FR2531811A1 - POLYSILOXANE POLYMERIC LAYERS FORMED BY PLASMA-INDUCED POLYMERIZATION FOR ELECTRONIC CIRCUITS - Google Patents

Abstract

THE INVENTION CONCERNS THE TECHNOLOGY OF INTEGRATED CIRCUITS. THE INVENTION RELATES TO THE USE OF POLYSILOXANE LAYERS DEPOSITED BY PLASMA45 USED AS PROTECTIVE AND DIELECTRIC LAYERS ON ELECTRONIC CIRCUITS. SUCH LAYERS ARE VERY ADVANTAGEOUS AS THEY ARE CHEMICALLY INERT, THEY HAVE EXCELLENT THERMAL STABILITY AND HAVE A LOW DIELECTRIC CONSTANT. ELECTRICAL CIRCUITS CARRYING THESE LAYERS CAN BE TREATED WITHOUT DAMAGE AT ELEVATED TEMPERATURES AND HAVE MINIMUM PARASITE CAPACITIES. APPLICATION TO VERY HIGH LEVEL OF INTEGRATION CIRCUITS.

Description

The invention relates to electronic circuits

  containing polymer layers used as cou-

insulation and dielectric.

  Continuous progress in technology

  very high integration circuits have led to a higher density of

  reduction of the dimensions fixed by the rules of

  This has revealed the need for dielectric

  In particular, the close proximity of various driving elements in a circuit with a very high level of integration has revealed the need for a dielectric material having a dielectric constant.

  low in order to reduce parasitic capacitances.

  high dielectric density is also required because of the

  short distance between various conductive elements in

  circuits with a very high level of integration The use of two levels of metallization, at most, creates the need

  of a dielectric material having other specific properties

  For example, it is desirable in some cases

  that the dielectric is applied at a relative temperature

  low so as not to adversely affect the

  multilevel circuit In other cases, the

  Such multi-level circuits are frequently

  intervene the use of relatively high temperatures

  the dielectric material must be stable. In addition, the reduced element sizes and the

  higher applied voltages found in new

  Devices with a very high level of integration may require new materials for vitrification and encapsulation.

  Desirable properties for diaper layers

  used in circuits with a very high level of integration are: low dielectric constants to minimize parasitic capacitances;

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  high thermal stability to allow treatment

  the circuit at elevated temperatures, a temperature of

  relatively low application rate to minimize

  rioration of the circuit under the effect of the application of

  high dielectric strength, good adhesion, good layer integrity (ie the absence of

  cracks) and chemical stability, in particular vis-à-

  water and water vapor.

  would be of great value, particularly for application to high density circuits, as the reduced parasitic capacitances

  higher speeds and that thermal stability

  would be more flexible in the treatment of such circuits. A number of studies have been done on

  the use of polymer layers in electronic circuits

  both as a layer of insulation and encapsulation

  as the most remarkable passivation layer.

  Among them are: in an article entitled "Correlation of Chemical and Electrical Properties of Plasma Deposited Tetramethylsilane Films", Journal of Applied Physics, 52 (2), 903 (1981), A Szeto and D Wo Hess. the properties of tetramethylsilane layers

  of interest in the manufacture of electronic circuits

  Similar studies have been carried out for

  polymer layers made by polymerization of hexamines

  plasma-induced thyldisiloxane These studies have been reported in the following articles: Maisonneuve et al, Thin Solid Films, 44, pages 209216 (1977); M Aktik et al, Journal of Applied Physics, 51 (9), pages 5055-5057 (1980); M Maisonneuve et al, Thin Solid Films, 33, pages 3541 (1976); and Jo E Klemberg-Sapieha, Applied

  Physics Letters, 37 (1), pp. 104-105 (1980).

  The invention provides an electrical semiconductor device comprising a semiconductor material and conductive elements, characterized in that it further comprises a polysiloxane layer formed by plasma-induced polymerization of at least one alkylalkoxysilane with the alkyl and alkoxyl groups containing up to 3 carbon atoms.

  A preferred embodiment of the invention

  curing an electrical circuit in which at least a portion of a surface of the circuit is covered by a plasma deposited polysiloxane layer, wherein the monomer is an alkylalkoxysilane; the alkyl and alkoxyl groups must not contain more than three carbon atoms; carbon On

  can be cited as characteristic examples

  thylmethoxysilane, dimethyldimethoxysilane, triethyl.

  ethoxysilane etc. The trimethylmethoxysilane monomer is preferred because of a low dielectric constant, a high breakdown voltage, low stresses in the layer and a reasonable degree of thermal stability as well as a strongly hydrophobic character. of

  polymer is useful for a wide variety of electrical circuits

  This circuit typically comprises a substrate, conductive elements, input connections, output connections, etc. These layers are very advantageous when at least some spacings between conductors are very small (i.e. of the order of 2 Mim or less) and circuit frequencies (or corresponding access times) are very high (i.e. greater than 5 M Hz) They are also very advantageous when using an aluminum metallization or another metallization in the case where thermal cycles

are required.

