This invention relates to methods of applying electrical connections and/or conditioning to a silicon or silicon-like wafer for chip diagnostic purposes.
In the field of integrated circuit processing, not only have device critical dimensions been steadily decreasing and circuit complexity been steadily increasing, but the size of the wafers in which the devices and circuits are built has grown greatly. Whereas wafer dimensions were three to four inch radius twenty years ago, typical wafer radii at present are twelve inches or more. The increased circuit complexity and wafer size presents new challenges in chip diagnostics and testing.
One typical method of diagnosing design flaws and/or fabrication problems in a circuit, or providing electrical conditioning to the circuit, involves applying electrical signals such as Vdd, ground, or other signals so as to power up or “turn on” the circuit, and then to observe such effects as photon emission or thermal effects to locate potential trouble spots. In the past, observation was generally done on a die-by-die basis with a microscope from the front side of the die, i.e., through the metal layers.
Other diagnostic techniques used for the investigation of integrated circuit defects and sensitive areas utilize a laser beam incident on the DUT. When a laser beam impinges on a material such as a semiconductor substrate or metal interconnect, it can cause thermal effects and/or photo-generated charge carrier effects, both of which can be utilized to localize many types of circuit defects. TIVA (Thermally Induced Voltage Alteration) and OBIRCH (Optical Beam Induced Resistance Change) are two methods developed to utilize thermal effects for circuit defect analysis. LIVA (Light Induced Voltage Alteration) is a method developed to utilize photo-generated charge carrier effects for circuit defect analysis. All of these methods involve a laser stimulus of the DUT, and electrical or thermal measurements of the response to the stimulus. The TIVA method is described in U.S. Pat. No. 6,078,183, issued on Jun. 20, 2000, the LIVA method is described in U.S. Pat. No. 5,430,305 issued on Jul. 4, 1995, and the OBIRCH method is described in U.S. Pat. No. 5,804,980 issued on Sep. 8, 1998. These three patents are hereby incorporated by reference in their entireties. Other methods which use laser stimulation to produce perturbations of mechanical, thermal, or electrical properties of the die include but are not limited to: Optical Beam Induced Current (OBIC), Laser Assisted Device Alteration (LADA), Soft Defect Localization (SDL), Externally Induced Voltage Alteration (XIVA), and Resistive Interconnection Localization (RIL). A general description of many of these methods is found in Microelectronics Failure Analysis Desk Reference, 5th Ed., ASM International, Materials Park, Ohio, 2004, Chapter 10, Laser and Particle Beam-Based Localization Techniques, pp 407-444.
Both the observation of photoemission or thermal response from the DUT resulting from electrical stimulation, and the electrical measurement of changes caused by optical stimulation, require reliable electrical connection to the sample.
Traditionally, the electrical connections have been provided by “micro-probes” which are mounted on supports and steered with mini-controls to contact the die in various locations such as the contact pads.
Microprobes have several disadvantages. They are expensive and easily damaged, and the connections made to the die are not permanent and not reliably good quality connections. In addition the microprobe supports are bulky, and it is physically difficult to fit more than 6-8 microprobes around the die.
Microprobes are also physically incompatible with the improved ultra-high NA objectives used for low emission observation, such as provided by the Credence 10-104001-00 lens set or 10-106015-00 lens set, unless observation is done from the die side opposite the micro-probes, i.e., the backside. Such objectives typically have bulky (large diameter) bodies and very small (2-10 mm) focal working distances that require the objective to be positioned very close to the DUT. The size and shape of microprobes renders the use of such objectives on the connection side, i.e. the frontside, of the DUT impossible. This is illustrated in
The recent trend in diagnostic testing is to test individual devices at the “whole wafer” level, rather than separating and testing individual dies. This has many advantages, including:
In addition to the disadvantages outlined above for the use of microprobes for on-wafer diagnostics, it is generally highly impractical to physically place more than 4 microprobes anywhere on a large wafer. This is because it is difficult for a microprobe to be positioned so as to reach across the radius of a wafer to make contact with a die. As a result, microprobes can only be placed on the side of the die of interest closest to the edge of the wafer. Generally no more than 4 microprobes can be fit on one side of the die. A loose die can be reached from both sides but then the physical bulk of the microprobe positioners limits the number of microprobes to 6-8.
A method for forming high-quality electrical connections to a die, whether in single-die or full-wafer environment, which avoided the disadvantages of microprobes and which could be used with the improved ultra-high NA objectives used for low photon emission observation, as well as for optically stimulated electrical measurements or other diagnostic methods, would be an important advancement in chip diagnostic technology.
It is therefore an object of this invention to provide an improved form of electrical connection to a die or wafer which does not utilize micro-probes.
It is a further object of this invention to provide a form of electrical connection to a die or wafer which is physically compatible with ultra-high NA objectives used for low photon emission observation.
It is a further object of this invention to provide a form of electrical connection to a die or wafer which is practical for contacting a die at any position on a full wafer.
