JPH0435517A - Surface acoustic wave convolver - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement of a monolithic surface acoustic wave convolver (hereinafter abbreviated as SAW convolver) composed of a piezoelectric film and a semiconductor.

[Summary of the Invention] The present invention provides a SAW convolver having a structure of piezoelectric film/insulator/low concentration S1 epitaxial layer/high concentration St substrate.
By using an aAs epitaxial layer, it is possible to improve the concentration characteristics compared to the conventional structure described above without reducing the convolution efficiency (hereinafter abbreviated as FT), and to reduce the thickness of the epitaxial layer. This eliminates the need for strict control as in conventional structures.

[Prior Art] FIGS. 9 and 10 are cross-sectional views showing the structures of two different conventional monolithic SAW convolvers,
In the figure, l is a highly concentrated semiconductor substrate, 2 is an insulator, 3 is a piezoelectric film, 4 is a gate electrode, 5 is a comb-shaped electrode of an input transducer, 6 is a back electrode, 7 is an input terminal, 8 is an output terminal, and 9 is a A high concentration semiconductor substrate, 10 represents a low concentration semiconductor epitaxial layer.

That is, FIG. 9 is characterized by a piezoelectric film/insulator/semiconductor structure, and FIG. 0 is characterized by a piezoelectric film/insulator/low concentration semiconductor epitaxial layer/high concentration semiconductor substrate structure. In the structure shown in FIG. 1O, the semiconductor epitaxial layer 10 and the high concentration semiconductor substrate are made of the same material, and the semiconductor substrate and the epitaxial layer have the same lattice constant, forming a so-called homojunction.

Comparing Figures 9 and 10, it is known that the structure in Figure 10 has a higher convolution efficiency FT, and the structure in Figure 10 is actually used. This is the current situation. The details regarding the convolver characteristics of the structure shown in FIG. 9 are described in the following references [1-2].

Literature [80 pieces per piece, Khuri-Yakub and G, S
, Kin.

A Detailed Theory of the
Monolithic Zinc Oxide on 5
ilicon ConvolverIEEE Tran
s, 5onics Ultrason,, vol, 5U
-24, No. 1゜January 1977, pp.
34-43 References [2 JJ, Elliott, et al.

A Wideband SAW Convolver
utilizing Sezawa waves 1n
the +1etal-ZnD-5iO,-5icon
figuration Appl, Phys, Lett, 32. May 197
8. pp, 515-516 For details regarding the convolver characteristics of the structure shown in Figure 10, please refer to the following references [3]-
[Mentioned in #4.

Literature [3 books] S. Minagawa, et al.

Efficient ZnO-5iO,-5i Sez
awa wavecnvolver.

IEEE Dingrans, 5onics IJltr
ason,, vol, 5IJ-32°No, 5. Sep
tember 1985. pp, 670-674 References [
4] Japanese Patent Application Laid-Open No. 63-62281 (Patent Application No. 61-2074)
57) The structure is similar to that in Figure 10, but instead of the low concentration epitaxial layer/high concentration semiconductor substrate as shown in the figure, the structure uses a high concentration epitaxial layer/low concentration semiconductor substrate (however, the structure uses a high concentration epitaxial layer/low concentration semiconductor substrate) For an example of such a structure, please refer to the following reference [5].

Literature [5 Kuroda, et al. Analysis of the propagation characteristics of surface acoustic waves in ZnO/GaAs structures,” Japan Society for the Promotion of Science 131st Committee, Materials of the Acoustic Wave Device Subcommittee Study Group, 1988 1 However, the structure of document [5] has the disadvantage that, like the structure of FIG. 9, the convolution efficiency FT is small and it is not practical as a convolver structure.

In other words, in the conventional structure, the convolution efficiency F
At present, the structure shown in FIG. 10 is in practical use because of its high T. In particular, it is known that a high FT can be obtained when using Sl as the ZnO1 semiconductor for the piezoelectric film in the structure shown in FIG.
n○/Sin.

/n-S i epitaxial layer/n"-34 substrate structure has been put into practical use. This is detailed in the above-mentioned documents [3] and [4].

