System and method for inductive wireless signaling

A transformer includes first and second semiconductor substrates. The first semiconductor substrate includes a first circuit, a first coil providing a first impedance, and a first capacitor coupled in parallel with the first coil. The second semiconductor substrate includes a second circuit, a second coil providing a second impedance and inductively coupled with the first coil, and a second capacitor coupled in parallel with the second coil.

BACKGROUND

In various integrated circuit (IC) applications, an important consideration is inter-chip communications. Traditionally, wires have been used to perform signaling between one integrated circuit (also referred to as chip) and another. As products are continually reduced in size, reduced in power consumption, and increased in bandwidth, wireless interconnection technologies have been gaining popularity.

One technique for wireless interconnection employs the principle of inductive coupling that is utilized in transformers. In a transformer, a changing current in a primary winding (coil) creates a changing magnetic flux in the transformer's core and thus changes a magnetic field through the transformer's secondary winding. The changing magnetic field induces a changing voltage in the secondary winding. This effect is referred to as mutual induction. Inductive coupled coils have been applied to inter-chip communication with a technology known as a through-chip interconnect (TCI). A current change in a first inductor used for transmission at a first semiconductor substrate (e.g., corresponding to a first chip) generates a voltage signal at a second inductor used for reception at a second semiconductor substrate (e.g., corresponding to a second chip). By generating appropriate voltage signals, wireless communication is realized.

A challenge associated with traditional signaling based on inductive coupling is ensuring a high coupling coefficient k, which is the ratio of output current to input current as pertaining to the coils at the receiver and transmitter. High-k transformers are desired for increasing sensitivity, which relates to the minimum detectable signal at the receiver, and for reducing power consumption.

One conventional technique for raising the coupling coefficient k is substrate thinning (decreasing the thickness of substrates at the respective chips). At a given frequency, decreasing the substrate thickness tends to increase the coupling coefficient k. However, such increase in k may cause the resulting substrate to be difficult to handle (e.g., from a manufacturing or processing perspective), may raise associated costs, and may lead to roughness that in turn results in undesirable variation (nonuniformity) in the coupling coefficient.

Another conventional technique for raising the coupling coefficient is to increase inductance. Increasing the number for turns in the coils increases the inductance and generally increases the coupling coefficient, except for resonance effects that may occur at specific frequencies. However, this approach increases device area and cost.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation.

FIG. 1is a circuit diagram of a transformer in accordance with some embodiments. A transformer100includes a first semiconductor substrate110aand a second semiconductor substrate110bthat are not directly connected together by electrically conductive wires. The substrates110a,110bmay correspond to first and second integrated circuits (ICs or chips). The substrates110a,110binclude respective circuits120a,120b. Embodiments may be used for three dimensional (3D) RF IC applications. For example, the second semiconductor substrate110bmay be stacked above the first semiconductor substrate110a, with the second coil140bpositioned above the first coil140a.

Signaling is performed wirelessly between the substrates110a,110bvia a through-chip interconnect (TCI) approach employing inductive coupled coils140aand140bat the respective substrates. A capacitor130ais coupled in parallel with inductive coil140a, and a capacitor130bis coupled in parallel with inductive coil140b. By coupling capacitors in parallel with coils, the coupling coefficient k may be improved (e.g., by more than 300% at some frequencies, and with come capacitance values) in various embodiments relative to known wireless signaling approaches, as described further below in the context ofFIG. 3. The capacitance at respective capacitors130aand130bmay be between 0 and 10 pF, the inductance at respective inductive coils140aand140bmay be between 0 and 10 nH, and the mutual inductance may be between 0 and 10 nH. In some embodiments, capacitance at respective capacitors130aand130bis about 2.7 pF, inductance at respective inductive coils140aand140bis about 60 pH, and the mutual inductance is about 36 pH. The amplitude of a transmitted signal may be enhanced, as may the amplitude of a received signal, as described further below in the context ofFIG. 4.

