Non-destructive internal inspection of MEMS bonded wafer pairs via acoustic micro imaging is useful in finding, characterizing and eliminating anomalies and defects.
During product development, acoustic inspection is helpful is modifying processes to avoid defects. During production, acoustic inspection spots rejects and identifies process drift.
The ultrasonic transducer that scans the wafer pair pulses UHF ultrasound into the top surface and receives the return echoes. Pulse-echo occurs thousands of times per second as the transducer moves across the surface. Each scanned x-y coordinate yields one pixel in the acoustic image which, in the high resolution typically used for MEMS wafers, consists of millions of pixels.
In many of the electronics items (such as plastic IC packages) that are imaged acoustically, there are multiple depths of interest, and the return echo signals used to make the acoustic image may be restricted to (gated on) a specific depth such as the die face or the lead frame depths.
MEMS wafer pairs may have one or several depths of interest at material interfaces. Gating on a specific interface may be needed, but refocusing of the ultrasound is not necessary.
The user of the acoustic micro imaging system is usually interested in the seal around each MEMS device, although he may also image the MEMS cavity itself to observe, for the example, the etching underneath the beam in an accelerometer. To see the extent of the beam freed by etching, the wafer pair would be imaged from the back side.
The ultrasound pulsed from the transducer into the bonded MEMS wafers is generally of a high frequency such as 230MHz or 300MHz. High ultrasonic frequencies provide high spatial resolution in the acoustic image. High frequencies are also less able than lower frequencies to penetrate deeply into materials, but this is no limitation in imaging MEMS wafers consisting of thin silicon that is virtually transparent to ultrasound.
|Figure 1. Ultrasound (red) encountering a solid-to-solid interface is partly reflected; ultrasound encountering a solid-to-gap interface is almost totally reflected, resulting in higher signal amplitude.|
When pulsed into the top surface, the ultrasound propagates downward into the wafer pair and is reflected back to the transducer by material interfaces. It is not reflected by the bulk silicon, glass or other material of the top wafer unless a crack in the wafer has created a material interface. When it reaches the MEMS structures between the two wafers, the pulse is reflected by two types of material interfaces, as shown in Figure 1 above :
Solid-to-solid interfaces (the interface between the die and a glass frit seal around the MEMS cavity, for example) : Such an interface reflects a portion of the ultrasonic energy back to the transducer; another portion crosses the interface and travels deeper into the sample.
Solid-to-gap interfaces (such as the interface between the top wafer and the MEMS cavity) . More than 99.99 percent of the pulse’s energy is reflected to the transducer from a solid-to-gap interface. Such interfaces in the seal itself indicate the presence of voids or delaminations.
In the acoustic image, solid-to-solid interfaces involving different materials are typically some shade of gray. Solid-to-gap interfaces are typically bright white because echoes from these interfaces have the highest amplitude.
In non-MEMS direct-bonded silicon wafers, there may be no reflection at all from the interface because the two materials have identical acoustic properties. If silicon is well bonded to silicon, the acoustic image may be entirely black.
Figure 2 below is the acoustic image of a MEMS wafer pair showing the rectangular seal around each of the devices. The integrity of the cavity seals is often the key target of acoustic imaging because even small defects in the seal can grow and cause the MEMS device to fail.
|Figure 2: Circles areas reveal gaps in the seals of a bonded MEMS wafer pair imaged acoustically.|
In the four locations marked by circles in Figure 2, the acoustic image displays gaps in the seals. The areas of the MEMS cavities appear white because the ultrasound encountered a solid-to-gap interface in these areas. They correspond to #2 in the diagram in Figure 1.
The intact portions of the seals are gray (like #1 in the diagram) because the ultrasound encountered two solid materials having different acoustic properties. The gaps in the seals do not mean that the seal material is entirely absent at these locations. It may be delaminated or may contain a void. Most of the seal material may be present below this point, but the seal itself is breached.
The wafer pair shown in Figure 3 below was imaged acoustically during the early stages of development of an Au-Au bonding method. The entire area of one wafer is completely plated with Au. On the other wafer, only the Au seal lines are present. The seals around each cavity should consist of the Au-Au bond, and the acoustic image of the seal should be dark gray because of the reflection of ultrasound from the Si-Au interface.
|Figure 3: Seals in Au-Au bonded wafer pair show many irregularities (arrows).|
In several locations, the Si-Au interface is very thin or absent, meaning that the seal is either breached or compromised. Three of these locations are marked by arrows in Figure 3. Some of the seals not marked by arrows are at best questionable.
Such anomalies are not unusual in the early stages of product development work where acoustic imaging can often accelerate the development of a bonding method or bonding parameters that will provide good manufacturability and high yield.
Figure 4 below is the acoustic image of a small portion of a bonded wafer pair on which the seal around each device is relatively wide to accommodate dicing. There are also structures within each device that make contact with the top wafer and are therefore black or gray.
|Figure 4. Bright voids in the relatively wide seals of this wafer pair have broken or nearly broken the seal.|
Within the black areas of the seal, there are numerous bright white areas representing small delaminations or voids. A few such defects are also present within the area of the device. In the circled area of the seal, these small defects form a continuous or nearly continuous pathway across the seal. The process of dicing would in this case probably result in two devices having breached seals.
In Figure 5 below , a phenomenon has occurred that is more often seen in Si-Si direct bonded wafers. A particle (red arrow) between the two wafers has separated the two surfaces to create the classic circular non-contact defect (outlined by red dots) often seen in direct bonded wafers.
|Figure 5: Foreign particle between two wafers has prevented seal bonding in this wafer pair.|
Over the entire area of the white circle, the two wafers are not in contact. The only item that is in contact with the top wafer is the particle at the center of the non-bonded circle. The particle appears dark because it represents a solid-to-solid interface.
The two adjacent devices would both be open to the outside after dicing. The particle in this case may be thicker than the seal material. The empty area around the particle also demonstrates that the features within the area of the seal are on the bottom wafer, and not—as might be the case because of the minuscule vertical dimen-sions—on the top wafer.
Acoustic imaging of MEMS devices before dicing provides the advantage of identifying gap-type defects before further value is added. Where necessary, the MEMS devices can also be imaged after dicing, particularly when it is desirable to learn whether damage is being caused by the dicing process. In either case, the high sensitivity of UHF ultrasound to internal features can contribute to higher reliability and greater yield.
Tom Adams is an acoustic imaging consultant for Sonoscan Inc.