Apparatus embodying doped substrate portion

A method of determining adhesion quality and apparatus embodying the method are disclosed. The apparatus includes a substrate, a seed layer, and a resonator. The substrate defines a cavity and has a doped portion proximal to the cavity. The seed layer is disposed above the cavity. The resonator includes a bottom electrode on the seed layer, a piezoelectric portion on the bottom electrode, and a top electrode on the piezoelectric portion. To test the quality of adhesion of the seed layer to the substrate, one or more electrical property is measured between the doped portion and the bottom electrode and compared to a threshold value.

BACKGROUND

The present invention relates to acoustic resonators, and more particularly, to resonators that may be used as filters for electronic circuits.

The need to reduce the cost and size of electronic equipment has led to a continuing need for ever-smaller electronic filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Further, many such devices utilize electronic filters that must be tuned to precise frequencies. Filters select those frequency components of electrical signals that lie within a desired frequency range to pass while eliminating or attenuating those frequency components that lie outside the desired frequency range.

One class of electronic filters that has the potential for meeting these needs is constructed from thin film bulk acoustic resonators (FBARS). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure can be suspended in air. A sample configuration of an apparatus10having a resonator12(for example, an FBAR) is illustrated inFIGS. 1A and 1B.FIG. 1Aillustrates a top view of the apparatus10whileFIG. 1Billustrates a side view of the apparatus10along line1B-1B ofFIG. 1A. The resonator12is fabricated above a substrate14. Deposited and etched on the substrate14are, in order, a bottom electrode layer15, piezoelectric layer17, and a top electrode layer19. Portions (as indicated by bracket12) of these layers—15,17, and19—that overlap and are fabricated over a cavity22constitute the resonator12. These portions are referred to as a bottom electrode16, piezoelectric portion18, and a top electrode20. In the resonator12, the bottom electrode16and the top electrode20sandwiches the PZ portion18. The electrodes16and20are conductors while the PZ portion18is typically crystal such as Aluminum Nitride (AlN).

When electric field is applied between the metal electrodes16and20, the PZ portion18converts some of the electrical energy into mechanical energy in the form of mechanical waves. The mechanical waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.

Resonators for applications in the GHz range may be constructed with physical dimensions on the order of less than 100 micrometers in lateral extent and a few micrometers in total thickness. In implementation, for example, the resonator12is fabricated using known semiconductor fabrication processes and is combined with electronic components and other resonators to form electronic filters for electrical signals.

During fabrication, in some instances, the resonator12delaminates from the substrate14resulting in an undesirable configuration. Such delamination, when found, is often found proximal to edges of the cavity22between the substrate14and the bottom electrode layer15. To determine whether or not the resonator12is delaminated from the substrate14, the apparatus10is cut along line1B-1B to expose its cross section. Then, the cross section is examined. However, even if delamination is discovered during the examination, it is difficult to determine whether the detected delamination occurred during the fabrication process or during the cutting step to expose its cross section. Further, the cutting step destroys the apparatus10rendering it useless.

Consequently, there remains a need for a better method to determine whether or not delamination has occurred between a substrate and a resonator on the substrate.

SUMMARY

The need is met by the present invention. In a first embodiment of the present invention, an apparatus includes a substrate, a seed layer, and a resonator. The substrate defines a cavity and has a doped portion proximal to the cavity. The seed layer is disposed above the cavity. The resonator includes a bottom electrode on the seed layer, a piezoelectric portion on the bottom electrode, and a top electrode on the piezoelectric portion.

In a second embodiment of the present invention, a method of testing an apparatus is disclosed, the apparatus including a substrate and a resonator fabricated on the substrate. Value of an electrical property is measured between a doped portion of the substrate and an electrode layer of the resonator. Then, the measured value is compared to a threshold value.

In a third embodiment of the present invention, a method of fabricating an apparatus is disclosed. A substrate is provided. A cavity is formed within the substrate. A portion of the substrate is doped, the portion being proximal to the cavity. A seed layer is fabricated on the substrate. A resonator is fabricated on the seed layer, the resonator having a bottom electrode and a top electrode sandwiching a piezoelectric portion.

