1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, the present invention relates to a resistor circuit to be used in a semiconductor device.
2. Description of the Related Art
Resistors to be used in a semiconductor device or a semiconductor integrated circuit include a diffused resistor in which impurities are implanted into a monocrystalline silicon semiconductor substrate, which have a conductive type opposite to that of the semiconductor substrate, and a polycrystalline silicon resistor formed of polycrystalline silicon having impurities implanted thereinto.
FIG. 2A is a plan view illustrating a related-art resistor circuit in which resistor elements formed of polycrystalline silicon are two-dimensionally arranged. FIG. 2B is a sectional view taken along the line A-A′ of FIG. 2A.
A high concentration impurity region 6 and a low concentration impurity region 7 are formed in a polycrystalline silicon film forming the resistor elements. A resistance value of the resistor element depends on a resistivity, a length, and a width of the low concentration impurity region 7, and the resistivity of the low concentration impurity region 7 is determined by an impurity concentration of the low concentration impurity region 7 having a high resistance. The high concentration impurity region 6 is used for ohmic connection with a metal wiring.
An intermediate insulating film 8 is formed on the resistor elements, and electrical connection of the respective resistor elements is made by metal wirings 10 via contact holes 9. The resistor circuit to be used in a semiconductor integrated circuit is formed by connecting the plurality of resistor elements illustrated in FIGS. 2A and 2B in series or in parallel on a surface of the same substrate as, for example, illustrated in FIG. 3 via the metal wirings.
The intermediate insulating film 8 formed on the resistor elements contains boron or phosphorus, and is planarized through heat treatment at 850° C. or higher to reduce a level difference due to the film pattern in the semiconductor integrated circuit. Further, after the metal wirings are formed, a film 11 as a protective film such as a silicon nitride film is formed thereon.
The resistor elements that are laid out to form the resistor circuit have the same shape in plan including a width and a length. This causes the respective resistor elements to equally suffer from variations in shape in an etching process that defines the shape, and resistance ratios can be held constant among the resistor elements.
At that time, when resistance values of the resistor elements and the ratios among the resistor elements are changed in accordance with requirement by the resistor circuit, the changes are realized by connecting in parallel or in series the resistor elements having the same shape as illustrated in FIG. 3. In this case, in order to realize a resistor circuit having resistance values of 4R, 2R, 1R, and 1/2R (R is a resistance value of one resistor element) in FIG. 3, four resistor elements connected in series, two resistor elements connected in series, one resistor element, and two resistor elements connected in parallel are connected. By adjusting the resistance values by resistor groups 201 to 204 including the plurality of resistor elements and forming the resistor circuit of the resistor groups 201 to 204, both of desired resistance ratios and high accuracy of the resistance ratios are attained.
Further, in order to enhance the accuracy of the resistance values, in addition to uniformization of the processed shape, reduction of influence of an ambient voltage and stabilization thereof are necessary. The reason is that, because a polycrystalline silicon thin film is a semiconductor, depletion and accumulation phenomena are caused by an ambient potential to change the resistance value. Means for solving this problem is also included in FIGS. 2A and 2B.
First, with reference to FIG. 2A, by forming the metal wirings over the respective resistor groups of the resistor circuit and applying a predetermined voltage, a voltage around the resistor element is stabilized and the extent of depletion and accumulation with regard to the resistor elements is fixed to a predetermined value. Further, as can be seen in FIG. 2B, the metal wirings over the resistor elements are formed over the respective resistor groups so as to cover the resistor elements via the intermediate insulating film 8.
Next, with regard to the potentials of the metal wirings over the resistor elements, by applying a potential from one terminal of each of the resistor groups, the potential applied is close to the potential of each of the resistor groups, and thus, the influence of the ambient voltage is minimized and the extent of the depletion and the accumulation is minimized.
On the other hand, no special measures are taken with regard to a lower side of the resistor groups in FIGS. 2A and 2B in the semiconductor substrate. Such a method is sometimes adopted that a well region or a polycrystalline silicon electrode is formed on the lower side for each of the resistor groups and a potential thereof is applied from one terminal of each of the resistor groups. An effect of maintaining the accuracy in this method is enhanced more as the voltage applied to the resistor circuit becomes larger (see, for example, Japanese Patent Application Laid-open No. Hei 09-321229).
Manufacture of a resistor element in a related-art semiconductor device has the following problem.
A metal wiring formed on a resistor element, which is formed of polycrystalline silicon, has a membrane stress that is determined by a specific linear expansion coefficient and a formation temperature. Therefore, when a metal wiring is formed with respect to each of the resistor groups, a stress corresponding to an area thereof is applied to the resistor group thereunder, and the resistance value of polycrystalline silicon changes due to a piezoresistance effect. As a result, the resistance values of the respective resistor groups deviate from a desired design value and a balance among the resistance ratios in the resistor circuit is lost.
The stress also varies depending on the kind of the metal. When a film that is liable to shrink due to heat such as a film formed of a high melting point metal is adopted, the influence described above becomes prominent.
Therefore, a related-art resistor circuit in which metal films are separately formed for the respective resistor groups has a problem in that enhancing the accuracy of the resistance ratios is difficult.