GB2253941A

GB2253941A - Hall-effect sensors incorporated in CMOS integrated circuits

Info

Publication number: GB2253941A
Application number: GB9205801A
Authority: GB
Inventors: Silvo Zlebir; Andrej Dipl Ing Belic
Original assignee: ISKRA STEVCI IND MERILNE IN UP
Current assignee: ISKRA STEVCI IND MERILNE IN UP
Priority date: 1991-03-18
Filing date: 1992-03-17
Publication date: 1992-09-23
Anticipated expiration: 2012-03-17
Also published as: FR2674375B1; GB2253941A8; SI9110476A; DE4208544A1; GB9205801D0; FR2674375A1; GB2253941B

Abstract

A Hall-effect sensor HS' incorporated in a CMOS integrated circuit IC' is formed with a well 2' as the sensor active layer in a substrate 1'. Heavily doped regions 31' to 34' in the well 2' are connected to respective sensor metal contacts 41' to 44'. The upper surface S' of the substrate 1' is covered by a field oxide layer 5' having a thickness between 0.8 mu m and 1.0 mu m. Over the field oxide layer 5' in a region 50' surrounding the sensor contacts 41' to 44', a polysilicon blocking layer 6' is provided to block the disturbing influence of ions migrating in the field oxide layer 5'. In another embodiment, the blocking layer is formed under the field oxide layer. <IMAGE>

Description

2253941 1 HALL-EFFECT SENSORS INCORPORATED IN CMOS INTEGRATED CIRCUITS

This invention relates to Hall-effect sensors incorporated in 5 CMOS integrated circuits.

Semiconductor Hall-effect sensors have previously been fabricated predominantly on silicon by bipolar technology. For that purpose a-. portion of an epitaxially grown layer, for example of the n-type, is used as the active layer of the Hall-effect sensor. This portion is isolated from other portions of the integrated circuit by a p-isolation wall, which is accomplished by the acceptor diffusion into the epitaxial layer, and is disconnected from the p-type substrate by a pnjunction. The metal electric contacts of the Hall-effect sensor are connected with n- type regions, which are heavily doped and are formed by the diffusion of donor impurities. Therefore the properties of the Hall-effect sensor prevailingly depend on the properties of the epitaxial layer. Normally the donor concentration therein is 1016 cm-3, and the thickness thereof is between 8 lAm and 15 pm, depending on the requirements put on other integrated circuit components. Therefore the electric power consumption in such a Halleffect sensor and in the rest of the integrated circuit fabricated by the bipolar technology is high. Moreover, the electric power consumption and the sensitivity of such a Hall-effect sensor are not reproducible since the thickness of the epitaxial layer is not constant. This represents a serious disadvantage when the sensor is to be used as a current input and a multiplier in a monolitic integrated circuit for an electric wattmeter.

Additionally to the above factor and to the piezoresistive effect, the stability of the Hall-effect sensor is influenced above all by surface effects. In the protective field effect S'02 layer usually covering an integrated circuit, various impurity ions, such as sodium ions, are present. Under the influence of the electric field and due to the thermal movement, the ions migrate to the active layer of the Hall- effect sensor and they induce therein an electric charge thus influencing the dependence of the resistivity on the depth t as measured with respect to the surface of the active layer of the Halleffect sensor, and therefore also the sensor sensitivity. In US Patent 2 C_ No. 4 66o o65 a Hall-effect sensor is described which is fabricated in bipolar technology and in which, before the formation of the protective oxide layer, a p-type layer is implanted. Thus a direct contact of the sensor active layer with the layer of the protective oxide on the surface is prevented. For the fabrication of the protective layer an additional step in the technological process is needed. Furthermore,. the active sensor layer is narrowed and the Hall-effect sensor, because of higher resistivity e (t), becomes highly nonlinear, which makes it unsuitable for a monolithic integrated circuit for an electric 10 wattmeter.

According to the present invention there is provided a Halleffect sensor incorporated in a CMOS integrated circuit, the sensor comprising:

a substrate of a first electrical conductivity type having a well of a second electrical conductivity type formed on the substrate; two first and two second mutually separated and heavily doped regions of the second conductivity type formed in the well such that respective lines joining the two first regions and the two second regions are mutually perpendicular, respective metal electric supply contacts and pick-up contacts being provided over the two first and two second regions, respectively; a field oxide layer covering the upper surface of the substrate except at the locations of the supply and pick-up contacts, the thickness of the field oxide layer being between 0.8 /Am and 1.0 jAm; and a blocking layer formed adjacent the field oxide layer in a region at least between the supply and pick-up contacts, for blocking the disturbing influence of ions migrating in the field oxide layer.