  The invention will be better understood on reading the

  description that will follow-of embodiments and by

  with reference to the accompanying drawings in which Figure 1 shows a graph showing the

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  same data relating to the dielectric constant for

  polysiloxane layers formed from various alkylalco-

  xysilanes; Fig. 2 shows a graph showing the same breakdown voltage data for

  polysiloxane layers formed from various alkylalco-

  xysilanes; Figure 3 shows a graph showing the same data regarding the thermal effect on layer thickness for polysiloxane layers formed from various alkylalkoxysilanes; Figure 4 shows a section of a part of a

  characteristic integrated circuit, making certain

  some details of such a circuit comprising a polysiloxane coating layer; Figure 5 shows a section of a part of a

  more complex integrated circuit, with a coating material

  formed according to one embodiment of the invention; and Fig. 6 shows a sectional view of a portion of an integrated circuit in which polysiloxane is used as a dielectric to separate two levels.

aluminum metallization.

  It has been discovered that certain polymers containing

  silicon and formed by plasma polymerization of cer-

  Some silicon-containing organic compounds provide polysiloxane polymer layers with exceptionally good properties for use in integrated circuits, particularly very high integration level circuits, with high densities of elements.

  circuitry and short access times (typically

  less than 106 or 10 seconds). These layers can

  can be advantageous for a wide variety of circuits.

  Such circuits preferably comprise a semiconductor material

  conductor (for example silicon, germanium, gallium arsenide, etc.), a substrate (which is often

  also the semiconductor material), conductive elements.

  (for example aluminum) and various doped regions.

  Various alkylalkoxysilanes can be used as monomers, provided that the number of atoms of

  carbon in the alkyl group and the number of carbon atoms

  in the alkoxyl group do not exceed three

  generally use up to 20% of substances

  this class to modify the properties of the polymer (fillers, crosslinking agents, property modifiers of various kinds, etc.) However, for most applications, the monomer must essentially

  consist of a substance of the class indicated above.

  More than one alkylalkoxysilane may be used for the monomer

  lane, although a single monomer is usually used.

  The preferred monomers are the alkylalkoxysilanes in which the alkyl groups are methyl groups. The most preferred monomer is trimethylmethoxysilane, since the resulting polymer has a very low constant.

  dielectric, high thermal stability and excellent

  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 logic and memory circuits.

  are often imposed in the manufacture of cir-

  embedded cookware, especially those containing aluminum conductive elements Adhesion is particularly important in encapsulation and

  when using multiple conductive layers.

  The thickness of the layers can vary within wide limits (it can often go down to 0.05 pm) and

  it usually depends on the particular application.

  A typical range is from 0.2 to 100 micrometers.

  In many circuit applications, thicknesses of 0.5 to 10 micrometers usually give satisfactory results. In many applications, the layer should be as thin as possible, without causing any adverse effects. it is desirable to minimize the thickness in order to minimize the capacitance effects, but to maintain a sufficient thickness to prevent voltage breakdown and diffusion across the layer.

  this point of view is often between 0.5 and 2 micro-

meters.

  A plasma discharge procedure is used to produce the polymeric layer from the monomer On

  achieves satisfactory results with various

  classics described in detail in a number of

  books and documents We can cite as typical books

  : Techniques and Applications of Plasma Chemis-

  try, edited by J R Hollaban and AT Bell (WileyInterscience, New York, 1974), particularly the

  chapter 5 by M Millard, page 177; and Plasma Polymeriza-

  edited by M Shen and A To Bell, ACS Symposium Series No. 108 (American Chemical Society,

Washington, DC, 197/9).

  The particular apparatus which has been used in the experiments described below is characteristic of the material used for the plasma-induced polymerization. The layers have been deposited in a parallel plate radial circulation reactor 40 cm in diameter. electrodes was about 2 cm and

  the operating frequency of approximately 13.56 M Hz.

  corresponding to the RF excited electrode as the grounded one (susceptor) were water-cooled at a temperature of about 3540 ° C.

  trade and which has not been the subject of any

  Complementary All chemicals used were

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  at room temperature and adjusted the flow rates using a needle valve to give a general pressure of 6.5 Pa in the absence of plasma. The pressure without monomer introduction was about 0.65 Pa. operate the plasma at about 50 V and layers were deposited on a pre-cleaned silicon substrate and

  mounted on the bottom plate connected to the mass.