These objectives are met by the wafer bond-out scheme disclosed herein.
a illustrates a prior art configuration for chip diagnostics utilizing microprobes.
b illustrates the structure of microprobes and their controls.
c shows the physical constraints on multiple microprobes.
d shows the difficulty in using microprobes with ultra-high NA objectives.
e illustrates a case where frontside emission observation is preferable to backside observation.
a illustrates a configuration for on-wafer bond-out according to the present invention.
b illustrates a close-up of the DUT with a large number of micro-bonds, according to the present invention.
A method and configuration for performing permanent electrical/mechanical attachment to a die which does not utilize micro-probes, and which enables observation of photon emission from the die frontside without interfering with the microscope objective lens, even for ultra-high NA objectives, is shown in
Die (20) is glued (from the backside) to standardized device carrier, i.e. circuit board, (22). An adhesive which is soluble in a benign solvent such as isopropyl alcohol (IPA) or acetone is required. A low-viscosity, cyanoacrylate-based adhesive is preferably used, such as (LocTite type 460). The substrate and DUT must be cleaned using IPA or Acetone and allowed to dry thoroughly. A small amount of sample adhesive is applied to the substrate and the DUT is attached and clamped in place using finger-tip pressure for approximately 30 seconds. A device prepared in this fashion is ready for wire bonding in approximately 5 minutes. Note; the adhesive can be dissolved using acetone if desired to separate DUT and carrier for reclamation of either component. The carrier (22) is a low cost consumable, which may, by way of example, have 24 contact pads (24). Each contact pad (24) is connected to a pin (26) extending from the backside of the carrier. Then, die (20) is electrically/mechanically connected to carrier (22) by bonding micro-wires (28) to points on the die and connecting those micro-wires to contact pads (24). Any manual bonder capable of wedge-bonding with Al—Si wire is suitable for this process. One tool of choice is the F&K Delvotec model 5330. Other suitable tools include the Hybond model 572A and WestBond Model 7476E. Each die (20) is thereby custom-wired to inexpensive standardized carrier (22). The custom-wired carrier is then plugged into a standard socket 23 such as a Zero Insertion Force (ZIF) socket, which may be mounted on a standardized DUT card 25 The ZIF socket allows the prepared DUT to be easily replaced and reused with little or no damage or war to either the DUT or the DUT card. The DUT card may, for example, have each line include an available test point and SMA connector. Electrical connections 29 are on the side of the DUT card opposite the socket. Since the wiring from the die to the carrier is customized, only a single DUT card is required.
This direct bond-out configuration enables frontside observation by data optics 28 such as ultra-high NA objectives without any physical interference with the lens due to the low profile and small physical size of the wire bonds. It also provides permanent bonding onto an inexpensive carrier with good electrical/mechanical connection and fast, one-time set-up, on the order of 30 minutes, which is equivalent to the set-up time for microprobing.
b illustrates an alternate configuration for single-die bond-out, which permits observation of the die from either the frontside or backside, without remounting the die. Device carrier 200 has hole 210 therethrough, and DUT 215 is mounted from its edges 220 so as to be suspended within hole 210, with both its frontside and backside exposed for observation. Suspension may be accomplished, by way of example, by pressure fitting 225 with vertical physical contacts 230, so as not to be in contact with or damage frontside or backside of DUT 215. Microbonds 235 can be formed between points on DUT 215 and bonding pads 240 on device carrier 200, as in the previously described case.
The bonding for both the cases illustrated in
A configuration is disclosed herein and illustrated in
Once the PC board 42 is mounted on the frontside of the wafer 44 with the die 46 exposed, permanent micro-bonds 48 can be made from the die to the PC board, as is described above for the single-die case. Wires can then be extended from the PC board pads to the outside edge of the wafer Ribbon cables or flexible PCB can be connected to the carrier prior to its mounting to the wafer. Such flexible conductors can be terminated in a multitude of ways (including but not limited to: PCB connection, other card edge connections, IDC connectors, other pin and socket connectors, individual wire splices, etc.).
In an alternative embodiment, the PC board can be elongated so as to extend beyond the edges of the wafer, in which case custom mother board connectors, including but not limited to coax or SMA, can be added directly to the PC board.
Using the configurations disclosed herein, bond-out schemes can be utilized either with single die or for on-wafer chip diagnostics. This allows for frontside photon emission observation with ultra-high NA objectives, and other applications such as TIVA, LIVA, OBIRCH. or other electrical or thermal responses to photon stimulation of the sample. The method provides secure, permanent, good electrical connections, and can be accomplished in about the same time required for setup of microprobes.
It is not intended that the invention be restricted to the exact embodiments disclosed herein. Modifications may be made without departing from the inventive concept. For example, other types of microbonding than wedge bonding may be used. Optical or thermal observation may be performed from the frontside or the backside of the wafer or DUT. The wafer bond-out configuration can be used with other types of measurements, for example electron beam probing of circuits. The scope of the invention should be construed in view of the claims.
Number | Date | Country | |
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60755827 | Jan 2006 | US |