In accordance with preferred embodiments of this invention, a Hall-effect sensor is provided which is incorporated in a CMOS integrated circuit and in which, by providing an appropriate layer adjacent to the field oxide layer that covers the part of the Halleffect sensor surrounding sensor contacts, and possibly by an appropriate connection of the Hall-effect sensor, disturbing influences of the ions migrating in this field oxide layer to be exerted on the sensor active layer may be blocked.

In a first embodiment, the blocking layer is an n-type layer 3 ir- formed under the field oxide layer in a region situated predominantly within the p-type well, the blocking layer only partly extending into the substrate.

In a second embodiment, the blocking layer is of highly conductive polysilicon formed over the field oxide layer, the blocking layer being connected to the integrated circuit earth; one pick up contact is connected to the virtual earth of the electric supply contacts of the sensor.

Advantageously, protection of the active layer of the Hall-sensor may be accomplished in that the protective layer is produced in a simple process step within the CMOS technology.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:

Fig. 1 represents a transversal cross-section of a Hall-effect sensor in an integrated CMOS circuit according to a first embodiment of the invention; Fig. 2a represents a transversal cross-section of a Hall-effect sensor in an integrated CMOS circuit according to a second embodiment of the invention; Fig. 2b is a schematic representation of the connection of the second embodiment of the Hall-effect sensor shown in Fig. 2a within the integrated circuit and of its connection to an external current generator; Fig. 3 is a graph of the concentration of the electric charge carriers vs the distance from the upper surface of the substrate in the middle of the Hall-effect sensor represented in Fig. 1; Fig. 4 is a graph of the concentration of the electric charge carriers vs the distance from the upper surface of the substrate in the middle of the Hall-effect sensor represented in Fig. 2a; and Fig. 5 represents the second embodiment of the Hall-effect sensor in operation.

First and second respective embodiments of Hall-effect sensors HS; HSI which are incorporated in integrated CMOS circuits IC and ICI, respectively, are represented in Fig. 1 and Fig. 2a, respectively. In view of the similarities between the embodiments, their common features will be described together. On a substrate 1; 1 1 of a first electrical 4 conductivity type, a well 2; 21 of a second electrical conductivity type is formed. In practice, the well 2; 21 is the active region of the Halleffect sensor HS; HS'. In the well 2; 2', four separated and heavily doped regions 31. 34; 3 1',.... 341 of the second electrical conductivity type are formed. The line connecting two regions 31, 32; 31', 32' is perpendicular to the line connecting two other regions 33, 34; 33', 341. Over the regions 31, 34; 31..... 341 metal electric contacts 41, 44; 411. 44' of the Hall-effect sensor HS; HS' are provided, forming supply contacts 41, 42; 4V, 42' and pick-up contacts 43, 44; 43', 441. The pick-up contacts 43, 44; 43', 44' and the related regions 33, 34; 33', 341 are situated in front of and behind the plane of the drawing, respectively, on the symmetry axis of the line connecting the supply contacts 41, 42; 411 42' and for this reason they are represented in Fig. 1 and Fig. 2a, respectively, by dotted lines.

The upper plane S; 51 of the substrate 1; 1' is fully covered by a field oxide 5'02 layer, 5; 51 expect at the locations of the metal contacts 41. 44; 411, 441 of the Hall-effect sensor H5; H51. The thickness of the field oxide layer 5; 5' is between

0.8 A m and 1.0 Atm. Adjacent to the region of the field oxide layer 5; 5' which surrounds the metal contacts 41,..., 44; 411,..., 441 of the Hall-ef feet sensor H5; HS 1, and depending on the actual embodiment, either under that region of the layer 5 or over that region of the layer 51, a layer 6; 61 is formed which blocks a disturbing influence of ions migrating in the field oxide layer 5; 51 upon the active layer of the Hall-effect sensor HS; H51.

C In the first embodiment of the Hpll-effect sensor HS of the invention the n-type layer 6 is formed in the region 50 under the field oxide layer 5 so that it is situated predominantly within the p-type well 2 and that with its periphery it partly extends into the substrate 1. Since in the first embodiment of the Hall-effect sensor HS the substrate 1 is of ntype as well, the layer 6 and the substrate 1 are electrically connected.

