NL8302845A - POLYMER FILMS FOR ELECTRONIC CIRCUITS. - Google Patents

Description

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Polymer films for electronic circuits.

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The invention relates to electronic circuits with polymer films and dielectric layers used as insulator.

The increasing advancements in VLSI technology have led to increased packing density and smaller design rules. This has created a need for new and improved dielectric materials for use in various device structures. For example, the strong proximity of several conductive elements in the VLSI circuit has created a need for low dielectric constant dielectric material to be parasitic. decrease capacities. Also required is high dielectric strength due to the close approximation of various conductive elements in the VLSI circuits. The use of two or more metallization levels creates the need for dielectric material with other unique properties. For example, it is desirable in some instances that the dielectric be applied at a relatively low temperature so as not to adversely affect the multi-level circuit. In other instances, the processing of these multilevel circuits often involves the use of relatively high temperatures, the dielectric material being stable. Also, the realized feature dimensions and applied higher voltages found in new VLSI devices may require new materials for vitrification and encapsulation.

Desired properties for dielectric layers used for VLSI circuits are low dielectric constants to minimize parasitic capacitances, high thermal stability to allow further processing of the circuit at high temperatures, relatively low application temperature to damage the circuit minimize application of the dielectric, high dielectric strength, good adhesion, good film integrity (ie freedom from cracking) and chemical stability, especially for water and water vapor. Such a dielectric material would be extremely valuable, and especially for application to high density circuits, because reduced parasitic capacitances would allow higher sensitivities and thermal stability would allow greater flexibility in handling such circuits.

A number of studies have been conducted on the use of polymer films in electronic circuits both as an insulator, as an encapsulation film and as a passivation film. The most noteworthy of these are: A. Szeto and D.W. Hess: "Correlation of Chemical and Electrical Properties of Plasma - Deposited Tetramethylsilane Films", Journal of Applied Physics, 52 (2), 903 (1981), which describes a study of the properties of tetramethylsilane films of interest in manufacture of electrical circuits. Similar studies have been performed for polymer films made by plasma-induced polymerization of hexamethyldisiloxane. These studies are described in the following articles: M. Maisonneuve et al., 15 Thin Solid Films, 44, pp. 209-216 (1977); M. Aktik et al, Journal of Applied Physics, 51 (9), pp. 5055-5057 (1980); M. Maisonneuve et al, Thin Solid Films, 33, pp. 35-41 (1976); and you. Klemberg-Sapieha, Applied Physics Letters, 37 (1), pp. 104-105 (1980).

According to the present invention there is provided an electrical semiconductor device comprising semiconductor material and conductive elements, characterized in that the device further comprises polysiloxane film made by plasma-induced polymerization of at least one alkyl alkoxy silane in which the alkyl and alkoxy groups contain up to 3 carbon atoms.

A preferred embodiment of the invention provides an electrical circuit in which at least part of a surface of the circuit is covered with a plasma-deposited polysiloxane film in which the monomer is an alkylalkoxysilane. The alkyl and alkoxy groups should not contain more than 3 carbon atoms. Typical examples are trimethyl methoxysilane, dimethyl dimethoxysilane, triethyl ethoxysilane, etc. Preferred is the monomer trimethyl methoxysilane because of the low dielectric constant, high breakdown voltage, low film stresses, and reasonable degree of thermal stability and high hydrophobicity. The polymer film is useful for a wide variety of electrical circuits, high-frequency, low-frequency, direct current, etc. A circuit typically includes an 8302345_

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substrate, conductor elements, input connections, output connections, I.

etc. These films are very useful when at least some of the jelly

the distances are very small (i.e. on the order of 2 µm or less} and I.

the switching frequencies (or corresponding access times) are very high I.

5 (i.e., greater than 5 MHz). Eat is also very useful in the case of aluminum

minium metalation is used or other metalization where I

thermal cycling treatment is required. I

For a better understanding of the invention, reference is made to I.

to the accompanying drawing, in which I

Fig. 1 shows a graph of the same data on the I.

dielectric constant for silicone films made I

from a variety of alkyl alkoxy silanes; I

Fig. 2 shows a graph of the same data on the I.

breakdown voltage for silicone films made from an I.

