JP3881420B2 - Low dielectric constant material, interlayer insulation film and IC substrate - Google Patents

Description

【０００９】
で表される。式中、W は分子の分子量、 N_A はアボガドロ数である。さらに、分極率は電子分極率と、分子振動に起因する振動分極率の和で与えられる。本発明の材料の様に共役電子を含まない場合、電子分極率は分子中の各化学結合に固有な電子分極率の総和にほぼ等しく、分子構造の影響は極めて小さい。一方、振動分極率は全ての基準振動モードからの寄与の総和として、下記の式（２）：
【００１０】
【数２】

【００１１】
で表される。ここで、μは双極子モーメント、a は基準振動モードを表し、 Q_a は基準振動座標、ω_a は基準振動数である。同一分子であっても、ω_a はそのコンフォメーションや形状により変化するため、振動分極率は分子構造の影響を受ける。特に、自由末端を有する鎖状分子では、最低基準振動モードが１０cm^-1以下の低振動数となり、式（２）から明らかなように振動分極率に対して支配的な寄与をする。さらに、その末端がOH基のように極性が強い場合には、式（２）の分子に相当する双極子モーメントの変化が大きくなるため、振動分極率は電子分極率の約３倍にも達する。したがって、比誘電率を低下させるためには、自由末端を持たず基準振動数の高い分子構造単位で材料を構成し、振動分極を抑制する必要がある。
【００１２】
本発明によれば、かかる分子構造は分子構造（１）および（２）である。図１および図２にそれぞれの具体例を模式的に示す。ただし、図２(ｄ) では各々の辺がSi-O-Si結合を表す。分子構造（１）および（２）は鎖状末端を持たないためω_a が高く、振動分極を低減する効果がある。分子構造（１）または（２）のいずれにも属さない分子構造は、図３に例示する自由末端を有する鎖状構造であるが、先述のとおり、このような分子構造は振動分極が大きいため、材料の低誘電率化の観点からは好ましくない。
【００１３】
上記分子構造の電子分極率および振動分極率を分子軌道法によって計算した。図１，図２(b) および図３(a) の分子に対する計算結果を表１に示す。ただし、Siの置換基は全てメチル基、図３(a) の分子の末端は-Si(CH₃)₂-OHとして計算した。さらに、表１に示した分極率はSi原子１個あたりの値である。表１から、分子構造（１）および（２）は振動分極率が小さく、自由末端を有する鎖状構造は振動分極率が大きいことが明らかである。
【００１４】
【表１】