Capacitors130aand130bmay be variable capacitors. Such variable capacitors may be tuned in some embodiments to provide the same capacitance (or approximately equal capacitances) to further increase the coupling coefficient. In some embodiments, capacitors130aand130bare metal-oxide-semiconductor (MOS) capacitors (MOScaps) or metal-insulator-metal (MIM) capacitors. The impedances provided by coils140aand140bmay be equal or approximately equal to one another, because a transformer does not need to provide the same output impedance as input impedance for TCI application, unlike other applications, such as power amplifiers, in which the impedance is transferred.

FIG. 2is a circuit diagram of a transformer illustrating switching functionality for bidirectional communications in accordance with some embodiments. Circuit120amay include a transmitting circuit (transmitter)150a, and circuit120bmay include a receiving circuit (receiver)155b. Transmitter150amay provide signals for transmission via inductive coupling, and receiver155bmay receive and/or process the signals. In a unidirectional communication configuration, a single transmitter150aat circuit120aand a single receiver155bat circuit120bsuffice. In an optional bidirectional communication configuration, circuit120aalso includes a receiver155a, and circuit120balso includes a transmitter150b. A switch160aat substrate110aselects one of transmitter150aand receiver155afor coupling to coil140a. Similarly, a switch160bat substrate110bselects one of transmitter150band receiver155bfor coupling to coil140b. In a first switching state, transmitter150ais connected to coil140a, and receiver155bis connected to coil140b, for transmission (signaling) from substrate110ato substrate110b. In a second switching state, receiver155ais connected to coil140a, and transmitter150bis connected to coil140b, for transmission from substrate110bto substrate110a.

Although each of switches160aand160bis shown inFIG. 2in a double pole, double throw (DPDT) configuration, other switching configurations may be used. For example, a single switch may control respective couplings at both substrates.

FIG. 3is a plot of coupling coefficient performance in accordance with some embodiments. With a conventional TCI system, e.g., one that does not employ capacitors configured in parallel with respective coils as in various embodiments, the coupling coefficient k is relatively constant over a range of frequencies from about 1 to 20 GHz. With tuned capacitors in some embodiments, the coupling coefficient is increased over a wide range of frequencies. For example, at 11 GHz, k is enhanced from 0.20 in conventional systems to 0.66 in some embodiments. In some embodiments, for frequencies higher than 11 GHz, the transformer becomes capacitive, and k may be larger than 1. At a resonant frequency, the effective coupling coefficient keffmay be expressed as keff=k*sqrt(C/(G2L(1−k2)), where C may depend on the operational frequency, G is an equivalent conductance of the transformer100, and L is specified by a coil design suited to particular area constraints. Details regarding this mathematical formulation may be found at Simburger et al., “A Monolithic Transformer Coupled 5-W Silicon Power Amplifier with 59% PAE at 0.9 GHz,” IEEE Journal of Solid-State Circuits, Vol. 34, No. 12, December 1999, p. 1883.

FIG. 4is a plot of received voltage in accordance with some embodiments. With a 1 V peak-to-peak sine wave at 11 GHz, the received voltage V, is enhanced from 60 mV peak-to-peak to 420 mV peak-to-peak in some embodiments. Thus, received voltage signal420in some embodiments exhibits greater signal strength than received signal410in conventional systems.

Embodiments may be used in various contexts where multiple chips are to be interconnected wirelessly. For example, a transformer as inFIG. 1may be used to wirelessly connect various components of a communication system.FIG. 5is a block diagram of a communication system500in accordance with some embodiments. Communication system500may be an RF communication system and includes a transmitter unit502and a receiver unit504. At transmitter unit502, an input signal510is mixed at a mixer512based on an output of an oscillator514and later processed by a power amplifier (PA)516and sent to a transmit antenna518. At receiver unit504, a signal received by a receive antenna520is processed by a low noise amplifier (LNA)522and mixed at a mixer524, based on an output of an oscillator526, to provide an output signal530. Through-chip interfaces (TCIs) may be provided between various system components, e.g., as TCIs513,515,517,521,523, and525. Thus, inter-chip signaling is provided in this embodiment between various components with a high coupling coefficient at each interface. At each TCI shown inFIG. 5, it is understood that the adjacent circuit elements form the circuits corresponding to circuits120aand120binFIG. 1.

