Patent Application
20240007078
A resonator is a device or system that naturally oscillates at frequencies called resonant frequencies. Resonators can be, for example, crystal resonators (also known as quartz resonators), inductance-capacitance (LC) resonators, or microelectromechanical system (MEMS) resonators. Resonators are generally passive devices that are combined with active circuitry to create an oscillator. The oscillator produces a periodic signal at the resonant frequency. A crystal oscillator, for example, is an electronic circuit that uses the mechanical resonance of a vibrating crystal to create an electrical signal with a very precise frequency. Crystal oscillators may be used to generate frequencies to keep track of time or to generate a clock signal for digital integrated circuits. MEMS resonators may be used in place of crystal resonators to keep track of time and to generate a stable clock signal for analog and digital integrated circuits.
In one example, a microelectromechanical system (MEMS) resonator includes a substrate, an array of ferroelectric capacitors on the substrate, and a three-dimensional metal stack above the array of ferroelectric capacitors.
In another example, a MEMS resonator includes a substrate, an array of ferroelectric capacitors, and a three-dimensional copper stack. The array of ferroelectric capacitors includes a first ferroelectric capacitor and a second ferroelectric capacitor. The first ferroelectric capacitor is on the substrate. The first ferroelectric capacitor includes a top plate. The second ferroelectric capacitor is on the substrate. The second ferroelectric capacitor includes a top plate. The three-dimensional copper stack is above the first ferroelectric capacitor and the second ferroelectric capacitor. The three-dimensional copper stack is electrically coupled to the top plate of the first ferroelectric capacitor and the top plate of the second ferroelectric capacitor.
In a further example, a MEMS resonator includes a substrate, a first ferroelectric capacitor, a second ferroelectric capacitor, and a three-dimensional copper stack. The first ferroelectric capacitor is on the substrate, and includes a top plate and a bottom plate. The second ferroelectric capacitor is on the substrate, and includes a top plate and a bottom plate. The bottom plate of the second ferroelectric capacitor is electrically coupled to the bottom plate of the first ferroelectric capacitor. The top plate of the second ferroelectric capacitor is electrically isolated from the top plate of the first ferroelectric capacitor. The three-dimensional copper stack is above the first ferroelectric capacitor and the second ferroelectric capacitor, and is electrically isolated from the top plate of the first ferroelectric capacitor and the top plate of the second ferroelectric capacitor.
The resonance frequency of solid-state unreleased microelectromechanical systems (MEMS) resonators integrated into standard complementary metal oxide semiconductor (CMOS) technology is highly temperature dependent. The dominant factor (aside from external package stress) in the temperature dependence is the silicon dioxide, or the back end of line (BEOL) dielectric, as its Young's Modulus exhibits strong dependence on temperature. For example, the Young's Modulus of silicon dioxide dielectric may increase with temperature on the order of 50-70 parts per million per degree Kelvin (ppm/K). This change in Young's modulus with temperature produces a strong positive temperature coefficient of frequency (TCF) in the MEMS resonator.
Various strategies have been employed to compensate TCF in MEMS resonators. A thin layer of silicon dioxide (having a positive temperature coefficient of Young's modulus (TCE)) may be added to a MEMS resonator having a negative TCE to counteract the positive TCE of the other layers of the circuit. Some silicon MEMS resonators add dopants, such as boron or arsenic, to the silicon to stabilize the TCF. Some MEMS resonators include columns of silicon dioxide inserted into the silicon substrate below the aluminum nitride of the resonator to compensate the aluminum nitride. These techniques may be complex, difficult to implement, and limited in application.
While the compensation techniques described above rely on keeping the resonance mode shape more or less the same, and introducing small amounts of material with a complementary characteristic, the unreleased MEMS resonators described herein implement a new resonator structure that changes the mode shape to extend the mode into the BEOL structure, allowing the mode to interact with copper metallization. The temperature coefficient of Young's modulus of copper is complementary to that of the BEOL dielectric. The MEMS resonators introduce copper metallization in the regions with the highest elastic energy concentration to achieve passive temperature compensation for the resonance frequency. The mode extension is achieved by extending the copper metallization to span higher metal layers available in CMOS integrated circuit technology.
On top of the silicon substrate 110, n+ and p+ wells are formed, providing a region for CMOS transistors implantation. In some implementations, the ferroelectric capacitors 102 may be formed on N-well layer 231 during the FEOL processing of the CMOS IC. Contacts 214, 216 and vias 212 are formed during BEOL processing of the CMOS IC. The silicon dioxide layers 232, 233, 234 provide electrical insulation around and between the first and second metal layers and contacts, such as contacts 214, 216. In other examples, various types of interconnect dielectric material layers may be used between multiple metal layers.
