JPH1197912A - High frequency transmission line - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a high freguency transmission circuit lowin loss and lowin cost by satisfying relation between use frequencies and the thickness of conductors with a specific expression, and using a microstrip line with specific high frequencies. SOLUTION: In the microstrip line, conductors 12 and 13 whose thickness is (t) are formed on the upper and lower faces of a dielectric 11. Relation between use frequencies (f) and the thickness (t) of the conductors 12 and 13 is selected so that 3/8×π<=τt<=7/8×π can be satisfied. The line is used with high frequencies in which outside impedance Zout decided by the structure of a transmission line can be more dominance than inside impedance Zin of the conductors 12 and 13. In the expression, τ=√(ωμσ/2), ω=2π fare angular frequencies, μis magnetic permeability, and σ is conductivity. Zin=Zs ×coth(τt)=Rin+j.Xin, and Zs=√(jωμ/σ). Zout=√(Lex/Cex).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of loss of a high-frequency transmission line used at a high frequency of microwave, millimeter wave or higher.

[0002]

2. Description of the Related Art Transmission lines used at high frequencies such as microwaves and millimeter waves, such as coaxial lines and planar signal lines,
It is broadly classified into a waveguide that guides a TE wave, a TM wave, or an electromagnetic wave in which these are mixed, such as a signal line that uses a TEM wave as a transmission wave, a waveguide, a dielectric line, and the like (hereinafter, collectively referred to as transmission). Tracks). These transmission lines are required to reduce signal disturbance and loss during transmission.
Among these, in order to reduce the disturbance of the signal, it is a condition that the characteristic impedance and the delay characteristic are maintained over a wide frequency range, and a special type of line satisfying these conditions has been devised. For example, in a microwave IC that has been actively developed recently, a transmission line such as a microstrip line or a coplanar line is often used.

[0003] In such a transmission line, a wide frequency range and low loss are required as basic performance. In particular,
In order to obtain a wide frequency range, it is necessary to suppress the generation of modes other than the transmission mode. To satisfy this condition, the transmission line using the TEM wave has a size sufficiently smaller than the wavelength of the highest frequency used. There is a need. For example, if a coaxial line is used at 60 GHz, a diameter of at most 2.3 mm (electrical length conversion) or less is required.

In general, there are two types of transmission loss in a transmission line: dielectric loss and conductor loss. In the millimeter wave region, conductor loss is dominant. The conductor loss (hereinafter referred to as loss) is basically determined by the conductivity of the metal used. From the viewpoint of practical aspects such as surface stability, gold or gold-plated copper for MMIC or copper for coaxial line is often used. Used. As described above, in order to form a transmission line having a wide frequency range, the signal line must be reduced in size, and in general, a reduction in size in inverse proportion to the highest operating frequency is required. In other words, there is a problem that the transmission line used up to a higher frequency must be smaller, and as a result, the effective surface area of the metal decreases and the loss increases.

The high-frequency resistance of a metal is known as surface impedance, and its value is Zs = √ (jωμ / σ) when the thickness t of the conductor is sufficiently thicker than the depth δs on the surface of the operating frequency. Where ω = 2πf: angular frequency, μ: magnetic permeability, σ: conductivity. In general, the conditions to be satisfied are as follows: depth of skin (δs) δs = √ (2 / (ωμσ)) (5) where ω = 2πf: angular frequency, μ: magnetic permeability, σ: a conductor having three times or more of conductivity It is said that the thickness is necessary. If the thickness is smaller than this, the resistance value increases, and even if the thickness of the conductor is further increased, the resistance value hardly decreases, and it is said that reduction in loss is a limit.

[0006]

As described above, the transmission line used up to a higher frequency needs to be miniaturized, and as a result, there is a problem that the effective area of the metal is reduced and the loss is increased. . In particular, in the future, higher frequency in proportion to the increase in the amount of information is inevitable, and reduction of the transmission line loss has become a major technical problem. However, the reduction in loss of these high-frequency transmission lines is said to be physically limited for the same reason as described above. By the way, attempts to reduce the loss by changing the form of the transmission line are also active, but none of them is practical in terms of cost.

An object of the present invention is to realize low loss and low cost in a transmission line using up to a high frequency.