  Thickness and refractive index were determined.

  layer by ellipsometry, with an Ellipsometer II type instrument (Applied Materials Corp),

  a check of the layer thicknesses with an instru-

  Nanospec (Nanometrics) Characteristics were determined

  stoichiometric ticks from backscatter measurements.

  Rutherford strain-stress measurements have been obtained using a laser beam method in which stresses are determined from induced variations in the

  radius of curvature of a substrate after the deposition of a layer.

  Contact angle measurements were made using a

goniometer of the Rame-Hart type.

  The electrical properties have been determined in evapo-

  firstly on the Al point layers and, in the case of dielectric constant determination, by establishing a C-V curve and measuring the capacitance by

  accumulation.

  by controlling 100 dots per layer and measuring the voltage required to pass a current of 2 p A. In these experiments, various monomers corresponding to molar ratios O / Si ranging from zero were used.

  (tetramethylsilane) or 1 (trimethylmethoxysilane) to 4 (tetramethylsilane) or

  Methoxysilane) Experiments have shown that the deposition rate is basically the same (about 6 nanometers per minute) for O / Si ratios ranging from one to four. The stress in the layers is practically equal to zero for equal O / Si ratios. at zerd and it increases from about 3 to 5 x 107 Pa when O / Si ratios increase from one to four Such stresses are quite reasonable for most applications, including

  use in circuits Poly-type polymers

  siloxane become less hydrophobic when the ratio O / Si increases from one to four, and the hydrophobic character is the

  more marked for tetramethylsilane and trimethylmethoxy-

  Silane The refractive index decreases moderately when the

O / Si ratio increases from 0 to 4.

  The dielectric behavior of the layer as a function

  its composition is of particular importance.

  Figure 1 shows a graphical representation of the

  dielectric strength as a function of the composition of the layer (expressed by means of the monomer starting material) As indicated above, a low dielectric constant is very advantageous in applications corresponding to the most recent circuits. The dielectric constant is minimal for an O / Si ratio of one (which corresponds to the trimethylmethoxysilane monomer) The polymer resulting from the plasma-induced polymerization of trimethylmethoxysilane is most preferable, to a large extent because of this low dielectric constant and other properties.

favorable of this polymer.

  The dielectric strength of the layers was also measured. The dielectric strength was very low for an O / Si ratio of 0 and 4. The average values were less than 100 V / pm. However, for an O / Si ratio

  equal to 1 and 3 (corresponding to trimethyl-

  methoxysilane and methyltrimethoxysilane), an average breakdown voltage greater than several hundred was obtained.

of volts per micrometer.

  A convenient way to evaluate characteristics

  breakdown of a layer consists in representing graph-

  the "abnormal" population according to the composition of the monomer The "abnormal" population is the percentage of

  controlled points which have lower breakdown voltages

  at 100 V / pm Such data is represented on the

  Figure 2 for various starting menomers The partial values

  Low values for O / Si ratios of 1 and 3 are very advantageous in applications involving electrical circuits. The values corresponding to the other O / Si ratios can be higher, because of the softness of the layers and the probe used in them. The thermal properties of several layers of polysiloxane were also examined. This was done by exposing the layer at 300 C for one hour in air and measuring the percentage reduction.

  the thickness of the layer The results of these experiments

  these are shown in Figure 3 The percentage reduction in thickness is here represented graphically in

  the O / Si ratio of the monomer used.

  the polymer layers have excellent stability

  temperature, the thermal stability of the polymer layer

  formed from trimethylmethoxysilane is particularly

  The thickness has decreased by only around 2.5%

  after exposure to the heat treatment described above.

  Although all the polymer layers formed at

  from monomers consisting of alkylalkoxysilane having

  excellent properties, especially for applications

  cations in electronics, the polymer layers formed in

  from trimethylmethoxysilane are exceptionally

  good properties for such applications The dielectric constant and the stress in the layer are minimal or almost minimal? while properties such as hydrophobicity, dielectric strength

  and thermal stability are maximum or near-maximum

  Additional experiments have shown that

  polymers formed by the polymerization of trimethylmetho-

  plasma-induced xysilane have excellent adhesion,

  as shown by the tear test with an adhesive tape

  sive, and resist a pattern definition treatment on a topographic surface It is easy to define a pattern in the layer using a CF 4 + 02 plasma, but the layer is very resistant to a plasma of 02 This allows stripping by plasma of a photographic reserve material

  in the presence of the layer of polysiloxaneo

  is particularly remarkable, as shown by the fact that the layer shrinks by only a few percent,

  even at 450 ° C in the presence of a formation gas or nitrogen.

