may be in a state of a single layer or multi-layer lamination. Substrates obtained by ion-doping the whole area or a partial area of these substrates are also the target of the etching treatment. In addition, when a material such as a silicon oxide film, a silicon nitride film and a organosilicon film or a metal film such as an aluminium film, a chromium film and a gold film exists on the surface or in the inner portion of the above-described target to be etched, such products are also included in the target of the etching treatment of the present invention.

EXAMPLES

(13) Hereinafter, the present invention will be more specifically described by way of examples and comparative examples. The etching target used for evaluation is a single crystal silicon (100) (sometimes referred to as just silicon (100)) wafer. One surface of this silicon (100) wafer is in the state where the whole surface thereof is covered with a protective film made of a thermal silicon oxide film, and the other surface of the wafer has a pattern shape, wherein the silicon surface (0.25 cm0.25 cm) is regularly exposed by removal of a part of the thermal silicon oxide film by dry etching. Immediately prior to the etching treatment, this silicon (100) wafer was immersed in an aqueous solution of 1% by mass of hydrofluoric acid at 23 C. for 15 minutes, and then rinsed with ultrapure water and dried. By this treatment with the aqueous solution of hydrofluoric acid, a natural silicon oxide film produced on the surface of the silicon surface-exposed portion of the pattern shape was removed, and then the next etching treatment was carried out.

(14) Method for Etching Single Crystal Silicon (100) Wafer and Method for Calculating Etching Rate (SiE.R.)

(15) 40 g of an etching liquid (described later) was put into a container made of polytetrafluoroethylene, and the container was immersed in a hot water bath to heat the etching liquid to 80 C. After the temperature of the etching liquid reached 80 C., a single crystal silicon (100) wafer (1 cm1 cm) and a thin piece of copper (0.5 cm0.5 cm, thickness of copper: 6000 ) were simultaneously immersed in the etching liquid and subjected to the immersion treatment for 30 minutes. After that, the single crystal silicon (100) wafer was taken out from the etching liquid, and rinsed with ultrapure water and dried. The single crystal silicon (100) wafer after subjected to the etching treatment was in a state where the pattern portion was depressed compared to the surrounding area due to etching of single crystal silicon, and by measuring the difference in height between the etched portion and the unetched portion, the etching depth of the single crystal silicon (100) surface for 30 minutes was obtained. The value obtained by dividing the etching depth by 30 was calculated as the etching rate of the single crystal silicon (100) surface (unit: m/min).

Examples 1-13 and Comparative Examples 1-4

(16) The etching treatment was carried out using etching liquids described in Table 1. The results are shown in Table 1. The structural formulae of cyclic compounds represented by general formula (I), which have a thiourea group and wherein N and N are linked used in Examples 1-13 are shown below. In Comparative Examples 1-4, in which none of the cyclic compounds represented by general formula (I), which have a thiourea group and wherein N and N are linked is contained, the etching rate was obviously lower compared to corresponding Examples 1-13.

(17) ##STR00006##

(18) TABLE-US-00001 TABLE 1 Table 1 Cu piece and Si E.R. Concentration of the Concentration of Concentration of compound having a Alkaline alkaline hydroxide hydroxylamine Compound having a thiourea group Si E.R. hydroxide (% by mass) (% by mass) thiourea group (% by mass) (m/min) Example 1 KOH 24 10 2-thiouracil 0.1 4.1 Comparative Example 1 KOH 24 10 not contained 1.5 Example 2 KOH 15 10 2-thiouracil 0.1 3.7 Comparative Example 2 KOH 15 10 not contained 3.0 Example 3 KOH 35 10 2-thiouracil 0.1 4.2 Comparative Example 3 KOH 35 10 not contained 2.6 Example 4 KOH 24 5 2-thiouracil 0.1 2.7 Comparative Example 4 KOH 24 5 not contained 2.0 Example 5 KOH 24 10 2-thiouracil 0.0001 4.0 Example 6 KOH 24 10 2-thiouracil 0.001 4.2 Example 7 KOH 24 10 2-thiouracil 0.01 4.1 Example 8 KOH 24 10 2-thiouracil 0.2 4.0 Example 9 KOH 24 10 2-thiouracil 0.5 4.0 Example 10 KOH 24 10 2-thiouracil 1 3.8 Example 11 KOH 24 10 ethylenethiourea 0.1 4.0 Example 12 KOH 24 10 propylenethiourea 0.1 4.0 Example 13 KOH 24 10 2-thiobarbituric acid 0.1 4.1 Immersion temperature: 80 C., immersion time: 30 minutes KOH: potassium hydroxide Si E.R.: silicon etching rate