DETAILED DESCRIPTION

The present invention will now be described with reference to the Figures which illustrate various embodiments of the present invention. In the Figures, some sizes of structures or portions may be exaggerated and not to scale relative to sizes of other structures or portions for illustrative purposes and, thus, are provided to illustrate the general structures of the present invention. Furthermore, various aspects of the present invention are described with reference to a structure or a portion positioned “on” or “above” relative to other structures, portions, or both. Relative terms and phrases such as, for example, “on” or “above” are used herein to describe one structure's or portion's relationship to another structure or portion as illustrated in the Figures. It will be understood that such relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in the Figures is turned over, rotated, or both, the structure or the portion described as “on” or “above” other structures or portions would now be oriented “below,” “under,” “left of,” “right of,” “in front of,” or “behind” the other structures or portions. References to a structure or a portion being formed “on” or “above” another structure or portion contemplate that additional structures or portions may intervene. References to a structure or a portion being formed on or above another structure or portion without an intervening structure or portion are described herein as being formed “directly on” or “directly above” the other structure or the other portion. The same reference number refers to the same elements throughout this document.

As shown in the Figures for the purposes of illustration, embodiments of the present invention are exemplified by an apparatus including a substrate, a seed layer on the substrate, and a resonator on the seed layer. The substrate defines a cavity and includes a doped portion proximal to the cavity34. The resonator includes a bottom electrode and a top electrode sandwiching a piezoelectric portion.

In this configuration, lamination or delamination of the seed layer from the substrate can be determined by measuring the value of an electrical property between the doped portion of the substrate and the bottom electrode of the resonator then comparing the measured value to a threshold value. Thus, the apparatus is not destroyed during the determination process. Further, if delamination is discovered, it can be attributed to the fabrication process since the apparatus remains intact.

FIG. 2Aillustrates a top view of an apparatus30according to a one embodiment of the present invention.FIG. 2Bis a cutaway side view of the apparatus30ofFIG. 2Acut along line2B-2B.FIG. 2Cis a cutaway side view of the apparatus30ofFIG. 2Acut along line2C-2C.

Portions of the apparatus30are similar to corresponding portions of the apparatus10ofFIGS. 1A and 1B. For convenience, portions of the apparatus30inFIGS. 2A through 2Cthat are similar to portions of the apparatus10ofFIGS. 1A and 1Bare assigned the same reference numerals. Referring toFIGS. 2A through 2C, the apparatus30according to the illustrated embodiment of the present invention includes a resonator32fabricated on a substrate14.

FIG. 3is a flowchart50illustrating the steps to form the apparatus30in accordance with another embodiment of the present invention. Referring toFIG. 3and toFIGS. 2A through 2C, to fabricate the apparatus30, a substrate14is provided. Step52. A cavity34is formed within the substrate. Step54. To form the cavity34, the substrate14is etched and filled with suitable sacrificial material such as, for example, phosphosilicate glass (PSG).

A portion33of the substrate14is doped to create a doped portion33of the substrate14having higher level of electrical conductance (thus lower level of electrical resistance) than the substrate14. Step56. The doped portion33can be doped with p-type dopants such as, for example, Boron, Aluminum, or Gallium. Alternatively, the doped portion33can be doped with n-type dopants such as, for example, Phosphorus, Arsenic, or Antimony.

The concentration of dopants within the doped portion33depends on the desired electrical property of the doped portion33. In the illustrated embodiment, in order to make the electrical connection by doping the silicon substrate14, the doped portion33is doped with Phosphorus for n-type doping within a range of 1e10 cm2to 1e20 cm2, for example, to approximately 8e13 cm2. As the doping level increases, the resistance of the doped portion33decreases. This resulted in the doped portion33having a resistance in the range of 100 to 400 ohms.

To achieve this level of doping, implant energy of 100 keV were applied to obtain a junction depth (thickness) of approximately one micrometer. In the illustrated embodiment, the doped portion33has lateral dimensions of approximately 10 micrometers on a side. Of course, other electrical properties (for example, lower resistance within the doped portion33) can be achieved by varying the doping concentration, dimensions of the doped portion33, or both.

Then, the substrate14, now including the filled cavity34is planarized using known methods such as chemical mechanical polishing. The cavity34can include an evacuation tunnel portion34aaligned with an evacuation via35through which the sacrificial material is later evacuated.