Variety of alkyl alkoxy silanes; I

Fig. 3 shows a graph of the same data on the I.

thermal influence on the film thickness for silicone films, which I

are endured from a variety of alkyl alkoxy silanes; I

Fig. 4 shows a cross-sectional view of a part I.

20 of a typical integrated circuit having certain characteristics of an I.

such circuitry includes one from a polysiloxane

material-made covering layer; I

Fig. 5 shows a cross-sectional view of part I.

of a more complex integrated circuit with a covering I

Material manufactured according to an embodiment of the invention

invention; and

Fig. 6 shows a cross-sectional view of part of an integrated circuit in which polysiloxane is used as a dielectric between levels around two levels of aluminum metallization.

30 to separate.

It has been found that certain silicon-containing polymers, I.

manufactured by plasma polymerization of certain oxygen-containing organosilicon compounds, silicone polymer films give with unusually good properties for use in integrated circuits, especially VLSI type circuits with high circuit density densities.

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elements and short access times (typically less than 10 or 10 sec).

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The films can provide advantages for a wide variety of circuits. Such circuits preferably contain semiconductor material (eg, silicon, germanium, gallium arsenide, etc.), substrate (often also the semiconductor material), conductor elements (for example, aluminum), and various doped regions.

Different alkyl alkoxy silanes can be used as monomers, provided that the number of carbon atoms in the alkyl group and the number of carbon atoms in the alkoxy group does not exceed 3. In general, up to 20% of substances outside this class can be used to modify the properties of the polymer (fillers, cross-linking agents, property modifiers of various kinds / etc.). However, for most applications the monomer should consist essentially of the class described above. More than one alkylalkoxysilane can be used as a monomer although usually only a single monomer is used. Preference is given to alkylalkoxysilane monomers in which the alkyl groups are methyl groups. Most preferred is the trimethylmethoxysilane monomer because the resulting polymer has a very low dielectric constant, high thermal stability and excellent adhesion properties. These properties are very advantageous for many circuit applications for a number of reasons. The dielectric constant often limits access times and clock frequencies in many memory and logic circuits. Thermal cycling treatments are often required in the manufacture of integrated circuits, especially those containing aluminum conductive elements. The adhesion is of particular importance in encapsulation applications and in cases where multiple conductive layers are used.

The thickness of the layers can vary over wide limits (often they are as thin as 0.05 µm) (and usually depends on the particular application. Typically, a thickness is from 0.2 to 100 µm. For many in circuit applications, thicknesses from 0.5 to 10 µm usually give satisfactory results For many applications, the film should be as thin as possible without causing adverse effects, for example, in some circuit applications it is desirable that the thickness be such minimizes capacitance effects to a minimum but maintains sufficient thickness to allow voltage breakdown and fa Tf Λ in o 'Sf öüy / d q0 ^ -------------- V. # 5 diffusion by preventing the film The optimum thickness from this point of view often occurs between 0.5 and 2 µm.

A plasma discharge procedure is used to produce the polymer film from the monomer. Satisfactory results are obtained with conventional procedures described in detail in a number of books and publications. Typical books are Techniques and Applications of Plasma Chemistry, published by J.R. Hollaban and A.T. Bell (Wiley-Interscience, New York, 1974), in particular M. Millard, Chapter 5, p. 177 and Plasma Polymerization, published by M. Shen and A.T. Bell, ACS Symposium Series No. 108 (American Chemical Society, Washington, D.C., 1979).

The equipment used in the experiments described below is typical of equipment used for plasma-induced polymerization. The films were deposited in a parallel plate, 40 cm diameter, radial flow reactor. The electrode spacing was about 2 cm and the operating frequency was about 13.56 MHz. Both the RF excited electrode and the grounded (susceptor) plates were cooled with water at a temperature of about 35-40 ° C.

The reagents were commercially available and used without further preparation. All chemicals used were liquids at room temperature and flow rates were adjusted with a needle valve to obtain a system pressure of 50 millitorr in the absence of a plasma. The pressure without introduction of monomer was about 5 millitorr. The plasma was operated at about 50 volts and films were deposited on a pre-cleaned silicon substrate mounted on the grounded bottom plate.