Examples 14-20 and Comparative Example 5

(19) The process was carried out in a manner similar to that in Examples 1-13 except that 0.0001% by mass of copper sulfate was contained in etching liquids described in Table 2 (no thin piece of copper was included). The results are shown in Table 2.

(20) In Examples 14-20, cyclic compounds represented by general formula (I), which have a thiourea group and wherein N and N are linked, had the performance of suppressing reduction in the etching rate not only in the case where a thin piece of copper coexisted in the etching liquid, but also in the case where copper had been dissolved in the etching liquid.

(21) However, in Comparative Example 5 (none of the cyclic compounds represented by general formula (I), which have a thiourea group and wherein N and N are linked was added), the etching rate was obviously lower compared to Examples 14-20, in which one of the cyclic compounds represented by general formula (I), which have a thiourea group and wherein N and N are linked was added.

(22) TABLE-US-00002 TABLE 2 Table 2 Addition of Cu and Si E.R. Concentration Concentration of the Concentration of of alkaline Concentration of compound having a copper sulfate Alkaline hydroxide hydroxylamine Compound having a thiourea group added Si E.R. hydroxide (% by mass) (% by mass) thiourea group (% by mass) (% by mass) (m/min) Example 14 KOH 24 10 2-thiouracil 0.1 0.0001 4.0 Example 15 KOH 24 10 2-thiouracil 0.01 0.0001 4.1 Example 16 KOH 24 10 2-thiouracil 0.001 0.0001 4.1 Example 17 KOH 24 10 2-thiouracil 0.1 0.0001 4.2 Example 18 KOH 24 10 ethylenethiourea 0.1 0.0001 4.3 Example 19 KOH 24 10 propylenethiourea 0.1 0.0001 4.1 Example 20 KOH 24 10 2-thiobarbituric acid 0.1 0.0001 4.5 Comparative Example 5 KOH 24 10 not contained 0.0001 1.2 Immersion temperature: 80 C., immersion time: 30 minutes KOH: potassium hydroxide Si E.R.: silicon etching rate

Examples 21-26 and Comparative Example 6

(23) Method for Etching Thin Piece of Copper and Method for Calculating Etching Rate

(24) 40 g of an etching liquid described in Table 3 was put into a container made of polytetrafluoroethylene, and the container was immersed in a hot water bath to heat the etching liquid to 80 C. After the temperature of the etching liquid reached 80 C., a solid copper film (2 cm2 cm, thickness of copper: 6000 ), the thickness of which had been measured in advance by a X-ray fluorescence spectrometer, was simultaneously immersed in the etching liquid and subjected to the immersion treatment for 60 minutes. After that, the thin piece of copper was taken out from the etching liquid, and rinsed with ultrapure water and dried. The thickness of the thin piece of copper was measured by the X-ray fluorescence spectrometer again to obtain the difference between the film thicknesses before and after the treatment, thereby obtaining the etching depth of the thin piece of copper for 60 minutes. The value obtained by dividing the etching depth by 60 was calculated as the etching rate of copper (unit: A/min). In the case where the cyclic compound represented by general formula (I), which has a thiourea group and wherein N and N are linked was contained in the etching liquid, the etching rate of copper was lower than 1 /min, but in the case where the cyclic compound represented by general formula (I), which has a thiourea group and wherein N and N are linked was not contained, the etching rate of copper was 10 /min or higher. Based on this, it was found that the cyclic compound represented by general formula (I), which has a thiourea group and wherein N and N are linked, exerts not only the effect of suppressing reduction in the silicon etching rate even when copper as the wiring material coexists, but also the effect of preventing dissolution of copper as the wiring material. The values of the silicon etching rate were equivalent to those of Tables 1 and 2 (omitted in the table).