[0008]

In order to solve the above-mentioned problems, the above-mentioned surface impedance as a high-frequency resistance was examined. Literature RECollin “Field theory of
guided waves ”, McGraw-Hill, New York states that
Electromagnetic waves that seep into metal

## EQU00002 ## It is known that the characteristic impedance Zs = .SIGMA. (J.omega ./. Sigma.) (6) Propagation is performed under the following condition: .tau. =. SIGMA. (J.omega..mu.) = (1 + j) /. Delta.s. In the direction of propagation, the conductivity of the metal becomes the dielectric constant, and the equivalent dielectric constant of the metal is extremely large.When electromagnetic waves enter the metal from the dielectric portion, refraction according to Snell's law is observed. As a result, no matter what angle the electromagnetic wave enters the metal surface, it travels in the metal in a direction substantially perpendicular to the metal surface. Therefore, the surface impedance Zin (also referred to as the internal impedance) of the conductor having the thickness t can be represented by a transmission line model having the thickness t with the ends open, and is as follows.

Zin = Zs × coth (τt) (7) The high-frequency resistance of the metal having the thickness t is the real part Re (Zin) of the internal impedance, which is obtained by substituting (6) into (7).

From the equation Zin = (1 + j) × Zs × coth {(1 + j) t / δs} Re (Zin) = Rin = Zs × (sinh (x) + sin (x)) / (cosh (x) −cos ( x)) (8) where x = 2 × t / δs where Zs is the thickness t of the conductor described above and the skin depth δ
s is the surface impedance when sufficiently thicker than s
The high-frequency resistance is said to not fall below this value. Therefore, the high-frequency resistance Re due to the thickness of the conductor
The change of (Zin) = Rin may be obtained by examining terms other than Zs in (7).

This function f (x)

= (Sinh (x) + sin (x)) / (cosh (x) -cos (x)) is x = π, that is, when t = (π / 2) × δs, f (x) = 0 .917 ... (9) Therefore, the minimum value Rin mi of the high-frequency resistance
n is

Rin min = Zs × sinh (π) / (1 + cosh (π)) ≒ 0.917 × Zs (10) That is, the resistance value Zs, which has been conventionally regarded as the limit,
Thus, the resistance is reduced by about 8% or more.

FIG. 5 is a graph showing how the high-frequency resistance changes with frequency when the conductor is made of gold, using the thickness of the conductor as a parameter. The a-line (√ (ωμ / 2σ) ∝√f), which is proportional to the 乗 power of the frequency, is the limit line for reducing the resistance, which has been conventionally known. The b, c, d, and e lines each have a thickness of 0.1 mm. Microstrip lines at 5, 0.7, 1.0 and 2.0 μm (w = 69 μm, h = 100
μm, εs = 12.9). In each of the b to e lines, a resistance value lower than the limit line for lowering the resistance in proportion to the half power of the frequency is obtained.
This resistance value is equivalent to using a conductive material having a conductivity 1.2 times lower than that of the conductive material, and the conductivity of gold (4.17 × 10 ⁷ s /
(at 20 ° C.) is improved to 5.0 × 10 ⁷ s / m (copper is 5.8 × 10 ⁷ s / m.)
m).

As discussed above, by setting the thickness t of the conductor to π / 2 times (approximately 1.5 times) the depth of the skin or about the same, a material having a maximum conductivity of 20% higher is used. The same effect as described above can be obtained. This result indicates that the optimum thickness is obtained when the thickness of the conductor is about half the value conventionally believed, and the resistance is reduced by 8% or more. Hereinafter, this effect is referred to as a “high-frequency resistance reduction effect”. According to this, even when the thickness of the conductor is reduced to about half of the conventional thickness, the resistance value substantially equal to the conventional value can be obtained.
Resources can be saved, material costs can be reduced, and process costs can be reduced.

FIG. 6 is a graph showing the change of Gmax and transmission loss α with respect to frequency in the above-mentioned microstrip line using the thickness of the conductor as a parameter. A line proportional to the square is shown). In FIG. 6, when the conductor t is thick (t = 2.0 μm), a low frequency range (（in this example)
At 6 GHz), the effect of reducing the high-frequency resistance appears on the conduction loss α, but this effect cannot be used because Gmax controls the loss. On the other hand, when the thickness t of the conductor is small (t = 0.5
μm), a high-frequency resistance reduction effect appears in a high frequency range (about 100 GHz) where Gmax and the transmission loss α almost match, and this effect can be used. That is, the effect of reducing the high-frequency resistance can be used only in a high frequency range where Gmax and the transmission loss α coincide with each other with an error of 10% or less.

