Michael Offenberg

Novel Process For A Monolithic Integrated Accelerometer

A monolithic sensor, fabricated using a technology that combines surface micromachining with a BiCMOS process, is described. The capacitive sensing element consists of a 10/spl mu/m thick layer of polysilicon that is deposited in an early stage of the IC fabrication sequence. A high degree of synergy is achieved and conflicting issues regarding the merging of the processes are avoided by us ing IC processing steps for manufacturing of the sensor element. Adhesion between parts of the sensor structure, during their release, is eliminated by using an HF vapor phase etching process for the sacrificial layer. The completed sensor has been characterized successfully.

Texture and stress profile in thick polysilicon films suitable for fabrication of microstructures

Abstract In this paper we report on results concerning the texture and mechanical stress of thick polycrystalline silicon films, grown in an epitaxial reactor. These phosphorus doped films were subjected to different annealing treatments at temperatures at or above 1100 °C in nitrogen and oxygen ambients, respectively. The morphology and texture of the films, respectively, were obtained using TEM and X-ray diffraction analysis. The samples texture does not present any significant changes related to the used annealing treatment. The stress gradient of released micromachined test structures was determined with optical interference measurements and compared with -Raman spectroscopy stress profiles and X-ray diffraction stress data using the sin 2 ÷-method obtained at different depths. We show that annealing in oxygen changes the stress state in the top regions of the polycrystalline films to a more compressive value, thus causing released beams to bend downwards. On the contrary, annealing in nitrogen atmosphere shifts the stress towards tensile values and released beams bend upwards.

Influence of anneals in oxygen ambient on stress of thick polysilicon layers

The stress and stress gradient in thick polysilicon layers used for high volume production of surface micromachined sensors depend strongly on the presence or absence of oxygen during high temperature annealing steps. This paper presents experimental results on the stress dependence applying various annealing processes. The stress and stress gradient can be changed reversibly by subsequent annealing steps in oxygen and nitrogen ambient, respectively, and this effect is independent of doping. The mechanisms responsible for this influence have been analyzed. It is concluded that the stress changes upon annealing can be attributed to the diffusion of oxygen atoms in and out of the thick polysilicon layers, as confirmed with secondary ion mass spectroscopy (SIMS) measurements. Applying this knowledge, with a careful selection of the annealing scheme, the stress state of the layers can be tailored according to the specific needs of the respective application.

Comprehensive study of processing parameters influencing the stress and stress gradient of thick polysilicon layers

Thick polysilicon layers (greater than 10 micrometer), grown in an epitaxial reactor, are highly desirable for surface micromachining applications. The mechanical properties of these layers were studied extensively by characterizing the stress and stress gradient. The stress profile and texture were insensitive to variations of deposition parameters both of the polysilicon seed layer and the epitaxial process, and were influenced to a small degree by doping with phosphorous. Annealing in oxygen ambient resulted in compressive stresses up to 80 MPa and stress gradients of -10 MPa/micrometer. Subsequent annealing in nitrogen reduced the stress and stress gradient, but it can be reversed by re-annealing in oxygen. A model based on diffusion of oxygen is presented explaining the influence of the annealing on stress. Oxygen atoms diffuse into polysilicon during an anneal in the oxygen ambient, introducing compressive stress. Upon annealing in the nitrogen ambient, oxygen is released from the polysilicon layers due to the partial pressure of oxygen at the annealing temperature. The diffusion of oxygen atoms out of the layers results in a partial reversal of the mechanical effects. This insight gives the possibility to tailor the stress of thick polysilicon layers within certain limits to the specific needs of the application.

Thick-polysilicon-based surface micromachined capacitive accelerometer with force-feedback operation

This paper describes the concept, the design and measurement results of a surface micromachined accelerometer. For the accelerometer presented here a polysilicon high rate deposition process was used to fabricate polysilicon layers with a thickness of 10 micrometers . The sensor is designed as an interdigital finger structure forming a differential capacitor, where the moveable fingers are mounted on a moveable mass and the fixed electrodes are anchored on the substrate. The air gap between the fingers is 1 micrometers . A force-feedback operation mode is realized to increase the sensor performance. A two chip solution with sensing element and signal processing circuit on separate chips was chosen as first approach towards a monolithic integrated accelerometer. The evaluation circuit was realized in BiCMOS technology. Sensors with a closed-loop sensitivity of 8mV/g and a bandwidth of about 10kHz have been fabricated. A resolution of 0.1g and a linearity error of less than 1% could be achieved for a measurement range of +/- 100 g.