【００１５】
さらに式（１）を用いて、分子構造（１）または（２）を構成する主鎖の割合と比誘電率の関係をプロットすると図４に示す関係が得られた。ただし、材料の密度を１．０７g/cm^３ と仮定し、分極率（電子分極＋振動分極）の値は表１に示す計算値の平均的な値を用いた。即ち、分子構造（１）および（２）の分極率として９．０、末端を有する鎖状構造の分極率として２０．０を用いた。図４より、主鎖の８０% 以上を分子構造（１）または（２）の構造にすると比誘電率は３．０未満となる。
【００１６】
次に、環状および多面体状の分子構造を含む材料を実際に合成する方法について述べる。本発明の材料は、例えば、R¹ R¹ Si(OR² )₂ (R¹ , R² はアルキル基)で表される原料と金属アルコキシドとから合成することができる。金属アルコキシドに対してモル比で３〜２０倍のR¹ R¹ Si(OR² )₂ を混合し、加水分解を行うと、R¹ R¹ Si(OR² )₂ と金属アルコキシドの反応に加えて、R¹ R¹ Si(OR² )₂ 同士の重合も起こり、結果として分子構造（１）または（２）を含む材料が合成される。
【００１７】
本発明の低誘電率材料の作製にアルコキシドを用いる場合、使用するアルコキシドは特に限定しないが、例えば、メトキシド、エトキシド、プロポキシド、ブトキシド等があげられる。また、アルコキシ基の一部をβ―ジケトン、β―ケトエステル、アルカノールアミン、アルキルアルカノールアミン、有機酸等で置換したアルコキシド誘導体も使用できる。
【００１８】
本発明における加水分解では、ゲル化の急激な進行を抑制するため、全アルコキシ基に対して２モル倍未満の水を添加して加水分解する。この際、無機酸、有機酸あるいはそれらの両方を触媒として使用してもよい。また、アルカリで溶液のpHを調整し、加水分解反応を制御してもよい。添加する水は、アルコール等の有機溶媒で希釈してもよい。
【００１９】
加水分解においては、シロキサンポリマー、アルキルアルコキシシランなどのSi原料およびアルコキシドを均一に分散、溶解できる有機溶媒が使用される。例えば、メタノール、エタノール、プロパノール、ブタノール等の各種アルコール、アセトン、トルエン、キシレン等である。
加水分解後、溶媒、加水分解で生成したアルコール等を常圧あるいは減圧下で留去する。
【００２０】
本発明の低誘電率材料をLSI 素子の層間などに用いられる絶縁膜とする場合、基板への塗布は、スプレーコート法、ディップコート法、スピンコート法等で行う。
低誘電率基板としてバルク体で用いる場合は、鋳型に流し込んで成形し、熱処理する。
【００２１】
塗布膜およびバルク体の熱処理は、７０〜５００℃で行う。７０℃未満であると、溶媒等が十分蒸発せず、固化できない。５００℃を越えると、有機成分の分解が始まる。
本発明による低誘電率材料は、このようにLSI 素子用層間絶縁膜、IC基板など各種電子部品に応用することができる。
【００２２】
【実施例】
本発明の絶縁膜を以下の実施例によって具体的に説明する。ただし、本発明はこれらの実施例のみに限定されるものではない。実施例および比較例の材料はジアルキルアルコキシシラン、シロキサンポリマーなどのSi原料と、架橋剤である金属アルコキシドから合成した。使用した金属アルコキシドは、Al(O-sec-C₄ H₉)₃ 、 Ti(OC₂ H₅ )₄ 、およびTa(OC₂ H₅ )₅ である。Al, Ti, Taのアルコキシドはアセト酢酸エチルで化学改質してから用いた。
【００２３】
【表２】