That is, according to the first aspect of the present invention, in a high-frequency transmission line composed of a center conductor and an outer conductor disposed with a dielectric material interposed therebetween, the relationship between the operating frequency f and the thickness t of the conductor is as follows: 3) × π ≦ τt ≦ 7/8 × π (1) where τ = √ (ωμσ / 2), where t: thickness of the conductor, ω = 2πf: angle Frequency, μ: magnetic permeability, σ: electrical conductivity, and the internal impedance Zin of the conductor represented by the following equation (2)

Zin = Zs × coth (τt) = Rin + j × Xin (2) where Zs = √ (jωμ / σ) where t: conductor thickness, ω = 2πf: angular frequency, μ: magnetic permeability, σ: External impedance Z determined by transmission line structure rather than conductivity
out Zout = √ (Lex / Cex) (3) It is characterized in that it is used at a high frequency in which:

Further, the invention according to claim 2 is characterized in that in claim 1, the thickness of the conductor is 0.5 μm or less.

Furthermore, the invention according to claim 3 is characterized in that the waveguide is surrounded by a conductor having a thickness satisfying the condition of claim 1, and is configured to propagate a specific wave inside the closed interior. And

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which a high-frequency transmission line according to the present invention is applied to a microstrip line, a coaxial line and a waveguide will be described below with reference to the drawings.

[First Embodiment] FIG. 1 is a schematic sectional view showing a configuration of a microstrip line according to a first embodiment. This microstrip line has a dielectric 11 (w =
10 μm, h = 2.7 μm, εs = 3, tan = 0.
001), conductors 12 and 13 having a thickness of 0.55 μm were formed on the upper and lower sides of the conductors. In the first embodiment, the thicknesses of the conductors 12 and 13 are made thinner than the conventionally-known values, and are reduced to about half of the conventional values so as to satisfy the above-mentioned formula (1). The thickness of the conductor layer (gold) is 0.55 μm.
However, in this microstrip line, the conductor 12,
In order to ensure the close contact between the dielectric 13 and the dielectric 11, the dielectric 11
50 nm of titanium and 50 nm of platinum are respectively laminated on the front and back surfaces (both are not shown), and the conductors 12 and 13 described above are formed thereon. Since the conductivity of the contact conductor layer made of titanium and platinum is lower than that of gold, the value obtained by actually measuring the conductivity of the “total conductor” (3.6 × 10 ⁷ s /
m) and the thickness (t) was set to 0.65 μm in the total film thickness.

FIG. 2 is a graph showing the change of the transmission loss α with respect to the frequency in this case, using measured values and calculated values. FIG.
In the graph, the calculated values are indicated by thin lines, and the measured values are indicated by thick lines. The line a including the calculated value is a line proportional to the half power of the frequency as in FIG. In FIG. 2, the effect of reducing the high-frequency resistance appears particularly in the range indicated by A, and the transmission loss lower than the value considered conventionally is reduced by about １ ／.
A thin conductor thickness was obtained.

As described above, if the conditions of the present invention are applied to the planar circuit used in the MMIC, not only the transmission loss is improved, but also the loss reduction effect is high even at half the thickness conventionally required. Therefore, it is possible to expect an effect such as a reduction in the material cost, a saving of the processing time by this, and an effect that the laminating property is improved because a sufficient effect can be obtained with a thin conductor.

[Embodiment 2] FIG. 3 is a schematic sectional view showing a configuration of a coaxial line according to Embodiment 2. This coaxial line uses fiberglass as the core material of the center conductor 21 and surrounds the conductor 22 made of copper with a thickness (t) of 0.2 around the core.
Coated to μm. Further, a conductor 2 also made of copper is formed on the inner surface of the outer conductor 23 made of fiberglass.
4 was coated to a thickness (t) of 0.2 μm. Note that an intermediate layer 25 made of foamable Teflon is formed between the conductors 22 and 25.