Next, a thin seed layer38is optionally fabricated on the planarized substrate14, between the substrate14and the bottom electrode layer15. Typically the seed layer38is sputtered on the planarized substrate14. The seed layer38can be fabricated using Aluminum Nitride (AlN) or other similar crystalline material, for example, Aluminum Oxynitride (ALON), Silicon Dioxide (SiO2), Silicon Nitride (Si3N4), or Silicon Carbide (SiC). In the illustrated embodiment, thickness of the seed layer38can range from approximately 10 Angstroms (or one nanometer) to approximately 10,000 Angstroms (or one micrometer) thick. Known sputtering techniques can be used to fabricate the seed layer38.

Then, above the seed layer38, the resonator32is fabricated, the resonator comprising a bottom electrode16and a top electrode20sandwiching a piezoelectric (PZ) portion18. Step60. To fabricate the resonator32, the following layers are fabricated: a bottom electrode layer15, a piezoelectric layer17, and a top electrode layer19. Portions (as indicated by bracket32) of these layers—15,17, and19—that overlap and are situated above the cavity34constitute the resonator32. These portions are referred to as the bottom electrode16, the piezoelectric portion18, and the top electrode20. The bottom electrode16and the top electrode20sandwiches the PZ portion18. The sacrificial material is removed from the cavity34following the fabrication of the resonator32.

The electrodes16and20are conductors such as Molybdenum and, in the illustrated embodiment, have thickness in a range from approximately 0.3 micrometer to approximately 0.5 micrometer. The PZ portion18is typically made from crystalline material such as Aluminum Nitride (AlN), and, in the illustrated embodiment, has thickness in a range from approximately 0.5 micrometer to 1.0 micrometer. The thickness of the various layers of the resonator32as well as lateral dimensions of the resonator32can vary widely depending on the desired application.

For example, the lateral dimensions of the resonator32can range from approximately 100 micrometers to over 100 micrometers. Of course, these measurements can vary widely depending on a number of factors such as, without limitation, the desired resonant frequency, materials used, the fabrication process used, etc. The illustrated resonator32having these measurements can be useful in filters in the neighborhood of 1.92 GHz. Of course, the present invention is not limited to these sizes or frequency ranges. The seed layer38provides for a better underlayer on which the resonator32is fabricated.

FIG. 4is a flowchart70illustrating a method of testing the apparatus32(ofFIG. 2A). Referring toFIG. 4and toFIGS. 2A through 2C, once fabricated, the apparatus30can be tested for delamination of the seed layer38from the substrate14by measuring the value of an electrical property between the doped portion33and the bottom electrode layer15(a portion of which form the bottom electrode16of the resonator32), Step72, then comparing the measured value to a threshold value to ascertain whether the measured value indicates delamination of the seed layer38from the substrate14, Step74.

For example, capacitance between the doped portion33and the bottom electrode layer15can be measured. When there is no delamination of the seed layer38from the resonator, the capacitance between the doped portion33and the bottom electrode layer15is determined by various factors such as, for example only, the piezoelectric material of the seed layer38, size of the area of contact between the doped portion33and the seed layer38, and thickness of the seed layer38. InFIG. 2A, the contact area is indicated by brackets reference numeral43.

In the illustrated embodiment, for the AlN seed layer38having thickness of five nanometers and contact area of approximately 200 square micrometers (for example, approximately ten micrometers wide line with approximately 20 micrometers lengthwise overlapped with the bottom electrode16), the capacitance is approximately 3.72 picoFarads when the seed layer38is laminated to the substrate14with no delamination. This is the capacitance value expected from the apparatus10if delamination does not exist and is referred herein to as the base capacitance value.

To test for delamination of the seed layer38from the substrate14, capacitance between the doped portion33and the bottom electrode layer15is measured. Then, the measured capacitance value is compared to a threshold capacitance. In the present example, the threshold capacitance can be set near 3.72 picoFarads plus some tolerance value.