【００２４】
実施例１〜３はジメチルジエトキシシランおよび表２に示した金属のアルコキシドを原料として作製した。金属アルコキシドとジメチルジエトキシシランの比は、実施例１では１：１０、実施例２では１：１０、実施例３では１：２０とした。これらをエタノール溶媒中で撹拌し、水のエタノール溶液を添加して加水分解し、ゾルを調製した。得られたゾルをアルミシャーレに流し込み７０℃，１５０℃の２段階で熱処理し、バルク体を作製した。バルク体の動的粘弾性測定を室温、周波数１１０Hzの条件で行い、貯蔵弾性率を求めた。また、バルク体の両面に電極を付与し、周波数１MHz で誘電率を測定した。
【００２５】
比較例４〜７は HO-[Si(CH₃ )₂-O]₄₀-Hで表されるシロキサンポリマーおよび表２に示した金属のアルコキシドを原料として作製した。金属アルコキシドとシロキサンポリマーの比は、比較例４では４：１、比較例５では２：１、比較例６では４：１、比較例７では３：１とした。これらをエタノール溶媒中で撹拌し、水のエタノール溶液を添加して加水分解し、ゾルを調製した。得られたゾルをアルミシャーレに流し込み７０℃、１５０℃の２段階で熱処理し、バルク体を作製した。バルク体の動的粘弾性測定を室温、周波数１１０Hzの条件で行い、貯蔵弾性率を求めた。バルク体の両面に電極を付与し、周波数１MHz で誘電率を測定した。
【００２６】
実施例１〜３および比較例４〜７の試料についてラマンスペクトルを測定した結果、実施例１〜３では６０５cm^-1 にピークが観測されたが比較例４〜７ではそのようなピークは観測されなかった。６０５cm^-1 のピークはSi−O の三員環によるものである（F. L. Galeener, Solid State Commun. 44, 1037 (1982)）。Si−Oの四員環についても４９０cm^-1 付近にピークを示すことが知られているが、Si−O の鎖状分子によるピークもほぼ同一のラマン周波数であるため、四員環以上の環状構造の存在を、ラマンスペクトルから検出することは難しい。しかし、表２に示すように、いずれの金属を用いた場合でも、実施例の試料は比較例の同じ金属に対応する試料より貯蔵弾性率が高い結果が得られた。この貯蔵弾性率の差は、作製したバルク体の構造の違いを反映しており、これらの結果は実施例の試料において、分子構造（１）および（２）に属する硬い分子構造単位の含有率が比較例の試料より高いことを示す。さらに表２に示すとおり、いずれの金属を用いた場合でも、実施例の材料は比誘電率が３．０未満であり、比較例より低い値であった。
【００２７】
【発明の効果】
本発明によれば、比誘電率が３．０未満の低誘電率材料が得られる。LSI 用層間絶縁膜、IC基板など、半導体素子および電気回路部品へこの低誘電率材料を適用することにより、電気信号の遅延が小さくなるため、デバイスの高速化に対応することができる。
【図面の簡単な説明】
【図１】 図１は、分子構造（１）の具体例を示す模式図である。
【図２】 図２は、分子構造（２）の具体例を示す模式図である。
【図３】 図３は、未架橋自由末端を有する鎖状構造の具体例を太線部分により示す模式図である。
【図４】 図４は、分子構造（１）または（２）を構成する主鎖の割合と誘電率の関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a low dielectric constant material such as an insulating film used between layers of an LSI element or an IC substrate used as an electric circuit component.
[0002]
[Prior art]
As LSI devices increase in speed and integration, signal delay due to capacitance between wirings and between layers is becoming a problem. In general, since the wiring delay is proportional to the square root of the relative dielectric constant of the insulating material, reducing the relative dielectric constant of the interlayer insulating film is an effective means for reducing the wiring delay.
[0003]
Conventionally, a method of forming a sol prepared by hydrolyzing tetraethoxysilane by a spin on glass (SOG) method is known as an interlayer insulating film. However, the molecular structure of the material thus prepared is a three-dimensional network structure of ≡Si—O—Si≡ and has no voids, and has a high dielectric constant of 4.0. One method for reducing the dielectric constant is to reduce the density of the material. As a method of reducing the density, there are a method of making it porous and a method of making the molecular structure sparse.
[0004]
When it is made porous, the dielectric constant is lowered from 4.7 to 2.3 according to the amount of pores [Aoi, 43rd JSAP Proceedings, 26p-N-5 (1996)]. However, since the porous film has a problem in hygroscopicity, it is difficult to use it for a normal semiconductor element or electric circuit component. On the other hand, a siloxane polymer having a skeleton represented by HO— [Si (CH ₃ ) ₂ —O] _n —H (n is an average of 40) was crosslinked using various metal alkoxides as a material capable of sparse molecular structure. There is something [Yamada et al., Proceedings of Autumn Symposium of the Ceramic Society of Japan, p1 (1996)]. With this material, crosslinking at the siloxane polymer portion cannot occur, so it is considered that a reduction in density is realized. In fact, the relative permittivity of the material thus produced has decreased to 3.2-3.7. However, when the design rule is 0.2 mm or less, it is considered that an insulating film having a relative dielectric constant of 3 or less is required, so that it is necessary to further reduce the dielectric constant.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to reduce the dielectric constant of an insulating material that can be applied to semiconductor elements, electrical circuit components, and the like.
[0006]
[Means for Solving the Problems]
The solution to the above problem is achieved by the following [1] to [5].
[1] -Si-O- (Si has a bond with an alkyl group other than a bond between O) and -MO- (M is B, Al, T i, Ge , Y, Zr, Nb, among Ta have a backbone and at least one element) units selected from the molecular structure of the main portion, the molecular structure (1): cyclic structure or the molecular structure (2): the ring-shaped structural unit low dielectric constant material, characterized in that but two-dimensional flat surface, the sterically linked structure (however, excluding the structure that four-membered ring structure and a ladder-like structure of Si-O is linked to a plane), a .
[2] The low dielectric constant material according to 1 above, wherein the molecular structure of 80% or more of the main chain is the molecular structure (1) or (2).
[3 ] An interlayer insulating film made of low dielectric constant material before Symbol 1 or 2 wherein.
[ 4 ] An IC substrate made of the low dielectric constant material as described in 1 or 2 above.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The relative dielectric constant is a physical property value representing macroscopic polarization of a material, and its origin is polarization due to orientation of molecules constituting the material and polarization induced by individual molecules. In a material having a three-dimensional network structure, orientational polarization is negligible. Therefore, the dielectric constant ε depends on the density ρ of the material and the polarizability α of the molecules constituting the material.
[0008]
[Expression 1]