The film thickness and refractive index were determined ellipsometrically with an ellipsometer II (Applied Materials Corp.), with the film thicknesses being cross-checked with a nanospec (Nanometrics). Film stoichiometry was determined using Rutherford backscatter measurements. Film tension measurements were obtained by an optically jacked laser beam method in which the tension is determined from changes induced in the radius of curvature of a substrate after film deposition. Contact angle measurements were performed with a Rame-Hart goniometer.

Electrical properties were obtained by first evaporating Al stripping on the films and in the case of the determination of 8302045

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6 the dielectric constant, by obtaining a C-V curve and measuring the capacitance in accumulation. Breakdown voltage measurements were obtained by probing 100 strips per film and measuring the voltage required to pass 2 µA current.

In these experiments, several monomers were used, corresponding to Ο / Si molar ratios of zero (tetramethylsilane) or 1 (trimethylmethoxysilane) to 4 (tetramethoxysilane). The experiments showed that the deposition rate is essentially the same for Ο / Si ratios from 1 to 4 (about 6 nm per minute). The film tension is essentially zero for Ο / Si ratios of zero and rises 8, from about 3 to 5 x 10 dynes per cm * as the Ο / Si ratios rise from 1 to 4. Such voltages are quite reasonable for most applications including circuit use.

The silicone polymers become less hydrophobic as the 15 µ / Si ratio increases from 1 to 4 with both tetramethylsilane and trimethylmethoxysilane being the most hydrophobic. The refractive index modestly decreases with an increasing ratio of Ο / Si from 0 to 4.

Of particular importance is the dielectric behavior of the film as a function of the film composition. Fig. 1 shows a graph 20 of the dielectric constant versus the film composition (in terms of the monomer starting material). As mentioned above, a low dielectric constant is very beneficial in modern circuit applications. The dielectric constant is minimal at a Ο / Si ratio of 1 (corresponding to the monomer trimethylmethoxysilane).

Largely because of this low dielectric constant and other good properties of this polymer, the plasma-induced polymerization of trimethylmethoxysilane is most preferred.

The dielectric strength of the films was also measured. The dielectric strength was very low for Ο / Si equal to 0 and 4. The mean values were below 100 volts per µm. However, for O / Si ratios 1 and 3 (trimethylmethoxysilane and methyltrimethoxysilane, respectively), the average breakdown voltage was above several hundred volts per µm.

A suitable way of evaluating the film's breakdown properties is to plot the "sport" population against the monomer composition. The "sport" population is the percentage of 83 0 2 3 4 5 µm 7 \ dots measured with breakdown voltages below 100 volts per µm. Such data are shown in Figure 2 for various starting monomers. The particularly low values for Ο / Si ratios of 1 and 3 are very advantageous in electrical circuit applications. The other 5 Ο / Si ratios may be higher due to the softness of the films and the gauge used in the measurements.

The thermal properties of various polysiloxanes were also examined. This was done by exposing the film to a temperature of 300 ° C for one hour in the air and measuring the percent reduction in film thickness. The results of these experiments are given in Figure 3. Herein, the percent reduction in thickness is plotted as a function of the ratio of O / Si of the monomer used. Although all polymer films show excellent thermal stability, the thermal stability of the polymer film made from trimethylmethoxysilane is particularly good. The thickness was reduced by only about 2.5% after being exposed to the heat treatment described above.

Although all polymer films made from alkylalkoxysilane monomers exhibit excellent properties, especially for electronic applications, the polymer films made from trimethylmethoxysilane show exceptionally good properties for such applications. The dielectric constant and the film tension are minimal or nearly minimal, while properties such as hydrophobicity, dielectric strength and thermal stability are maximal or substantially maximal. Additional experiments have shown that polymer films made by plasma-induced polymerization of trimethylmethoxysilane have excellent adhesion as demonstrated by the Scotch tape tensile test and withstanding a patterning process on a topographic surface. The film is easily patterned using a CF 2 O plasma, but is very resistant to O plasma. This allows plasma stripping of resist material in the presence of the silicone film. Thermal stability was particularly impressive as evidenced by the fact that even at 450 ° C in the presence of forming gas or nitrogen, the film showed only a few 35 percent contraction.