In the second embodiment of the Hall-effect sensor HS' of the invention the layer 6' is formed of a highly conductive polysilicon and is placed over the field oxide layer 5' in the region 50'which is essentially located between the metal contacts 41...... 44'ofthe Hall-effect sensor HS'. The layer 6' is connected to the mass of the integrated circuit IC. The pick-up electric contact 43' (Fig. 2b) of the Hall-effect sensor HS' must be con- nected to the virtual mass (i.e. earth) of the supply electric contacts 4V, 421 of the liall-effect sen- sor HS'. Namely, the polysilicon layer 6' only exerts a blocking action when the supply contact 41' of the Hall-effect sensor HS' is connected to an external current generator G of the sine current I (Fig. 2b). The second supply contact 42' of the Hall-effect sensor HS' is connected to the output of an operational amplifier OA, which is also fabricated in the integrated circuit 1C. The noninverting input of the operational amplifier OA is connected to the mass, its inverting input, however, is connected to the first pick-up contact 43' of the Hall-effect sensor HS'. Thereby the connection of the contact 43' to the virtual mass of the energization in a wattmeter or in an electric meter fabricated with the Hall-effect sensor HS' incorporated in the CMOS integrated circuit IC is provided. The Hall current 1 H originating from the pick-up contact 44' is conducted further into the integrated circuit IC.

The sheet resistivity of the well 2; 2' functioning as the active layer of the Hall-effect sensor HS; HS' is approximately 2 to 4 kfl/,o. This value of the sheet resitivity is most favourable with regard to the electric power consumption and to the linearity of the Hall-effect sensor HS; HS'. Since the electron Hall mobility g. is aproximately 3 times the hole Hall mobility g p the well 2' as the active layer in the preferred embodiment of the Hall-effect sensor HS' is of the n-type.

Both embodiments of the Hall-effect sensor HS; HS' according to the invention are fabricated in a process which is completely compatible with the CMOS technology. On the substrate 1; 1'with the concentration of the dopant of the first electrical conductivity type of the order of 1015 CM3 to 1016 CM-3 an appropriate dopant of the second electrical 6 conductivity type is implanted and it is let to diffuse in order to produce the well 2; 2' of the second electrical conductivity type also having a concentration 1011 cm-3 to 1016 CM-3. At the depth t of about 3 gin to 6 gm, measured from the ilpper plane S; S' of the substrate 1; 1' towards its interior, the well 2; 2' is separated from the substrate region by a pn-junction. The graph of the concentration N(cm-3) of mobile carriers of electric charge vs the depth t is represented in Fig. 3 and 4 for the first and for the second, respectively, embodiment of the Halleffect sensor HS; HS' according to the invention. On the appropriate locations in the well 2; 2' by the implantation of a suitable dopant and by its diffusion to the depth of approximately 1 gm, the heavily doped regions 31,..., 34; 31..... 34' of the second electrical conductivity type with the concentration of mobile carriers of electric charge 1022 cm- 3are formed. Over the regions 31,..., 34; 31...... 34' the metal electric supply pondas 41, 42; 4V, 42' and the metal electric pick-off contacts 43, 44; 43', 44' of the Hall-effect sensor HS; HS' are fabricated. Thereafter by the process step, which is common in the integrated circuit fabrication, the surface of the silicon single crystal is thermally oxidized. The thus created field oxide SiO 2 layer 5; 5' protects the surface of the integrated circuit IC; 1C.

In the fabrication process of the second embodiment of the Hall-effect sensor HS' only the step of depositing the polysilicon layer 6' on the field oxide layer 5' is still to be added, namely in the region 50' which is situated essentially between the metal contacts 41...... 44'of the Hall-effect sensor HT.

In the fabrication of the first embodiment.of the Hall-effect sensor HS, however, the duration of thermal oxidization of the surface of the integrated circuit IC is extended. Thereby in the region 50 just under the field oxide layer 5 a rearrangement of the dopants takes place so that there the n-type layer 6 of several tenths of a micrometer appears. Namely, the field oxide SiO 2 absorbs acceptor dopants, e.g. boron, and in the region 50 under the field oxide layer 5 donor dopants, e.g. phosphorus, being originally present in the substrate 1 become prevailing. The complete graph of the concentration of mobile electric charge carriers vs the depth t for the first embodiment of the Halleffect sensor HS is shown in Fig. 3.

7 1 C' In the following the protecting function of the layer 6; Cwill be described, in which layer the disturbing influence of the migration of the ions in the field oxide layer 5; S' caused at elevated temperatures and by the electric field is to be blocked. Also the variation of the surface charge at the inner field oxide surface originating from this ion migration exerts a disturbing influence on the active layer of the Halleffect sensor HS; HS'.

In the first embodiment of the Hall-effect sensor HS (Fig. 1) the well 2 is of the p-type and the substrate 1 is on the highest positive potential. The migration of the ions from the field oxide layer 5 towards the active layer of the Hall-effect sensor, Le.towards the well 2, is prevented by a reverse pn-junction at the border between the n-type layer 6 and the p-type well 2. The linearity of the Hall-effect sensor HS, however, is not impaired since the thicknes of the protecting layer 6 is small.