[0009]
It is represented by Wherein, W is the molecular weight of the molecule, N _A is Avogadro's number. Furthermore, the polarizability is given by the sum of the electronic polarizability and the vibration polarizability due to molecular vibration. When conjugated electrons are not included as in the material of the present invention, the electronic polarizability is almost equal to the sum of the electronic polarizabilities inherent to each chemical bond in the molecule, and the influence of the molecular structure is extremely small. On the other hand, the vibration polarizability is the sum of contributions from all the reference vibration modes, and the following equation (2):
[0010]
[Expression 2]

[0011]
It is represented by Here, μ represents a dipole moment, a represents a reference vibration mode, Q _a represents _a reference vibration coordinate, and ω _a represents a reference frequency. Even in the same molecule, ω _a varies depending on its conformation and shape, and hence the vibration polarizability is affected by the molecular structure. In particular, in a chain molecule having a free end, the lowest reference vibration mode has a low frequency of 10 cm ⁻¹ or less, and as shown in the equation (2), it makes a dominant contribution to the vibration polarizability. Furthermore, when the terminal is strongly polar, such as an OH group, the change in dipole moment corresponding to the molecule of formula (2) becomes large, so the vibrational polarizability reaches about three times the electronic polarizability. . Therefore, in order to reduce the relative dielectric constant, it is necessary to configure the material with a molecular structural unit having no free end and having a high reference frequency to suppress vibration polarization.
[0012]
According to the invention, such molecular structures are molecular structures (1) and (2). FIG. 1 and FIG. 2 schematically show respective specific examples. However, FIG. 2 (d) in each side represents an Si-O-Si bond. Since the molecular structures (1) and (2) do not have chain ends, ω _a is high, and there is an effect of reducing vibration polarization. The molecular structure that does not belong to either the molecular structure (1) or (2) is a chain structure having a free end illustrated in FIG. 3, but as described above, such a molecular structure has large vibrational polarization. From the viewpoint of reducing the dielectric constant of the material, it is not preferable.
[0013]
The electronic polarizability and vibration polarizability of the molecular structure were calculated by the molecular orbital method. Figure 1, Table 1 shows the calculation results for the molecules shown in FIG. 2 (b) and Figure 3 (a). However, all substituent groups are methyl groups Si, end of the molecule of Fig. 3 (a) was calculated as -Si (CH _{3) 2} -OH. Furthermore, the polarizabilities shown in Table 1 are values per Si atom. From Table 1, it is clear that the molecular structures (1) and (2) have small vibration polarizabilities, and the chain structure having free ends has large vibration polarizabilities.
[0014]
[Table 1]

