The present invention relates to an arrangement in semi¬ conductor accelerometers with micromachined semiconductor springs and with a mass in the free end portion of the spring. Such accelerometers may be piezoresistive and will then comprise a resistor or a plurality of resistors connected to form a resistance bridge and with the resistors placed in regions of the spring which are subjected to mechanical tensions when the spring is bent as a consequence of the mass being subjected to acceleration. Such accelero¬ meters may also be capacitive with the free mass constituting one electrode of a capacitor and with the distance between capacitor plates varying when the accelerometer is subjected to acceleration.

Semiconductor accelerometers are small and may be produced by the same kind of manufacturing process as used in production of transistors and integrated cicuits. The springs and, if desired, the mass are provided by the aid of known methods for etching semiconductors, and both springs and masses may consequently be made very small. Silicon is the semiconductor material of most interest to build such devices. A semi¬ conductor spring in such an accelerometer will typically have a width of approximately 0.1 mm, a length of 0.1 - 1 mm, and a thickness of 0.01 - 0.1 mm.

Accelerometers are often critical components in the system of which they are part. The spring members are mechanically the weakest parts of such an accelerometer and they may, thus, have strong influence on its reliability and quality. There is, thus, a demand for a simple manner to ensure that such a device is always intact and not damaged in any way, either by a totally or partially ruptured semiconductor spring.

It is an object of the present invention to provide a very simple concept for enhancing the intrinsic security of such a device, and according to the present invention it is proposed to provide a spring rupture indicator along the length of the spring(s), between the point of suspension and the mass.

The spring rupture indicator according to the invention is a resistance loop which is doped in such a manner that the resistor has a type of conductivity (e.g. p-doped) which is the opposite to the main portion of the spring members proper (e.g. n-doped).

Alternatively, the spring rupture indicator may be a resistance loop which is etched out from a resistance film, which is provided on top of a thin insulating film on the surface of the spring members.

In a further variant the spring rupture indicator may be a conductor loop which is etched out from a conductive film, which is provided on top of a thin insulating film on the surface of the spring elements.

The conductor loop may consist of a metal film material, e g chromium, molybdenum, tantalum, titanium, tungsten, or, if desired, aluminium, applied to and etched out on the surface of the spring members with the thin insulating film between the conductor and the semiconductor.

The invention shall be disclosed in more detail with reference to the accompanying Figure 1, which shows a piezoresistive semiconductor accelerometer with a spring and with a mass mounted at the end, and Figure 2, which shows a capacitive semiconductor accelerometer.

In Figure 1 the resistance bridge known per se, is indicated by resistors Rl, R2, R3, and R4. For sake of clarity the

connections to said resistors are not indicated in the drawing.

Accelerometer 1 comprises an etched silicon spring 2 with the resistance bridge just mentioned, generally designated by numeral 3 and with a weight 4 provided at the free end portion 2' of the spring. As will appear from the Figure, the accelerometer will preferably have greater thickness at the clamping portion 5 than at the spring portion 2 proper. Consequently, resistance bridge 3 will be placed in a rather passive area, however, the resistance bridge will be subjected to tension when weight 4 causes silicon spring 2 to bend.

In case of a rupture or defect of silicon beam causing it no longer to be influenced as intended by weight 4, a check measurement by the aid of the resistance bridge will show no indication of a defect accelerometer. It is obviously unpractical to make check measurements of such an accelero¬ meter with high loads of force from load 4, as this may at the worst also cause spring 2 to crack or snap, so that the accelerometer Is in fact defect after such a test.

The present invention is valuable, in particular to avoid such a situation of uncertainty.

According to the invention a resistance or conduction loop is provided, which comprises two branches 6, 6' extending along a substantial portion of spring 2, at least from resistance bridge 3 and to end 2' of spring 2. At the end portion 2' of spring 2 said branches 6, 6' are connected with a cross piece 6". At its free ends loop 6, 6', 6" has contact pads 7, 7' which may be connected with a check circuit, if desired, in connection with resistance bridge 3.

In case of a crack or rupture of silicon spring 2 a corresponding defect or break will occur in the loop, so

that either a break or a considerably increased resistance value will be recorded between measurement points 7, and 7'.

The loop may, if desired, be diffused simultaneously with the resistance bridge, or the resistance loop may consist of a film material which is provided on top of the spring.

It will be understood that the shown etched silicon spring 2 with clamping portion 5 may be designed in any desired manner, e g by letting both clamping member and spring exhibit uniform thickness, apart from a narrower portion at the resistance bridge proper. Such an alternative design of the accelerometer proper is, however, of no concern to the understanding of the invention and its realization.

The principle may e.g. also be used in capacitive semi¬ conductor accelerometers, as shown in Figure 2. In Figure 2 numeral 8 denotes an electrically insulating member, e.g. consisting of glass. Attached to the latter in areas 17 and 17' is a semiconductor member with suspension regions 9 and 9', and with a mass 10, and spring members 11 and 11'. A capacitor is here built with a metal electrode 12 consti¬ tuting one plate of the capacitor and with mass 10 consti¬ tuting the other plate of the capacitor. 13 is here an electric contact to the main portion of the semiconductor material. On top of the semiconductor an electrically insulating passivation 15 is provided. A resistive or conductive loop 16-16' is in this case placed to traverse spring members 11 and 11' in the form of a film. Structure 16-16' may be provided in the shape of a loop with both bonding points on one side 16 or 16', or it may be a strip with one bonding point at 16 and the other at 16'.