In the second embodiment of the Hall-effect sensor HS' (Fig. 2a and 2b), however, in order to protect the sensor active layer against disturbances originating from the migration of ions in the field oxide layer 5% besides the actual structure shown in Fig. 2a, there are also required the supplying of the Hall-effect sensor by the sine current generator G and the connection of of the sensor pick-up contact 43' to the virtual mass of this supplying circuit (Fig. 2b). The substrate 1' is connected to the potential -Vs and the polysilicon layer 6' is connected to the mass. The magnetic field B is directed perpedicularly to the sensor surface. The electric current I between the regions 31' and 32' and across the sensor active layer, i.e. across the well T, gives rise to the appearance of the electric field E in the field oxide layer 5' (Fig. 5). Since the sine (with respect to virtual fflass) current is supplied to the sensor and since the sensor pick-up contacts 43, 44' are situated on the symmetry axis of the line connecting the regions 31% 32% the electric field E in the middle between the contacts 4 1', 42' is equal to 0 and moreover, the time mean value of the electric field E anywhere is also equal to 0. The migration time constant is much longer than the mains voltage period. Therefore the effects of the migration are mutually cancelled. It should be noted that the layer 6' should not be of metal since at temperature variations it would cause a strain in the silicon within the sensor active layer and thereby inter alia it would influence the sensitivity, the resistance and the offset of the Hall-effect sensor HS'.

8

Claims

C'.

A Hall-effect sensor incorporated in a CMOS integrated circuit, the sensor comprising: a substrate of a first electrical conductivity type having a well of a second electrical conductivity type formed on the substrate; two f irst and two second mutually separated and heavily doped regions of the second conductivity type formed in the well such that respective lines joining the two first regions and the two second regions are mutually perpendicular, respective metal electric supply contacts and pick-up contacts being provided over the two first and two second regions, respectively; a field oxide layer covering the upper surface of the substrate except at the locations of the supply and pick-up contacts, the thickness of the field oxide layer being between 0.8 1,-m and 1.0 Jam; and a blocking layer formed adjacent the field oxide layer in a region at least between the supply and pick-up contacts, for blocking the disturbing influence of ions migrating in the field oxide layer.
2. A Hall-effect sensor according to claim 1, wherein the blocking layer is formed under the field oxide layer in a region predominantly within the well, the blocking layer only partly extending into the substrate.
3. A Hall-effect sensor incorporated in a CMOS integrated circuit, the sensor being substantially as hereinbefore described with reference to Figs. 2a, 2b and 5 of the accompanying drawings.

f

3. A Hall-effect sensor according to claim 1 or claim 2, wherein the blocking layer is of the first electrical conductivity type.

4. A Hall-effect sensor according to claim 1, claim 2 or claim 3, wherein the first electrical conductivity type is n-type and the second electrical conductivity type is p-type.

5. A Hall-effect sensor according to claim 1, wherein the blocking layer is of highly conductive polysilicon and is formed over the field oxide layer in the region between the supply and pick-up contacts, wherein the blocking layer is connected to the integrated circuit earth, and wherein one electric pick-up contact is connected to the 9 C virtual earth of the electric supply contacts of the sensor 6. A Hall-ef feet sensor according to claim 1 or claim 5, wherein the first electrical conductivity type is p-type and the second electrical conductivity type is n-type.

7. A Hall-effect sensor incorporated in a CMOS integrated circuit, the sensor being substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

8. A Hall-effect sensor incorporated in a CMOS integrated circuit, the sensor being substantially as hereinbefore described with reference to Figs. 2a, 2b and 5 of the accompanying drawings.

-to- Amendments to the claims have been filed as follows 1. A Hall-effect sensor incorporated in a CMOS integrated circuit, the sensor comprising: 5 a substrate of a first electrical conductivity type having a well of a second electrical conductivity type formed on the substrate; two first and two second mutually separated and heavily doped regions of the second conductivity type formed in the well such that respective lines joining the two first regions and the two second regions are mutually perpendicular, respective metal electric supply contacts and pick-up contacts being provided over the two first and two second regions, respectively; a field oxide layer covering the upper surface of the substrate except at the locations of the supply and pick-up contacts. the thickness of the field oxide layer being between 0.8 pm and 1.0 pm; and a blocking layer of highly conductive polysilicon formed over the field oxide layer in a region at least between the supply and pick-up contacts, for blocking the disturbing influence of ions migrating in the field oxide layer.

2. A Hall-effect sensor according to claim 1, wherein the blocking layer is connected to the integrated circuit earth, and wherein one electric pick-up contact is connected to the virtual earth of the electric supply contacts of the sensor.