[0015]
Further, when the relationship between the ratio of the main chain constituting the molecular structure (1) or (2 ) and the relative dielectric constant is plotted using the formula (1), the relationship shown in FIG. 4 is obtained. However, assuming that the density of the material is 1.07 g / cm ³ , the average value of the calculated values shown in Table 1 is used as the value of the polarizability (electronic polarization + vibration polarization). That is, 9.0 was used as the polarizability of the molecular structures (1) and (2 ), and 20.0 was used as the polarizability of the chain structure having terminals. From FIG. 4 , the relative dielectric constant is less than 3.0 when 80% or more of the main chain has a molecular structure (1) or (2 ) .
[0016]
Next, will be described a method for actually synthesized material containing a ring-shaped contact and polyhedral molecular structure. The material of the present invention can be synthesized, for example, from a raw material represented by R ¹ R ¹ Si (OR ² ) ₂ (R ¹ and R ² are alkyl groups) and a metal alkoxide. When R ¹ R ¹ Si (OR ² ) ₂ having a molar ratio of 3 to 20 times the metal alkoxide is mixed and hydrolyzed, it is added to the reaction between R ¹ R ¹ Si (OR ² ) ₂ and the metal alkoxide. Thus, polymerization of R ¹ R ¹ Si (OR ² ) ₂ occurs, and as a result, a material containing the molecular structure (1) or (2) is synthesized.
[0017]
When an alkoxide is used for producing the low dielectric constant material of the present invention, the alkoxide used is not particularly limited, and examples thereof include methoxide, ethoxide, propoxide, butoxide and the like. Further, an alkoxide derivative in which a part of the alkoxy group is substituted with β-diketone, β-ketoester, alkanolamine, alkylalkanolamine, organic acid or the like can also be used.
[0018]
In the hydrolysis in the present invention, in order to suppress rapid progress of gelation, the hydrolysis is performed by adding less than 2 moles of water to all alkoxy groups. At this time, an inorganic acid, an organic acid, or both of them may be used as a catalyst. Further, the hydrolysis reaction may be controlled by adjusting the pH of the solution with an alkali. The water to be added may be diluted with an organic solvent such as alcohol.
[0019]
In the hydrolysis, an Si solvent such as a siloxane polymer and an alkylalkoxysilane and an organic solvent capable of uniformly dispersing and dissolving the alkoxide are used. For example, various alcohols such as methanol, ethanol, propanol and butanol, acetone, toluene, xylene and the like.
After hydrolysis, the solvent, alcohol produced by hydrolysis, and the like are distilled off under normal pressure or reduced pressure.
[0020]
When the low dielectric constant material of the present invention is used as an insulating film used between the layers of an LSI element, the substrate is applied by spray coating, dip coating, spin coating, or the like.
When a bulk substrate is used as the low dielectric constant substrate, it is cast into a mold and heat treated.
[0021]
The heat treatment of the coating film and the bulk body is performed at 70 to 500 ° C. If it is lower than 70 ° C., the solvent and the like are not sufficiently evaporated and cannot be solidified. When the temperature exceeds 500 ° C., decomposition of organic components starts.
Thus, the low dielectric constant material according to the present invention can be applied to various electronic components such as an interlayer insulating film for LSI elements and an IC substrate.
[0022]
【Example】
The insulating film of the present invention will be specifically described by the following examples. However, the present invention is not limited to only these examples. The materials of Examples and Comparative Examples were synthesized from Si raw materials such as dialkylalkoxysilanes and siloxane polymers and metal alkoxides as a crosslinking agent. Metal alkoxide used is, Al (O-sec-C 4 H 9) 3, T i (OC 2 H 5) 4, and Ta (OC ₂ H _{5) 5.} Al, Ti and Ta alkoxides were used after chemical modification with ethyl acetoacetate.
[0023]
[Table 2]