FIG. 6is a flow diagram of a process in accordance with some embodiments. After process600begins, at step610, a first semiconductor substrate110ais formed. The first substrate110aincludes a first circuit120a, a first coil140a, and a first capacitor130acoupled in parallel with the first coil140a. At step620, a second semiconductor substrate110bis formed. The second substrate110bincludes a second circuit120b, a second coil140b, and a second capacitor130bcoupled in parallel with the second coil140b. At step630, the first and second substrates110a,110bare aligned so that respective coils140aand140bare inductively coupled. Aligning the substrates110a,110bmay include stacking the substrates vertically. The substrates may be stacked directly by a suitable adhesive, e.g., glue. The substrates may be stacked either with respective inactive faces facing each other (bottom-to-bottom), with respective active faces facing each other (face-to-face), or with respective active faces facing in the same direction (face-to-bottom). Any of these stacking configurations may be used for 3D ICs applications.

FIG. 7is a cross-sectional view of stacked chips in accordance with some embodiments.FIG. 7shows two chips stacked vertically and oriented in (facing) the same direction in this example. Semiconductor substrate layers702and706may each be formed from silicon. Inductors705and709are disposed in respective layers704and708that may each be formed from a dielectric material, e.g., silicon dioxide, a low-K dielectric material (where κ denotes dielectric constant), or an extra low-κ (ELK; sometimes the term “extreme low-κ” is used) dielectric material.FIG. 7shows, in an example configuration, an electrode713of a first capacitor at layer708and an electrode711of a second capacitor at layer704. Various types of capacitors, e.g., MOS capacitors or metal-insulator-metal (MIM) capacitors, may be used.FIG. 7shows an example configuration with MIM capacitors.

Advantageously, embodiments provide wireless inter-chip signaling without the need to develop entirely new processes, without additional manufacturing cost (e.g., due to additional process steps such as substrate thinning), without incurring additional chip area, without the need for peripheral circuits such as buffers, and without additional power consumption.

In some embodiments, a transformer includes first and second semiconductor substrates. The first semiconductor substrate includes a first circuit, a first coil providing a first impedance, and a first capacitor coupled in parallel with the first coil. The second semiconductor substrate includes a second circuit, a second coil providing a second impedance and inductively coupled with the first coil, and a second capacitor coupled in parallel with the second coil.

Some embodiments use a single transformer for inter-chip signaling. In other words, a single-input single-output configuration is used, instead of using a multiple-input single-output configuration employing multiple transformers to combine RF power.

In some embodiments, a communication system includes a transmitter unit, a receiver unit, and semiconductor substrate interface modules. The transmitter unit includes a first oscillator configured to provide a first clock signal, a first mixer configured to mix an analog input signal with the first clock signal to provide a first mixed signal, a power amplifier configured to amplify the first mixed signal to provide a transmission signal, and a transmit antenna configured to transmit the transmission signal. The receiver unit includes a receive antenna configured to receive the transmission signal, a low noise amplifier configured to amplify the received transmission signal, a second oscillator configured to provide a second clock signal, and a second mixer configured to mix an output of the low noise amplifier with the second clock signal to provide an analog output signal. The semiconductor substrate interface modules are configured to provide wireless communication between the first oscillator and the first mixer, between the first mixer and the power amplifier, between the power amplifier and the transmit antenna, between the receive antenna and the low noise amplifier, between the low noise amplifier and the second mixer, and between the second mixer and the second oscillator. Each interface module includes a pair of inductively coupled coils and a pair of capacitors. Each capacitor is coupled in parallel with a corresponding coil.

Although examples are illustrated and described herein, embodiments are nevertheless not limited to the details shown, since various modifications and structural changes may be made therein by those of ordinary skill within the scope and range of equivalents of the claims.