JPS59117819A - Series resistance type digital-analog converter - Google Patents

Abstract

PURPOSE:To improve the accuracy of D/A conversion without so much increase in the entire area by widening the area of through-holes at the end of a resistance layer and leaving the area of the through-hole at the middle as it is. CONSTITUTION:A through-hole N5' of the same size as that of a through-hole N5 at the end of the resistance layer 1 is added to the outside of the through- hole N5 so as to bring both through-holes N5, N5' in contact with each other. Then, the parasitic resistance RN5 of this part is reduced to nearly a half in a simple consideration (in terms of the area). The reason why the N5' is provided at the outside of the N5 is that it is not necessary to change the method setting the value of the resistor R4 depending on the interval between the through-holes N4 and N5. Since the area of the through-holes of the end of the resistance layer is widened and the area of the through-hole at the middle is kept unchanged, the accuracy of D/A conversion is improved without giving so much increase in the entire area.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a resistor series type D/A converter, and is particularly directed to reducing the influence of contact resistance when configuring this as an integrated circuit from a layout standpoint.

Conventional technology and problems In a resistor series D/A converter integrated with an IC, multiple diffused resistors or polycrystalline silicon resistors are arranged, connected in series, and lead lines are derived from many intermediate points. , a reference voltage is applied to both ends, and the lead line is selected by turning on/off a switch in accordance with the digital input, and a target voltage level (analog value) is outputted by resistor division. FIG. 1 shows an example of a pattern in which resistors are arranged in a 4×3 matrix, and FIG. 2 shows its equivalent circuit. In FIG. 1, 1 is a resistance layer made of a diffusion layer (or a polycrystalline silicon layer, etc.). Although not shown, in the case of a diffusion resistance, impurities are ion-implanted or diffused into a semiconductor substrate in a predetermined shape, and in the case of a polycrystalline silicon resistance, It is formed by depositing polycrystalline silicon on a semiconductor substrate via an insulating film and patterning it, and the surface is covered with the insulating film. Through holes N1-N5 for contacts are provided at equal intervals in the insulating film corresponding to both ends and multiple locations in the middle of these diffusion layers, thereby forming four series resistors R1 to R4 with equal resistance values. be done. 4 indicated by a broken line is a wiring layer connected to the resistance layer 1 through through holes N1-N3. The same applies to the diffusion layer 2.3, and a resistance voltage divider circuit is formed in which three resistance layers are connected in series by a wiring layer 4 connected to the resistance layer through a through hole, and a number of lead lines are led out from the series resistance layer. is configured. That is, 12 resistors R1 to RI2 connected in series and having the same resistance value are connected to the first reference voltage V1 at one end and the second reference voltage 2 at the other end, and are connected to the through holes N2 to N15 in these resistance layers. The wiring layer 4 drawn out from the circuit is selectively connected (depending on the digital input) to the output terminal OUT by a switch 5l-812. The maximum output of this D/A converter is digital human power 1
100 and the switch S1 is turned on, and the minimum output is obtained when the digital input is 0001 and the switch S1 is turned on. Then, by selecting the switches to be turned on in the order of 81□1 3111 3101 . . . , the output voltage increases from 0 step by step (in steps of 1/12) to 11 (Vl-V2)/12. Since there are 16 combinations of 4-bit binary signals, this can be accommodated by adding one more resistance layer to form a 4.times.4 matrix. Typically, a D/A converter is provided with an 8.times.8 or 16.times.8 matrix, or 64 or 128 step voltage divider circuit.

This example shows this in a simplified manner.

By the way, in the layout shown in FIG.
Through hole Nl where ~3 and metal wiring layer 4 are in contact
, N2. . . . , parasitic resistances RNI to RN15 occur due to the contacts, respectively. Of these, RNI, RN5. RN6. RNIO, RNII, RNI
5 is interposed in series with the current bus from the reference voltage v1 to V2, so if this value is large, the divided voltage will be distorted, causing an error in the output voltage. Normally, the dimensions of through-holes Nl and N2 are the resistance R+ + R21 formed between them.
are set uniformly to make the values equal, and the area is reduced to increase density. For this reason, the value of the parasitic resistance may reach about 30Ω, and in this case, if the value of the voltage dividing resistors R1, R2, etc. is as small as about several hundred Ω (resistances R1, R2, etc.). (If there are a large number of resistors, the value of each resistor will tend to be small, but the overall resistance will still be quite high), and the value of the parasitic resistor cannot be ignored.