[Problems to be Solved by the Invention] However, the conventional structure shown in FIG. 10 also has drawbacks. In order to make the FT of the device sufficiently high and the temperature characteristics good, the thickness L of the epitaxial layer must be set to the maximum depletion width Wmax by Wmax(L≦Wmax+
The thickness needs to be about 2 μm. This is Si
In this case, it is shown that the thickness L of the epitaxial layer needs to be less than several μm (this point is also explained in detail in document [4]).

In practice, when forming a low-concentration Si epitaxial layer with a thickness of several μm or less on a high-concentration Si substrate, impurity diffusion occurs from the high-concentration substrate side to the epitaxial layer. layer)
It is not easy to ensure the reproducibility of the thickness L, and as a result, variations in device characteristics increase, which may cause a decrease in device manufacturing yield. In other words, in the conventional structure, even in the structure shown in FIG. 10, which has the highest FT, in order to increase the FT and improve the temperature characteristics,
There is a drawback that the yield may decrease.

[Object of the Invention] An object of the present invention is to provide a surface acoustic wave convolver that has high convolution efficiency, good temperature characteristics, and high manufacturing yield.

[Means for Solving the Problems] In order to achieve the above object, the present invention replaces the Si layer in the conventional monolithic SAW structure with GaAs.
The above-mentioned problems are solved by layering the structure.

[Effects] The mobility of GaAs used in the shrimp layer of the above SAW convolver structure is several times higher than that of 81, and therefore the loss in the shrimp layer can be made smaller than that of the conventional structure. It is possible to improve the convolution efficiency FT and the temperature characteristics.

[Embodiment] FIG. 1 is a sectional view showing the structure of a SAW convolver according to an embodiment of the present invention.

In the figure, 11 is a high concentration Si substrate, ]2 is a GaAs substrate.
Shrimp layer, 2 is an insulator, 3 is a piezoelectric film, 4 is a gate electrode, 5
is the comb-shaped electrode of the input transducer, 6 is the back electrode, and 7 is the comb-shaped electrode of the input transducer.
is an input terminal, and 8 is an output terminal.

The above structure is similar to the conventional structure shown in Fig. 10, but
In the figure, a high concentration semiconductor substrate 9 and a low concentration semiconductor layer 10 are shown.
are made of the same material, whereas in the structure shown in FIG.
) The shrimp layer 12 is made of a different material, which is a fundamentally different point.

In this case, as mentioned above, in the conventional structure shown in FIG.
The lattice constants of the shrimp layer and the substrate are the same, and a homojunction is formed, whereas in the structure shown in Figure 1, the materials of the shrimp layer and the substrate are different, so the lattice constants are different, and a heterojunction is formed. That will happen.

That is, in the structure shown in FIG. 1, a high concentration St substrate is used as the substrate, and a GaAs shrimp layer is used as the shrimp layer.

The technology for forming the GaAs layer on the 81 substrate is possible using techniques such as MOCVD, optical CVD, or MBE, which have been established in recent years, and techniques that combine these techniques.

The graphs in Figures 2 to 6 show the conventional structure (see Figure 10).
An example of comparing the characteristics in the case of the structure shown in FIG. 1 according to the present invention with the characteristics in the case of the structure shown in FIG. However, this applies to the following structure.

Conventional structure: Gate electrode...A1 Piezoelectric film...Zn○ (5μm) insulator...
......S10. (0.1μm) Shrimp layer-n-
S i (N d = 5 x 10°'an-')
Substrate---n-S i (N d = l
I O"am-") Structure of the present invention: Gate electrode...A1 Piezoelectric film...Zn○ (5 μm) Insulator...
・・・・・・SjO, (0,1 μm) Shrimp layer-n-G
aAs (Nd=5x 10"am-') substrate ---
−−−−−−n” −S i (N d = 1 x
10"an-") Here, Nd is the impurity (donor) density of each semiconductor layer. Moreover, the numerical values 5 μm and 0.1 μm are the thicknesses of each layer.

