JPS6028399B2 - charge transfer device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a charge transfer device having an SOS-MOS structure that has directionality in charge transfer.

A charge transfer device creates a depletion layer region on the surface of a silicon substrate by applying a voltage to an electrode of a MOS structure, accumulates minority charges, and moves the charged electrode.
Some methods move the depletion layer region along the surface of the silicon substrate and sequentially transfer accumulated charges. As a means for moving a charging electrode in a charge transfer device, a method using a three-phase clock and a method using a two-phase clock are used.

The method using a two-phase clock is easy to excite and is suitable for small-sized devices, but on the other hand, special measures are required to prevent reverse flow of charges.

In a charge transfer device, when a certain voltage is applied to the electrodes, the depth of the potential generated in the surface area of the silicon substrate is inversely related to the threshold voltage V.
It is known that th usually follows the following relationship.

Vth NiF (NB.tox) NiVG1■'S, [1'
Here, F represents a function of the impurity concentration NB of the silicon substrate and the thickness x of the oxide formed on the substrate surface, VG represents the voltage applied to the above electrode, and ■'s represents the depth of the above potential. It's the amount.

Since charge transfer occurs in the deep direction of the potential,
Therefore, in order to give directionality to charge transfer and prevent charge backflow, it is necessary to use some means to threshold the value voltage Vth.
All you have to do is change the structure and create a structural asymmetry.
FIGS. 1 and 2 show an example of a secondary charge transfer device made in this manner.

In the example of FIG. 1, the thickness bx of the oxide film 2 formed on the silicon substrate 1 is partially different for each element, and in the example of FIG.
There is an impurity concentration N in a part of the silicon substrate 1 under
A large portion 4 of B is provided.

Therefore, from the relationship of equation '1}, in the example of FIG. 1, the oxide film is applied to the thick part, in the example of FIG. 2, the impurity concentration is large, and to the silicon substrate under the electrode.
A portion with a large value voltage is generated, and the resulting asymmetry of the potential level causes directionality in the movement of charge when a two-phase clock is applied to terminals (2), (2), and (2).

However, partially changing the thickness of the oxide film or changing the impurity concentration of a part of the silicon substrate requires complicated processes and technical difficulties in the former case, and requires a mask in the latter case. As a result of requiring multiple types, the manufacturing process becomes extremely complicated, and in any case, there is a drawback that this tends to lead to an increase in price.

An object of the present invention is to provide a charge transfer device that eliminates the drawbacks of the prior art, and as a means thereof, S
A charge transfer device having an OS-MOS structure is characterized in that the threshold voltage is controlled by selectively changing the thickness of the silicon epitaxial growth layer to give directionality to charge transfer. In the SOS-MOS structure, a layer of silicon is epitaxially grown on an insulating substrate such as saphire or spinel, and the surface of this silicon epitaxially grown layer is oxidized to form an oxide film.

The threshold voltage yth of the silicon epitaxially grown layer in such an SOS-MOS structure gradually increases as its thickness t decreases.

FIG. 3 is an example of data showing the relationship between such threshold voltage Vth and thickness t. This is considered to be based on the fact that lattice defects in a silicon single crystal layer formed by epitaxial growth are most numerous near the interface and gradually decrease as the thickness increases. The present invention utilizes this phenomenon to control the threshold voltage of the silicon epitaxial growth layer, thereby creating directionality in charge transfer.

Hereinafter, the present invention will be described in detail with reference to Examples.

FIG. 4a is a sectional view showing the structure of an embodiment of the charge transfer device of the present invention.

In FIG. 4a, reference numeral 11 denotes a substrate made of Safya or the like, on which a silicon epitaxial growth layer 12 is formed.The surface of the growth layer 12 is uniformly oxidized to form an oxide film 13, and further on Electrodes 14-,, 14-2, 14-3, 14-4,... made of metal such as aluminum
... is attached. The thickness of the growth layer 12 is partially different, with a thin part 12-,, and a thick part 12-2,
have a thickness of t2, and the thin portions and thick portions are arranged alternately, and electrodes are provided for each set of these and are repeatedly arranged in the same direction.

Now, the terminal ■, connected to every other electrode 14-,, 14-3, . . . and the electrodes 14-2, 14-4, .
A two-phase clock is applied to each of the terminals connected to terminals 2 and 2 at once.

When a clock voltage is applied to terminal 2, but not to terminal 2, the potential in the silicon substrate is equal to the part 1 of the thin growth layer under the electrode connected to terminal 2.
2-, and the thick growth layer portion 12-2. However, this relationship is similar to the relationship between the threshold voltage Vth and Vt for the thickness and thickness shown in FIG.
Compatible. Therefore, electrodes 14-,, 14-3,...
As shown in Fig. 4b, the potential generated in the growth layer below ... occurs in stages, and is deep in the thick growth layer part 12-2, where minority charges are present in the depletion layer. accumulate. Next, a clock voltage is applied to terminal ■2,
When no voltage is applied to terminal ○, a similar potential is generated in the growth layer under the electrode connected to terminal ○2, as shown in Figure 4 (d), but in the transition period, as shown in Figure 4 (c). This gives rise to different distributions in stages.

Therefore, the charge accumulated in the portion of the thick growth layer under the electrode connected to terminal 2 easily moves to the portion of the thick growth layer under the electrode connected to terminal 2. In this way, charges are sequentially transferred to the right as the two-phase clock voltages at the terminals 1 and 2 are turned on and off, and the device shown in FIG. 4a operates as a charge transfer device.

The charge transfer device shown in FIG. 4a is manufactured in the following order. First, a silicon epitaxial growth layer is formed to a uniform thickness on a substrate such as Safya. Next, a resist layer is provided at a position corresponding to the thick growth layer, and then etching is performed to thin the thin growth layer. After the resist layer is removed, it is oxidized to form an oxide film with a uniform thickness over the entire surface of the grown layer. After coating this with a metal such as aluminum, the metal film is cut at the boundary between the elements to form an electrode. As explained above, the charge transfer device of the present invention selects the thickness of the grown layer by utilizing the fact that the threshold voltage of the silicon epitaxial growth layer in the SOS-MOS structure differs depending on the thickness of the grown layer. By controlling the method, a charge transfer device can be formed with a simple structure, and its manufacturing process is simple, so its practical value is extremely great.

[Brief Description of the Drawings] FIGS. 1 and 2 are diagrams showing the basic structure of a conventional charge transfer device, and FIG. 3 is a diagram showing the relationship between threshold voltage and thickness in a silicon epitaxial growth layer. FIG. 4 is a sectional view showing the structure of an embodiment of the charge transfer device of the present invention. 1: Silicon substrate, 2: Oxide film, 3: Electrode, 4: Portion with high impurity concentration, 11: Substrate such as Safya, 12:
Silicon epitaxial growth layer, 12-,: thin growth layer portion, 12-2...: thick growth layer portion, 13: oxide film, 14-,, 14-2, 14-3, 14-4,・・・
·:electrode. Gar bait, 2 captive moths, 3 figure moths, 4 boxes

Claims (1)

[Claims] 1. an epitaxial layer formed on an insulating substrate; an insulating film with a uniform thickness formed on the epitaxial layer;
The epitaxial layer has a plurality of electrodes provided on the insulating film, the epitaxial layer has thick parts and thin parts alternately, and the electrodes are provided on the thick part and the electrodes are provided on the thin part. A charge transfer device characterized in that the charge transfer device is configured such that signals of different phases are applied to the electrodes.