JPH02305460A - Semiconductor integrated circuit - Google Patents

Abstract

PURPOSE:To lessen a capacitor in forming area and to enable a chopper type comparator to operate at a high speed by a method wherein the capacitor accumulating an analog voltage is composed of a first and a second electrode, and a semiconductor is biased by a power source independent of that of a circuit which deals with a digital signal. CONSTITUTION:A certain conductivity type semiconductor substrate 14, a reverse type conductivity type semiconductor layer 15 formed on the substrate 14, a capacitor first electrode 17 formed of a first layer polysilicon provided onto the semiconductor layer 15 through the intermediary of a field oxide film 16, and a capacitor second electrode 19 formed of a second layer polysilicon provided onto the first electrode 17 through the intermediary of an insulating film 18 are provided. The semiconductor layer 15 is biased by a power source independent of that of a circuit which deals with a digital signal. By this setup, digital noises are prevented from affecting the capacitor of an analog circuit and the capacitor is not required to be made unneessarily large, so that the capacitor can be made small in forming area and a chopper type comparator can be made to operate at a high speed.

Description

Detailed Description of the Invention (a) Field of Industrial Application The present invention relates to a semiconductor integrated circuit in which a circuit that handles digital signals and a circuit that handles analog signals coexist, and in particular, the structure of a capacitor used in an analog circuit. Regarding.

(b) Conventional technology In general, in integrated circuits that convert analog signals such as video signals into digital signals and perform digital signal processing, analog and digital circuits are mixed on one semiconductor substrate. It is formed.

Conventionally, in the above-mentioned integrated circuit, an A/D conversion circuit using a chopper type comparator is often used.

Figure 2 is a circuit diagram of a chopper type comparator, (
1) is the analog input voltage ■1. (2) is an analog gate that selects reference voltages V and E, (3) is a capacitor whose one electrode is connected to the output of analog gates (1) and (2), and (4) is an analog gate that selects I. ) is an inverting amplifier circuit whose input is connected to the other electrode of the capacitor (3), (5) is an analog gate connected between the input and output of the inverting amplifier circuit (4), and (6) is an inverting amplifier circuit ( 4) is an inverter to which the output is applied.

To briefly explain the operation of the circuit in Fig. 2, during the sample period, the analog gate (1) is OFF, and the analog gate (2) is OFF.
) and the analog gate (5) are turned on, the voltage across the capacitor (3) becomes VTII-VIEP (V T□
is the threshold voltage of the inverting amplifier circuit (4). In this state, the analog gate (1) is turned on in the next comparison period.
, when analog gates (2) and (5) are turned off,
The input voltage of the inverting amplifier circuit (4) is connected to the analog input terminal■
The voltage obtained by superimposing the voltage charged in the capacitor (3) becomes (VIJI VREF) +VTH. Therefore, depending on whether V,, -VRE, becomes positive or negative, the output of the inverting amplifier circuit (4) is shifted by 11 degrees in the direction of "L" level or 'H' level, and Vls from the inverter (6). The comparison result between VIIEF and VIIEF is output.

The capacitor (3) used in the chopper type comparator is formed as shown in Figure 3! L. A first electrode (9) made of a first layer of polysilicon is provided on a field oxide film (8) formed on the main surface of an N-type semiconductor substrate (7) by the well-known LOGO5 technique. Furthermore, the tth
On the t-electrode (9), the second to first electrodes (
11) is formed by the second layer polysilicon. Further, on the second electrode (11), a third tTh (13) made of aluminum is placed on the first electrode (9) via an oxide film (12).
A contact hole 11 is connected to the contact hole. With this structure, a capacitor formed by the first electrode (9) and the second electrode (11) and a capacitor formed by the 21st electrode (11) and the third electrode (]3) are connected in parallel. becomes.

(c) Problems to be solved by the invention The semiconductor substrate (7) shown in Figure 3 is biased to the power supply voltage, and a capacitor also exists between the semiconductor substrate (7) and the first electrode (9). do. Normally, the power supply voltage is commonly applied to the digital circuit section and the analog circuit section, so if a fluctuation in the power supply voltage occurs, such as when many MOS transistors are turned on at the same time in the digital circuit section,
There is a drawback that the voltage charged in the capacitor (3) of the chopper type comparator changes due to the capacitor between the semiconductor substrate (7) and the first t-pole (9), and the closing degree of the chopper type comparator deteriorates. In order to reduce the influence of such digital noise, it is possible to increase the capacitance of capacitor (3).
This is undesirable because it occupies a large area on the semiconductor substrate and slows down the operating speed of the chopper type comparator.