The graphs in FIGS. 2 to 6 are the results of simulations of characteristics when the frequency of the input signal is 215 MHz. For calculation formulas for simulation, please refer to the following two references:

Literature [6 books] S., Mitsutsuka et al.

“Propagation 1oss of 5urf
ace acoustic waves On a mo
nolithic metal-insulator-
Semiconductor 5structure Journal of Appl, Phys,, vol.
, 65. No, 2. January 1989, pp, 6
51-661° Reference [7] S, Minagawa, et al.

Efficient monolithic ZnO
/Si Sezawa Wave Convolve
r”, 1982 Ultrasonics Symp,
Proc, IEEE Cat, #82C) (18
23-41982, pp. 447-451 The graphs in FIGS. 2 and 3 compare the bias characteristics of convolution efficiency FT.

The same figure also shows the C-■ characteristic (relationship between the capacitance C between the gate electrode and the ground and the gate bias applied to the gate) for reference. In addition, in the same figure, the thickness of the shrimp layer L
The case where Wmax+1 μm is shown. here,
Wmax is the maximum depletion layer width, and the value when Nd=5×10” is the following value at room temperature.

Comparing the graphs in Figures 2 and 3, we find that in the case of the structure of the present invention, not only is the maximum value of FT F711aX slightly larger, but also the range of bias where FT becomes a larger value. It can be seen that the area is wide. Further, in the case of the structure of the present invention, it has been shown that the FT maintains a good value even if the bias is slightly deviated, and the present invention is advantageous over the conventional structure in this respect as well.

The graph in Figure 4 shows the thickness L of the shrimp layer and F 711aX
This shows the relationship between The horizontal axis is L-Wtaax. Looking at the same graph, in the conventional structure, F 7111aX suddenly decreases as the thickness L of the shrimp layer increases, whereas in the structure of the present invention, the dependence of F 711aX on L is small, and the thickness of the shrimp layer increases. Even if L increases by about 5 μm, F-thin WaX will only decrease by about 4 dBm (when the gate length is 40+m+). Even if there is some variation in the thickness L, there is no significant difference in F ymaX, which indicates that the yield during manufacturing can be improved in this respect.

Next, the graphs in FIGS. 5 and 6 compare the temperature dependence of F7max. Looking at the same graph, it can be seen that the structure of the present invention has a smaller temperature change in F 711aX, and therefore has better temperature characteristics than the conventional structure. In particular, it can be seen that in the conventional structure, the temperature characteristics deteriorate significantly even if the thickness L of the shrimp layer increases a little, whereas in the structure of the present invention, the dependence of the temperature characteristics is considerably smaller than in the conventional structure. This point also shows that in the present invention, even if there is some variation in the thickness L of the shrimp layer, there is little variation in temperature characteristics, and it is effective in improving the yield.

As shown in the graphs in Figures 2 to 6 above,
According to the present invention, the convolution efficiency FT is high;
It is possible to obtain a SAW convolver that has good temperature characteristics and can improve manufacturing yield.

In addition, in the graphs of FIGS. 2 to 6, n-type GaAs and D
Although an S1 type substrate is assumed, such an n-type semiconductor is advantageous when implementing the present invention. This is because in the case of GaAs, carrier mobility is higher than $1 because it is not holes but electrons. To give a numerical example, if the mobility of electrons is μe and the mobility of holes is μh, 380CIII/VS ILxahs As in the numerical example above, the mobility is higher when electrons are the majority carrier, so shrimp In the present invention, n-type Ga
It is for the above reasons that it is advantageous to use As and n-type Si.

Although the graphs in FIGS. 2 to 6 are examples in which ZnO is used as the piezoelectric film, it is also possible to use AIN as the piezoelectric film. In addition to using Sio as the insulating film, SiN, Al, and O are also used.

It is also possible to use These insulating films can be formed by sputtering, CVD, or the like. Also, G
By anodizing the aAs/Si substrate, Q a
It is also possible to form an insulator by forming a GaAs layer on the As surface.