The change of the transmission loss α with respect to the frequency in this case is almost the same as in FIG. 2, and the description is omitted here. Also in the coaxial line of the second embodiment, the characteristic is 10
At 0 GHz, the loss was improved by about 8% compared to the conventional product.
In addition, since a light and durable material such as fiberglass can be easily used as a structural material as compared with a conventional product, it is possible to provide a high-performance, lightweight, and inexpensive ultrahigh-frequency coaxial line.

Third Embodiment FIG. 4 is a schematic sectional view showing a configuration of a waveguide according to a third embodiment. In this waveguide, a conductor 32 made of copper is coated on the inner surface of a cylindrical portion 31 made of fiberglass under the same film thickness condition as in the second embodiment.

Also in this case, the manner of change of the transmission loss α with respect to the frequency is substantially the same as that of FIG. 2, and the description is omitted here. Also in the waveguide of the third embodiment, the loss was improved by about 8% as compared with the conventional product. In addition, it is possible to provide a lighter and cheaper waveguide than conventional products. In the case where a large amount of power is transmitted through this waveguide, heat dissipation becomes a problem. However, a cylindrical high heat radiation ceramic such as AlN is used for the cylindrical portion 31 to form the conductor 32 having a film thickness satisfying the above conditions. By doing so, a low-loss power waveguide can be obtained. In the present embodiment, the waveguide 32 is formed by forming the conductor 32 having the above-described film thickness condition on the cylindrical portion 31 made of AlN by using the direct bonding technology already put into practical use. As a result, a waveguide with less high-frequency loss and excellent heat dissipation can be provided.

It should be noted that various materials can be used for the cylindrical portion 31 depending on the application. For example, fiberglass or plastic can be used for general purposes, and ceramics such as AlN and alumina can be used for electric power. Further, the cross-sectional shape of the waveguide may be rectangular as in this embodiment, or may be round.

Although the embodiment in which the high-frequency transmission line according to the present invention is applied to a transmission line, a waveguide, and the like has been described above, the above-described high-frequency resistance reduction effect can be obtained only by the various transmission lines shown here. Instead, the present invention can be applied to transmission lines using thin film conductors in general. For example, even when applied to a strip line, a coplanar line, or the like, the same operation and effect can be obtained even if the degree is different.

[0027]

As described above, according to the high-frequency transmission line according to the present invention, even when the thickness of the conductor is reduced to about half of the conventional thickness, a resistance value substantially equal to the conventional value can be obtained. It is possible to realize low loss and low cost in a transmission line used up to the frequency.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a configuration of a microstrip line according to a first embodiment.

FIG. 2 is a graph showing a change in transmission loss with respect to frequency in the first embodiment, which is represented by measured values and calculated values.

FIG. 3 is a schematic sectional view showing a configuration of a coaxial line according to a second embodiment.

FIG. 4 is a schematic sectional view showing a configuration of a waveguide according to a third embodiment.

FIG. 5 is a graph showing how a high-frequency resistance changes with frequency when a conductor is made of gold, using the thickness of the conductor as a parameter.

FIG. 6 is a graph showing the state of change of Gmax and transmission loss α with respect to frequency, using the thickness of a conductor as a parameter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Dielectric 12, 13 conductor 21 Center conductor 22, 24 conductor 23 Outer conductor 31 Tubular part 32 Conductor

Claims (3)

[Claims] In a high-frequency transmission line composed of a center conductor and an outer conductor arranged with a dielectric material interposed therebetween, a relationship between a working frequency f and a thickness t of the conductor satisfies the following expression (1). 3/8 × π ≦ τt ≦ 7/8 × π (1) where τ = √ (ωμσ / 2), where t: conductor thickness, ω = 2πf: angular frequency, μ: magnetic permeability, σ: electrical conductivity, and the internal impedance Zin of the conductor represented by the following equation (2): Zin = Zs × coth (τt) = Rin + j × Xin (2) where Zs = √ (jωμ / σ) where t : The thickness of the conductor, ω = 2πf: angular frequency, μ: magnetic permeability, σ: higher than the conductivity, the external impedance Zout Zout = √ (Lex / Cex) (3) determined by the structure of the transmission line becomes higher. A high-frequency signal line used at a frequency. 2. The high-frequency signal line according to claim 1, wherein said conductor has a thickness of 0.5 μm or less. 3. A high-frequency transmission line which is surrounded by a conductor having a thickness which satisfies the condition of claim 1, and propagates a specific wave inside the closed interior.