(d) Means for solving the problem The present invention was created in view of the above points, and -
A 4-electrode type semiconductor substrate and an inverted 4-electrode type semiconductor substrate formed on the semiconductor substrate.
Electric model half 1! a first electrode of a capacitor formed of first layer polysilicon provided on the semiconductor layer via a field oxide film; and a second layer of polysilicon provided on the first electrode via an insulating film. and a 21st electrode of a capacitor made of polysilicon, and by biasing the semiconductor layer with a power supply independent of the power supply of the circuit that handles the digital signal, the voltage accumulated in the capacitor of the Chompa type comparator due to digital noise can be reduced. The purpose is to prevent fluctuations in

(E) Effect According to the above-mentioned means, there is a semiconductor layer biased to a voltage that is separate and independent from the power supply of the digital circuit section, and this semiconductor layer is present directly under the first, second, and third electrodes. This prevents the first, second, and third electrodes from being affected by fluctuations in the power supply voltage applied to the semiconductor substrate.

(F) Embodiment FIG. 1 is a sectional view showing an embodiment of the present invention. The semiconductor substrate (14) contains N-type impurities, has a digital circuit and an analog circuit (not shown) formed on its main surface, and has a common voltage VDD for the digital circuit and the analog circuit.
is applied. A P-WELL (15) containing P-type impurities is formed in the semiconductor substrate (14) on which the capacitor of the chopper comparator is formed, and P-WE L L
A field oxide film (I6) is formed by the well-known LOGO5 technique except for a part of (15). A first t-pole (17) is formed on the field oxide film (16) by selectively etching the first layer polysilicon forming the gate of the MOS transistor. A silicon oxide film (18) is provided by a CVD method to cover this first electrode (17), and a silicon oxide film (18) is further provided.
A second electrode (19) is formed at a position overlapping with the first electrode (17) via selective etching of the second layer polysilicon for wiring. This second''
One pole (19) is the inverting amplifier circuit (4) shown in Figure 2.
The gate is extended and connected to the gate made of the first layer of polysilicon. Further, a silicon oxide film (20) is formed on the second layer electrode (19) by the CVD method, and a third electrode (21) made of aluminum is formed through the silicon oxide film (20). (18) is connected to the first electric scraper (17) through the contact hole formed in (20). This third electrode (21) extends over the silicon oxide film (20) and is connected to each output of the analog gates (1) and (2) shown in FIG. On the other hand, P-
A P+ type contact region (22) is formed in the WE L L (14), and a silicon oxide film (18) (20) is formed.
Power supply type fJii (2
3). @The source electrode (23) is the third electrode (2
1) At the same time, it is made of aluminum and has a ground power supply AV5. is extended and connected to a power supply band (not shown) to which is connected. The ground power supply AvSs is provided completely independently of the ground power supply D Vss of the digital circuit.

According to the configuration shown in FIG. 1, the first electrode (17) and the second electrode (
19) and a second electrode (19).
and a capacitor formed by the 31st electrode (21) are connected in parallel to form a capacitor of a chopper type comparator. Furthermore, the first electrode (17) and P-WELL (1
4) A capacitor is formed between P-WELL and P-WELL.
(14) is biased to the analog ground voltage A V s s , so the power supply voltage V
Fluctuations in oo D V ss have no effect on the voltage stored in the capacitor.

(G) Effects of the Invention According to the above-described invention, the influence of digital noise on the capacitor of an analog circuit is eliminated, and there is no need to increase the capacitance of the capacitor more than necessary, which reduces the area in which the capacitor is formed. A semiconductor integrated circuit with a built-in high-speed, high-precision chopper-type comparator can be realized.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the present invention, FIG. 2 is a circuit diagram of a chopper type comparator, and FIG. 3 is a sectional view showing a conventional example.

Claims (3)

[Claims] (1) In a semiconductor integrated circuit in which a circuit that handles digital signals and a circuit that handles analog signals coexist, a semiconductor substrate of one conductivity type, a semiconductor layer of the opposite conductivity type formed on the semiconductor substrate, and a semiconductor layer on the semiconductor layer. The first layer is provided through a field oxide film.
A first electrode formed of a layer of polysilicon, and a second electrode formed of a second layer of polysilicon and provided on the first electrode with an insulating film interposed between the first electrode and the second layer of polysilicon. A semiconductor integrated circuit comprising a capacitor that stores an analog voltage with two electrodes, and wherein the semiconductor layer is biased with a power source independent of a power source of a circuit that handles the digital signal. (2) The semiconductor integrated circuit according to claim 1, wherein the second electrode is connected to a first layer of polysilicon extending from a gate electrode of the inverter. (3) A third electrode provided on the second electrode via an insulating film, connected to the first electrode, and extending and connected to a switching element that selectively outputs an analog input voltage and a reference voltage. 2. The semiconductor integrated circuit according to claim 1, further comprising an electrode, and the third electrode is one electrode of a capacitor.