OBJECTS OF THE INVENTION The present invention attempts to improve the above-mentioned problems from the layout perspective without significantly increasing the overall area.

Structure of the Invention The present invention comprises forming a plurality of resistance layers such as diffusion layers or polycrystalline silicon layers on a substrate, and forming through holes in an insulating film covering each resistance layer for contact between the resistance layers and wiring layers. A reference voltage is applied to both ends of the series resistance layer by providing a plurality of each, connecting the plurality of resistance layers in series by wiring through the through holes, and leading out a large number of lead wires from the series resistance layer. D/ that allows resistance division in stages
In the A converter, the through ball located at the end of each resistance layer and used for connection with the reference voltage source or other resistance layer is characterized by having a larger area than the through hole in the middle of each resistance layer. However, this will be explained in detail below with reference to the illustrated embodiment.

Embodiments of the invention FIGS. 3(A) and 3(B) are plan views showing different embodiments of the present invention. The example shown in FIG.
(Same goes for Nl, N6.NIO, Ni1.N15) A through hole N5' of the same size is added to the outside of Nl, N6.NIO, Ni1.N15, and both through holes N5. Contact is made through N5'. As a result, the parasitic resistance RN5 in this part can be simply considered (in terms of area) as 1
/2. The reason why N5' is provided outside N5 is that it is not necessary to change the method of setting the value of resistor R4 based on the distance between through holes N4 and N5. The same figure (B
) is the through hole N6 (N'l, N5
．． Nl O, Nl 1. The same applies to Nl 5), which is expanded outward for the same reason. Even in this case, the distance between the through holes N7 and N6 is maintained at a length that forms the resistor R5.

In addition, another method can be considered in which the contact resistance value is corrected using resistance segments. That is, (R++RN1)
=R2=R3= (Ra+RN5)= (R5+RN6
)=...-R11-(R1°+RN15). However, this method is difficult to implement because the contact resistance value is unstable during manufacturing.

Effects of the Invention As described above, in the present invention, the through holes Nl, N5 . The area of N6 etc. is widened and the through hole N2. N3. Since the area of , etc. is left as is,
The D/A conversion accuracy can be improved without significantly increasing the overall area (although there is a slight increase in area since each resistor layer is lengthened to correspond to the enlarged through hole).

[Brief explanation of the drawing]

Figures 1 and 2 are a planar pattern diagram and an equivalent circuit diagram showing an example of a resistor series type D/A converter, and Figure 3 (A
)(B) is a plan pattern diagram of a main part showing each embodiment of the present invention. In the figure, 1 to 3 are resistance layers, 4 is a wiring layer, Nl, N5,...
... is a through hole at the end, N2. N3...
. is a through hole in the middle, and R1, R2, . . . are voltage dividing resistors. Applicant Fujitsu Ltd. Representative Patent Attorney Minoru Aoyagi Figure 1 ■1 - Figure 2

Claims (1)

[Claims] (11) A plurality of resistance layers such as diffusion layers or polycrystalline silicon layers are formed on a substrate, and an insulating film covering each resistance layer has a through hole for contacting the resistance layer and a wiring layer. A plurality of resistor layers are connected in series by wiring through the through holes, and a large number of lead wires are led out from the series resistor layer and are added to both ends of the series resistor layer. D/A that enables step-by-step resistance division of reference voltage
A resistor characterized in that in a converter, a through hole at the end of each resistor layer used for connection with a reference voltage source or another resistor layer has a larger area than a through hole in the middle of each resistor layer. Series type D/A converter. (2) The resistor series type D/A converter according to claim 1, wherein the end portion of the resistive layer is extended, and a through hole with a large area is provided at the end portion and the extended portion.