The above has been described for the structure shown in Figure 1, but in principle it is also possible to have a structure in which the insulator 2 in the structure shown in Figure 1 is omitted, as shown in Figure 7. It is. 1st
The insulator in the structure shown in the figure is provided to stabilize the MOS characteristics of the semiconductor, and the basic operation of the convolver is that if a depletion layer is stably formed in the semiconductor, Basically, the presence or absence of an insulator has little effect on the convolution efficiency FT. Therefore, if the piezoelectric film 3 has sufficient insulating properties, a structure without an insulator as shown in FIG. It is also possible to do this.

In addition, Figure 1 and 11!71! In the structure of the present invention shown in +, in order to improve the crystallinity of the GaAs shrimp layer,
A structure in which a strained superlattice is provided at the GaAs/high concentration Si interface may be used. FIG. 8 shows a structure in which a strained superlattice layer 13 is provided in the structure of FIG. Since the layer 13 is an extremely thin layer, it hardly affects the characteristics of the convolver. However, as mentioned above, G a A
Since the crystallinity of the s-epi layer is improved, it is expected that the stability of device characteristics will increase and that it will contribute to an improvement in yield. It goes without saying that the strained superlattice can be applied to the structure shown in FIG.

[Effects of the Invention] As described above, the present invention has better convolution efficiency and better temperature characteristics than a monolithic SAW convolver with a conventional structure, and also has improved manufacturing yield. A high SAW convolver can be obtained.

Further, as an application of the SAW convolver according to the present invention,
It can be applied to all devices using SAW convolvers. Specifically, it can be widely applied to spread spectrum communication devices, correlators, radars, image processing, Fourier exchanges, etc.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a monolithic SAW convolver showing one embodiment of the present invention, No. 21! l is a graph showing the bias characteristics of the convolution efficiency of the conventional structure, Fig. 3 is a graph showing the bias characteristics of the convolution efficiency of the structure of the present invention, and Fig. 4 is a graph showing the relationship between the thickness of the shrimp layer and the maximum value of the convolution efficiency. Graph showing the relationship. Figure 5 is a graph showing a comparison of the temperature characteristics of the maximum convolution efficiency of the conventional structure and the present invention. Figure 6 is the conventional structure and the present invention structure (the epitaxial layer is different from Figure 5). FIG. 7 is a cross-sectional view of a monolithic SAW convolver showing another embodiment of the present invention, and FIG. 8 is a graph showing the temperature characteristics of the maximum convolution efficiency of the present invention.
The cross-sectional views of the convolver, Figures 9 and 10, show the conventional S
FIG. 3 is a cross-sectional view showing an AW convolver structure. ]...Semiconductor substrate, 2...
・Insulator, 3...Piezoelectric film, 4...
...Gate electrode, 5...Comb-shaped electrode,
6・・・・・・・・・Back electrode, 7・・・・・・・・・
Input terminal, 8... Output terminal, 9...
...High concentration semiconductor substrate, 10...
Low concentration semiconductor epitaxial layer, 11...
・Highly doped 81 substrate, 12...GaAs epitaxial layer, 13...strained superlattice.

Claims (4)

[Claims] (1) High concentration Si substrate and Ga formed on the substrate
It is characterized by comprising an As epitaxial layer, an insulator formed on the epitaxial layer, a piezoelectric film formed on the insulator, and an input transducer and an output gate formed in contact with the piezoelectric film. surface acoustic wave convolver. (2) High concentration Si substrate and Ga formed on the substrate
A surface acoustic wave convolver comprising an As epitaxial layer, a piezoelectric film formed on the epitaxial layer, and an input transducer and an output gate formed in contact with the piezoelectric film. (3) High concentration Si substrate and Ga formed on the substrate
The high concentration layer includes an As epitaxial layer, an insulator formed on the epitaxial layer, a piezoelectric film formed on the insulator, and an input transducer and an output gate formed in contact with the piezoelectric film. Si substrate and Ga
A surface acoustic wave convolver characterized in that a strained superlattice is interposed at the interface of an As epitaxial layer. (4) The GaAs epitaxial layer and high concentration Si
The surface acoustic wave convolver according to any one of claims 1 to 3, wherein both substrates are n-type semiconductors.