[0024]
Examples 1 to 3 were prepared using dimethyldiethoxysilane and the metal alkoxide shown in Table 2 as raw materials. The ratio of metal alkoxide and dimethyl diethoxy silane, in Example 1 to 1:10, in real施例2 1:10 and 1:20 in the third embodiment. These were stirred in an ethanol solvent and hydrolyzed by adding an ethanol solution of water to prepare a sol. The obtained sol was poured into an aluminum petri dish and heat-treated at two stages of 70 ° C. and 150 ° C. to prepare a bulk body. The dynamic viscoelasticity measurement of the bulk body was performed under conditions of room temperature and a frequency of 110 Hz, and the storage elastic modulus was obtained. Electrodes were applied to both sides of the bulk body, and the dielectric constant was measured at a frequency of 1 MHz.
[0025]
In Comparative Examples 4 to 7, siloxane polymers represented by HO— [Si (CH ₃ ) ₂ —O] ₄₀ —H and alkoxides of metals shown in Table 2 were used as raw materials. The ratio of metal alkoxide and siloxane polymer, in Comparative Example 4 4: 1, in Comparative Example 5 2: 1, in Comparative Example 6 4: 1, in Comparative Example 7 3: 1. These were stirred in an ethanol solvent and hydrolyzed by adding an ethanol solution of water to prepare a sol. The obtained sol was poured into an aluminum petri dish and heat-treated at two stages of 70 ° C. and 150 ° C. to prepare a bulk body. The dynamic viscoelasticity measurement of the bulk body was performed under conditions of room temperature and a frequency of 110 Hz, and the storage elastic modulus was obtained. Electrodes were applied to both sides of the bulk body, and the dielectric constant was measured at a frequency of 1 MHz.
[0026]
As a result of measuring the Raman spectra of the samples of Examples 1 to 3 and Comparative Examples 4 to 7 , a peak was observed at 605 cm ⁻¹ in Examples 1 to 3 , but such a peak was observed in Comparative Examples 4 to 7. There wasn't. The peak at 605 cm ⁻¹ is due to a three-membered ring of Si—O (FL Galeener, Solid State Commun. 44, 1037 (1982)). The Si-O four-membered ring is also known to show a peak in the vicinity of 490 cm ^-1 , but the peak due to the chain molecule of Si-O has almost the same Raman frequency. The presence of the structure is difficult to detect from the Raman spectrum. However, as shown in Table 2, when any metal was used, the sample of the example had a higher storage elastic modulus than the sample corresponding to the same metal of the comparative example. This difference in storage elastic modulus reflects the difference in the structure of the produced bulk body, and these results indicate that the content of the hard molecular structural units belonging to the molecular structures (1) and (2) in the samples of the examples. Is higher than the sample of the comparative example. Further, as shown in Table 2, the material of the example had a relative dielectric constant of less than 3.0, which was lower than that of the comparative example, regardless of which metal was used.
[0027]
【The invention's effect】
According to the present invention, a low dielectric constant material having a relative dielectric constant of less than 3.0 can be obtained. By applying this low-dielectric constant material to semiconductor elements and electrical circuit components such as LSI interlayer insulation films and IC substrates, the delay of electrical signals is reduced, so that higher device speeds can be accommodated.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a specific example of a molecular structure (1).
FIG. 2 is a schematic diagram showing a specific example of the molecular structure (2).
FIG. 3 is a schematic view showing a specific example of a chain structure having uncrosslinked free ends by a bold line portion.
FIG. 4 is a graph showing the relationship between the ratio of the main chain constituting the molecular structure (1) or (2 ) and the dielectric constant.

Claims (4)

-Si-O- (Si has a bond with an alkyl group other than a bond between O) and -MO- (M is selected B, Al, T i, Ge , Y, Zr, Nb, from among Ta had a backbone and at least one element) units, the molecular structure of the main portion, the molecular structure (1): cyclic structure or the molecular structure (2): the ring-shaped structural unit is a two-dimensional flat surface, the sterically linked structure (however, four-membered ring structure and a ladder-like structure of Si-O excluding the connecting structure in plan view) a low dielectric constant material, characterized in that a.   2. The low dielectric constant material according to claim 1, wherein the molecular structure of 80% or more of the main chain is the molecular structure (1) or (2). Claim 1 or 2 interlayer insulating film made of low dielectric constant material according. Claim 1 or 2 IC substrate made of a low dielectric constant material according.

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