Fig. 4 shows a cross-sectional view of a typical 8SC2343 8 integrated circuit 40 with various features as indicated by + + + marks (such as p channels, n channels, etc.) · The circuit is suitably described as a CMOS (Complementary Metal Oxide Semiconductor) circuit with a single tub, single polysilicon-5 structure, which exhibits 5 µm design rules. The exact features of the integrated circuit are not critical to an understanding of the invention and will be described only briefly. The circuitry of the circuits is made of relatively heavy n-doped silicon (typically 18 end doped with phosphorus in a concentration range of roughly 10 atoms per cubic centimeter). This area is marked in a diagram as ψ * ψ “n. . A lighter doped region covers the n region (labeled as n) and several other regions are labeled such as regions, n + channels and p + channels. The CHANSTOP region is used to electrically isolate one region from another. An oxide region 41 is also part of the circuit. This oxide is commonly called SiO4 egg and is often referred to as a field oxide or FOX region. A polysilicon region 42 and aluminum region 43 are also shown, as well as a glass region 44 (usually phosphor glass). The entire circuit is covered with a layer of plasma polymerized silicone 45. Usually, the thickness varies between 0.5 and 2 µm.

Fig. 5 shows a side view of a more complex CMOS circuit 50 with P and N as well as p-type channels (p1) and n-T.uB 'X'Uti cn type channels (n1). There are also areas of heavy p-type doping + cft + (p) and heavy n-type doping (n). Certain areas have very thin oxide layers 51 which are usually made of SiO 2 and some thicker oxide layers 52 (again usually SiO 2) which are often referred to as field oxide or FOX. There is also a layer 53 of TaSi2 (often polycrystalline silicon is used in the same capacity) and several conductive layers 54 usually made of aluminum. Layers 55 of phosphor glass are also used in the structure. As the capping layer 56, a polysiloxane layer of plasma-induced polymerization of trimethylmethoxysilane is deposited over the entire structure. The film is used as a protective layer and usually has a thickness of about 1 µm.

Fig. 6 shows a slightly more complex structure 60 with many of the features shown in FIG. 6 including gate oxide 61, field oxide 62, TaSi2 layer 63, phosphor glass layer 64 and aluminum layer 65. Also shown. * 0 \ ^

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c, 9 shows the silicone layer 66 covering a large part of the circuit. A particular difference between this circuit and the circuit shown in Figure 5 is the use of an aluminum metal top layer for selectively contacting an aluminum-5 metal bottom layer. Here, the silicone is used not only as an encapsulation layer, but also as a dielectric layer between the levels to separate the top level of aluminum from the rest of the circuit.

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Claims (13)

Electric semiconductor device, comprising semiconductor material and conductive elements, characterized in that the device further comprises silicone film made by plasma-induced polymerization of at least one alkyl alkoxy silane in which the alkyl and 5 alkoxy groups contain up to 3 carbon atoms. Device according to claim 1, characterized in that the alkyl group is a methyl group. Device according to claim 2, characterized in that the alkoxy group is a methoxy group. Device according to claim 3, characterized in that the alkylalkoxysilane is trimethylmethoxysilane. Device according to claim 1, characterized in that the conductive elements comprise aluminum. 6. Device according to claim 1, characterized in that the thickness of the silicone film is between 0.05 and 100 µm. Device according to claim 6, characterized in that the thickness of the silicone film is between 0.5 and 10 µm. Device according to claim 7, characterized in that the thickness of the silicone film is between 0.5 and 2.0 µm. Device according to claim 1, characterized in that at least two of the conductive elements have a distance of less than 2 µm. 10. Device according to claim 1, characterized in that the semiconductor material is selected from silicon, gallium arsenide and germanium. 11. Device as claimed in claim 10, characterized in that the semiconductor material is silicon. Device according to claim 11, characterized in that the electric semiconductor device is a memory circuit. Device according to claim 11, characterized in that the electric semiconductor device is a logic circuit. 83 02 04 5