The ferroelectric capacitors 102 are fabricated during the FEOL processing of the CMOS IC. A first conductive layer that forms the bottom plates 204 may be deposited on substrate 200. The ferroelectric layer that forms the ferroelectric capacitors 102 is then deposited over the first conductive layer. A second conductive layer is then deposited over the ferroelectric layer. An etch process is then performed to form the individual plates of the ferroelectric capacitors 102. In another example, each layer may be patterned and etched individually. The first and second conductive layers that form the plates of the linear array of the 210 are a metallic alloy in this example.
The TCF of a MEMS resonator composed of a composite of different materials may be computed based on the TCE of each material weighted by the local strain energy density. See S. Wang, W-C Chen, B. Bahr, W. Fang, S-S Li and D. Weinstein, “Temperature coefficient of frequency modeling for CMOS-MEMS bulk mode composite resonators,” IEEE TUFFC, vol. 62, no. 6, pp. 1166-1178, June 2015.
Applying equation (1), an example MEMS resonator based on the capacitor array 100 may have TCF of about +85 ppm/K. A TCF value for each of various materials that may be applied to fabricate such a MEMS resonator (and used to compute the TCF of the MEMS resonator) is shown in Table 1.
To reduce the TCF of the capacitor array 100, the MEMS resonators described herein, add a stack of interconnected copper layers above the capacitor array 100 during BEOL processing. The copper structure added to the MEMS resonator balances the elastic energy between SiO2 and copper to reduce the TCF of the MEMS resonator. In some implementations, the TCF of a MEMS resonator that includes a stack of interconnected copper layers above the capacitor array 100 may be zero, or near zero.
In the MEMS resonator 300, the top plate 206 of the ferroelectric capacitors 102 are electrically coupled through the stack 302. The bottom plates 204 of the ferroelectric capacitors 102 are coupled to isolated sections of the layer 304 through vias 202. The layer 304 may be a polysilicon layer. Thus, in the MEMS resonator 300, the bottom plates 204 of the ferroelectric capacitors 102 are isolated and may be driven through the vias 202 coupled to the bottom plates 204.
In the MEMS resonator 600, the bottom plates 204 of the ferroelectric capacitors 102 are electrically coupled to the polysilicon layer 604 through the vias 202. The vias 202 may be tungsten vias. The bottom plates 204 of the ferroelectric capacitors 102 are electrically connected through the polysilicon layer 604. The top plates 206 of the ferroelectric capacitors 102 are coupled to isolated sections of the copper segment 606 through vias 212. Thus, in the MEMS resonator 600, the top plates 206 of the ferroelectric capacitors 102 are electrically isolated from one another, and from the stack 602, and may be driven through the vias 212.
In the MEMS resonator 300, the MEMS resonator 600, and other examples of a MEMS resonator that include a metallic stack to compensate TCF, the arrangement of the copper layers and the arrangement of the vias connecting the copper layers may be selected by analysis of energy density simulations. In the energy density simulations, regions with highest strain energy may be identified, as these are the regions with dominant impact on the TCF of the resonator. For example, if the TCF of the resonator is positive, the regions with the highest energy density are likely to be in the BEOL oxide. With identification of these regions, the copper vias and metallization may be moved to increase the elastic energy density in the copper, hence compensating the TCF of the resonator. Defining the metallization structure is an iterative process. With each change in the metallization, the mode shape changes, and therefore, multiple simulation iterations are performed to determine the metallization structure that compensates the TCF. TCF may be computed by simulating the structure at different temperatures, providing a result that is equivalent to using equation (1) and the TCE properties of the materials. Equation (1) may be applied to guide the movement of material needed to achieve a desired TCF.
In this description, the term “couple” or “couples” may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action: (a) in a first example, device A is coupled to device B; or (b) in a second example, device A is coupled to device B through intervening component C if intervening component C does not alter the functional relationship between device A and device B, such that device B is controlled by device A via the control signal generated by device A. Also, in this description, the recitation “based on” means “based at least in part on.” Therefore, if X is based on Y, then X may be a function of Y and any number of other factors.
A device that is “configured to” perform a task or function may be configured (e.g., programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or reconfigurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof.
A circuit or device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described circuitry or device. For example, a structure described as including one or more semiconductor elements (such as transistors), one or more passive elements (such as resistors, capacitors, and/or inductors), and/or one or more sources (such as voltage and/or current sources) may instead include only the semiconductor elements within a single physical device (e.g., a semiconductor die and/or integrated circuit (IC) package) and may be adapted to be coupled to at least some of the passive elements and/or the sources to form the described structure either at a time of manufacture or after a time of manufacture, for example, by an end-user and/or a third-party.
Circuits described herein are reconfigurable to include additional or different components to provide functionality at least partially similar to functionality available prior to the component replacement.
Uses of the phrase “ground” in the foregoing description include a chassis ground, an Earth ground, a floating ground, a virtual ground, a digital ground, a common ground, and/or any other form of ground connection applicable to, or suitable for, the teachings of this description. In this description, unless otherwise stated, “about,” “approximately” or “substantially” preceding a parameter means being within +/−10 percent of that parameter.
Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.