FIG. 5illustrates various measured capacitance values depending on the degree of delamination. Referring toFIGS. 2A through 2Cand5, reference numeral80indicates the base value of approximately 3.72 picoFarads that can be measured if no delamination exists. If the seed layer38delaminates from the substrate14, air is introduced between the seed layer38and the substrate14. An air-capacitance curve82illustrates the capacitance between the doped portion33and the bottom electrode layer15introduced by air between the seed layer38and the substrate14within the illustrated range of delamination gap. A combined-capacitance curve84illustrates the capacitance between the doped portion33and the bottom electrode layer15when the base capacitance value is combined with the air-capacitance curve.

As illustrated by the combined-capacitance curve84, the capacitance between the doped portion33and the bottom electrode layer15drops significantly even with slight delamination of the seed layer38from the substrate14. TABLE 1 below lists the values of the air-capacitance curve82and the combined-capacitance curve84in a list format for AlN seed layers having varying thicknesses.

For example, for a five nanometers thick seed layer38, even a 0.01 nanometer delamination can be detected by setting the threshold capacitance value at approximately 3.65 picoFarads. That is, if the capacitance between the doped portion33and the bottom electrode layer15is below 3.65 picoFarads, then the delamination is deemed to exist.

Alternative or in addition to measuring capacitance between the doped portion33and the bottom electrode layer15, resistance can be measured to ascertain whether delamination has taken place between the seed layer38and the substrate14. In general, AlN has resistivity of approximately 1e10 ohm per meter.

For the resistivity, a 5 nanometer thick layer of AlN with contact area43of approximately 200 square angstroms with the doped portion33has resistance of approximately 250 Giga ohms. Adding approximately 200 ohms of resistance of the doped portion33results in a negligible change in the base resistance of between the doped portion33and the bottom electrode layer15.

Delamination of the seed layer38from the substrate14introduces air gap and increases resistance between the doped portion33and the bottom electrode layer15. In fact, even a slight delamination results in a significant increase in the resistance between the doped portion33and the bottom electrode layer15. By setting an appropriate threshold resistance value, even slight delamination can be detected.

TABLE 2 below lists approximately resistive value of the doped portion30and laminated the AlN layer38at various thickness of the seed layer38. These values can be used as the base resistive value.

In another alternative embodiment of the present invention, impedance between the doped portion33and the bottom electrode layer15is used to determine whether or not delamination exists between the seed layer38and the substrate14. The impedance is a combined value of the capacitance and the resistance between the doped portion33and the bottom electrode layer15.

Depending on implementation or application, different electrical property can be used for the determination of delamination of the resonator32from the substrate14. For example,FIGS. 6A through 6Cillustrate an apparatus30ahaving same configuration as the apparatus20ofFIGS. 2A through 2Cbut for the seed layer38. The apparatus30aofFIGS. 6A through 6Cinclude portions similar to corresponding portions of the apparatus30ofFIGS. 2A through 2C. Portions of the apparatus30aofFIGS. 6A through 6Cthat are similar to corresponding portions of the apparatus30ofFIGS. 2A through 2Care designated with the same reference numerals. The apparatus30aofFIGS. 6A through 6Clacks the seed layer38present in the apparatus30ofFIGS. 2A through 2C.

For the apparatus30ofFIGS. 2A through 2C, base resistivity is relatively high (in the order of hundreds of Giga ohms or more) mostly due to the electrical insulating properties of the seed layer38. For this reason, for this embodiment, capacitance may be more preferred as the electrical property to be measured and used to determine delamination of the resonator32from the substrate14compared to the apparatus30aofFIGS. 6A through 6C. In comparison, for the apparatus30aofFIGS. 6A through 6C, base resistivity is relatively low (in the order of hundred or hundreds of ohms) mainly because the apparatus30alacks the electrical insulating seed layer38. For this reason, for this embodiment, resistance may be more preferred as the electrical property to be measured and used to determine delamination of the resonator32from the substrate14compared to the apparatus30ofFIGS. 2A through 2C.

From the foregoing, it will be apparent that the present invention is novel and offers advantages over the current art. Although specific embodiments of the invention are described and illustrated above, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. For example, the resonator doped portion33can have alternative shapes, placements, or both such as completely surrounding the cavity34. Further, differing configurations, sizes, or materials may be used for values portions of the apparatus30but still fall within the scope of the present invention. The invention is limited by the claims that follow.