  Fig. 4 shows a section of a typical integrated circuit 40 with various elements indicated by the legends (such as channels p, n channels, etc.). The circuit can conveniently be described as a CMOS circuit.

  (complementary metal-oxide-semiconductor), with a structure

  a single box and a single level of polysilicon

  The precise elements of the integrated circuit are not of great importance for the understanding of the invention and will be described only briefly. The substrate of the circuits consists of silicon doped in such a way. relatively

  strong with the N type (typically

  This region is marked n + on the diagram. A region with a weaker doping (marked n-) covers the region n + and various other regions carry markings denoting by the phosphorus region in the concentration range of the order of 1018 atoms per cubic centimeter. PCAISSON box regions, N channels and p channels are used. The "CHANNEL STOP" region is used to electrically isolate one region from another. The circuit also comprises an oxide region 41. and is often referred to as a field oxide region. Also shown is a polycrystalline silicon region 42 and a region thereof.

  aluminum 43, as well as a glass region 44 (usually

  phosphor glass) The entire circuit is covered by

  green by a layer 45 of polysiloxane polymerized by plasma.

  The thickness of the layer usually varies between 0.5 and

  2 pm. FIG. 5 shows a side section of a more complex CMOS circuit 50, with PCAISSON and NCAISSON regions, as well as p-type channels (Pca) and N-type channels (nca). There are also regions having a Fort

  p (p +) type doping and a high N (n +) Cer

  some regions have very thin oxide layers 51,

  usually consist of Si O 2 and some have

  52 thicker oxide films (still usually

  If O 2), which is often called field oxide, there are also

  a layer of Ta Si 2 53 (silicone is often

  polycrystalline copper for the same purpose) and various conductive layers 54 which are usually made of aluminum. Phosphorus glass layers 55 are also used in the structure.

  depositing on the entire structure a layer of poly-

  siloxane by polymerization of trimethyimethoxysilane

  induced by plasma This layer is used as a

  protector and it usually has a thickness

about one micrometer.

  FIG. 6 shows a somewhat more complex structure 60, and a large number of the elements shown in FIG. 6 comprise a gate oxide 61, a field oxide 62, a layer of Ta Si 2 63, a layer of glass at phosphorus 64 and an aluminum layer 65 The polysiloxane layer 66 which covers a large part of the circuit is also seen. A particular difference between this circuit and

  the circuit shown in FIG. 5 resides in the use

  An aluminum top metal layer for selective contact with a lower aluminum metal layer is used here. The polysiloxane is used here not only as an encapsulating layer, but also as a dielectric for to separate the

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  aluminum level higher than the rest of the circuit.

  It goes without saying that many modifications can be made to the device described and shown,

  without departing from the scope of the invention.

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Claims (10)

1. An electric semiconductor device comprising a semiconductor material and conductive elements, characterized in that it further comprises a polysiloxane layer formed by plasma-induced polymerization of at least one alkylalkoxysilane, with the groups alkyl and alkoxyl containing up to 3 carbon atoms.   2 Device according to claim 1, characterized   in that the alkyl group is a methyl group.   3 Device according to claim 2, characterized   in that the alkoxyl group is a methoxyl group.   4 Device according to Claim 3, characterized   in that the alkylalkoxysilane alkyl is trimethylmethane   trimethoxysilane. Device according to claim 1, characterized   in that the conductive elements consist of aluminum.   Device according to claim 1, characterized   in that the thickness of the polysiloxane layer is   between 0.05 and 100 pm 7 Device according to claim 6, characterized   in that the thickness of the polysiloxane layer is between 0.5 and 10 Mm.   8 Device according to Claim 7, characterized   in that the thickness of the polysiloxane layer is between 0.5 and 2.0 jams   9 Device according to claim 1, characterized   in that at least two of the conductive elements have a   less than two micrometers.   Device according to claim 1, characterized in that the semiconductor material is selected from   silicon, gallium arsenide and germanium.   11 Apparatus according to claim 10, characterized   in that the semiconductor material is silicon.   Device according to claim 11, characterized 25318 i 11 th   in that it consists of a memory circuit.   Apparatus according to claim 11, characterized   risé in that it consists of a logic circuit.