(25) TABLE-US-00003 TABLE 3 Table 3 Cu E.R. Concentration of the Concentration of Concentration of compound having a Alkaline alkaline hydroxide hydroxylamine Compound having a thiourea group Cu E.R. hydroxide (% by mass) (% by mass) thiourea group (% by mass) (/min) Example 21 KOH 24 10 2-thiouracil 0.001 <1 Example 22 KOH 24 10 2-thiouracil 0.01 <1 Example 23 KOH 24 10 2-thiouracil 0.1 <1 Example 24 KOH 24 10 ethylenethiourea 0.1 <1 Example 25 KOH 24 10 propylenethiourea 0.1 <1 Example 26 KOH 24 10 2-thiobarbituric acid 0.1 <1 Comparative Example 6 KOH 24 10 not contained 16.2 Immersion temperature: 80 C., immersion time: 60 minutes KOH: potassium hydroxide Cu E.R.: etching rate of solid copper film

Examples 27-31 and Comparative Examples 7-8

(26) Method for Measuring the Concentration of the Compound Having a Thiourea Group in Etching Liquid Before and after Storage with Heating

(27) A portion of an etching liquid described in Table 4 was divided and diluted with ultrapure water (concentration: about 0.00001% by mass). After that, it was subjected to the high-performance liquid chromatography measurement to measure the concentration of the compound having a thiourea group in the etching liquid, and it was regarded as the additive concentration before storage with heating. After that, 100 g of the etching liquid was put into a container made of polytetrafluoroethylene, and the container was immersed in a hot water bath at 50 C. and held for 72 hours. 72 hours later, the etching liquid was taken out, and the concentration of the compound having a thiourea group in the etching liquid after storage with heating was measured by means of high-performance liquid chromatography to obtain the concentration of the compound having a thiourea group decreased during storage with heating.

(28) In the case of the cyclic compounds represented by general formula (I), which have a thiourea group and wherein N and N are linked used in Examples 27-31, the concentration was very slightly changed during storage with heating, but in the case of the compounds, which have a thiourea group and wherein N and N are not linked used in Comparative Examples 7-8, the concentration was significantly reduced. Based on this, it is understood that in the Examples, it is possible to provide an etching liquid, wherein stability of the etching liquid is not impaired. It is considered that this is because the compound having a thiourea group to be used in the present invention has a cyclic structure, leading to difficulty in decomposition.

(29) TABLE-US-00004 TABLE 4 Table 4 Concentration of the compound having a thiourea group before and after storage with heating Concentration of Concentration of Concentration of the compound the compound the compound having a Concentration having a having a thiourea group of alkaline Concentration of thiourea group thiourea group decreased during Alkaline hydroxide hydroxylamine Compound having a before heating after heating storage with heating hydroxide (% by mass) (% by mass) thiourea group (% by mass) (% by mass) (% by mass) Example 27 KOH 24 10 2-thiouracil 0.1045 0.1038 0.0007 Example 28 KOH 24 10 2-thiouracil 0.0118 0.0117 0.0002 Example 29 KOH 24 10 ethylenethiourea 0.1010 0.1006 0.0004 Example 30 KOH 24 10 propylenethiourea 0.1024 0.1017 0.0007 Example 31 KOH 24 10 2-thiobarbituric acid 0.1004 0.0982 0.0022 Comparative KOH 24 10 thiourea 0.1061 0.0007 0.1054 Example 7 Comparative KOH 24 10 N-methylthiourea 0.1030 0.0006 0.1024 Example 8 Immersion temperature: 50 C., immersion time: 72 hours KOH: potassium hydroxide

INDUSTRIAL APPLICABILITY

(30) According to the present invention, in silicon etching in which copper is included as a wiring material, it is possible to provide an etching liquid, wherein the etching rate of silicon is not reduced, copper as the wiring material is not etched and stability of the etching liquid is not impaired, and this is industrially useful.