The present invention generally relates to integrated semiconductor Micro-Electro-Mechanical System (MEMS) devices, and more particularly to integrated MEMS detection devices that are highly sensitive to changes of mass and/or energy to which they are exposed.  MEMS devices according to the present invention are useful for a variety of applications in control, testing and analytical systems. 
MEMS devices are currently widely used in control and measurement systems and the area of their application is becoming ever broader due to the easy integration of such devices into electronic components and modules. 
A MEMS beam or cantilever can be brought to a stable oscillation at its resonance frequency, which is dependent on the geometry, effective mass, temperature and the elasticity modulus of the materials involved.  Registration of a measured value is typically done by measuring the electrical resistance of a piezo-resistor or the resonance frequency of the system.  Using this measurement technique, a registration of very small changes of the effective mass, or weak interactions leading to change of the geometrical parameters of the system can be achieved. 
Typically, mechanical vibrations are generated by an external source, for example, a piezo-crystal, in direct contact with the cantilever measured.  However, such an approach does not provide satisfactory results for many applications where high precision and accuracy are desired. 
Accordingly it is desirable to provide integrated MEMS detection devices that provide reliable measurement results with high precision and accuracy. 
The present invention provides integrated MEMS detection devices, and methods using the same, that provide high precision and accurate measurement results for a variety detection applications. 
According to the present invention, integrated semiconductor piezo-resistive MEMS detector devices (e.g., sensors) are provided which include a piezo-resistive detector located at a fixed end of a MEMS cantilever, a bi-layer resonance actuator including two thin film layers, each layer having a different thermal expansion coefficient than the other layer (e.g., SiO2 and Al), wherein one of the actuator layers serves as a heating element and the other serves as an insulating layer between the heating element layer and the cantilever, and a sensing element located at the free end of the cantilever.  The sensing element serves, dependent on the particular application, as a gravitational mass, an absorber of energy, a gas- or vapor-adsorber, etc.  The detector devices are preferably made of/integrated onto a piezo-resistive material, such as, e.g., Silicon (Si). 
According to the present invention, registration of the resonance frequency of the cantilever is performed both before and after an interaction (e.g., exposure to energy, materials/mass changes due to chemical reactions or/and physical interactions, etc.)  and the change in resonance frequency due to the interaction is used to determine a parameter associated with the interaction, such as for example, a change in the flux of energy (intensity of light) exposing the sensing element, an amount of radiation absorbed or an amount of a substance adsorbed/desorbed.  Rather than using complicated circuitry to bring the cantilever into a state of oscillation at its resonance frequency, a bi-layer actuator including a heating element layer powered by an alternating current (a.c.)  source is used to create a stable vibration that is measured by the piezo-detector (e.g., piezo-resistor) to thereby register the resonance frequency peak. 
According to an aspect of the invention, an integrated micro-electro-mechanical detection device is provided that typically comprises a cantilever having a free end and a fixed end coupled to a substrate, a piezo-resistive detector coupled to the fixed end of the cantilever, and a sensing element located on the free end of the cantilever.  The detection device also typically includes an actuator including a first thin film deposited on the cantilever and a second thin film deposited on the first thin film.  In operation, application of an alternating current to the second thin film causes the free end of the cantilever to vibrate at substantially the same frequency as the applied alternating current, wherein an output voltage of the piezo-resistive detector is proportional to the phase of the applied alternating current. 
According to another aspect of the invention, a method is provided for measuring the resonance frequency of a cantilever in a micro-electro-mechanical detection device having a sensing element coupled to a free end of the cantilever and a piezo-resistive detector element coupled to a fixed end of the cantilever.  The detection device also typically includes a bi-layer actuator formed by depositing a layer of insulating material on the cantilever and by depositing a layer of resistive material on the insulating layer.  The method typically comprises applying an alternating current to the resistive material, wherein the free end of the cantilever vibrates at substantially the same frequency as the applied alternating current, adjusting the applied alternating current over a range of frequencies, and determining the resonance frequency of the free end of the cantilever by measuring an output voltage of the piezo-resistive detector element as a function of the frequency of the applied alternating current. 
According to yet another aspect of the invention, a method is provided for determining a parameter of an interaction using a micro-electro-mechanical detection device having a sensing element coupled to a free end of a cantilever and a piezo-resistive detector element coupled to a fixed end of the cantilever.  The detection device also includes a bi-layer actuator formed by depositing a layer of insulating material on the cantilever and by depositing a layer of resistive material on the insulating layer.  The method typically comprises applying an alternating current to the resistive material wherein the free end of the cantilever vibrates at substantially the same frequency as the applied alternating current, adjusting the applied alternating current over a range of frequencies, and measuring an output voltage of the piezo-resistive detector element as a function of the frequency of the applied alternating current so as to determine a first resonance frequency of the cantilever.  In one embodiment, the method also typically comprises subjecting the sensing device to a physical interaction, wherein the physical interaction results in a change of mass of the sensing element, repeating the steps of adjusting and measuring so as to determine a second resonance frequency of the cantilever after said interaction, and thereafter calculating the change of mass of the sensing element using the first and second resonance frequencies. 
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention.  Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings.  In the drawings, like reference numbers indicate identical or functionally similar elements.