WO2004072585A2

WO2004072585A2 - Wafer bond strength evaluation apparatus

Info

Publication number: WO2004072585A2
Application number: PCT/US2004/003602
Authority: WO
Inventors: Terry L. Alford; Martin Mitan
Original assignee: Arizona Board Of Regents
Priority date: 2003-02-05
Filing date: 2004-02-05
Publication date: 2004-08-26
Also published as: WO2004072585A3

Abstract

An apparatus and method for measuring bond strength of a bonded wafer that allows for more reliable and reproducible measurements, thereby allowing a better comparison of bond strengths of various wafers. A wafer receptacle (2) holds the bonded wafer (28) to be measured securely in place. A Z-axis adjustment device (10) with precision actuator is adjusted to align a blade with bond interface of the bonded wafer. An X-axis adjustment device (8) with precision actuator (18) connected to a spring is adjusted to introduce the blade into bond interface creating a crack, the length of which can be measured and used to determine bond strength. The stability and precision introduced by the Z-axis and X-axis adjustment devices also increase the reliability and reproducibility of the bond strength measurement.

Description

WAFER BOND STRENGTH EVALUATION APPARATUS Cross Reference to Related Applications

This application claims priority from provisional patent application Serial No. 60/445,343 filed February 5, 2004 in the name of Terry L. Alford et al. entitled "Wafer Bond Strength Evaluation Apparatus," which is hereby incorporated by reference.

Field of the Invention

This invention relates to an apparatus and method for accurately measuring bond strength of a bonded wafer.

Background Bonded wafers are used in numerous microelectronic, optoelectronic and micromechanical applications. Bonded wafers are particularly useful in the semiconductor industry. Wafer bonding allows materials with different compositions and different properties to be combined, which produces a wafer that exhibits a combination of properties that a single material does not possess alone. This in turn creates design flexibility. An important characteristic of bonded wafers is the strength of the bond between them. In designing and engineering microelectronics, the bond should be as strong as possible. Therefore, as different materials are bonded into wafers and different processes are used to bond those wafers, it is important to be able to accurately measure the strength of that bond in order to evaluate the overall performance of the particular materials bonded and the process by which the wafers were bonded. The strength of a bond is measured in terms of the surface energy that keeps the two materials in the wafer bonded together.

The technique most-often used to measure the bond strength of a wafer is known as the crack technique, introduced by Maszara. In this technique, a blade or knife is inserted along the bond interface of the wafer and forced into the wafer until the wafer cracks. The resulting crack is measured, and the length of this crack is used to measure the surface energy keeping the wafer bonded, according to the following equation:

7= 3EtY 8L⁴

where E is Young's modulus, t is the wafer thickness, 2y is the thickness of the blade, and L is the crack length.

Most often, the crack propagating method is done manually. However, errors are easily introduced into the measurement of surface energy, and thus bond strength, because the measurement is affected by how the blade is inserted in the bonded wafer (e.g., at different angles), and by the measurement conditions. When wafers are cracked manually, there is considerable variance in how the blade is inserted and other measurement conditions, such as the stability of the blade and the amount of force used to propagate the blade into the wafer bond. These variances significantly decrease the reliability and reproducibility of bond strength measurements. This in turn makes it harder to evaluate the effect different materials and different bonding processes have on bond strength.

There is a need therefore for an apparatus and a method of measuring the strength of bonds between wafers using the crack propagation technique that allows for increased reliability and reproducibility. It is the object of this invention to provide an apparatus and a method to do just that. Brief Description

Broadly, the invention relates to an apparatus that significantly increases the reliability and reproducibility of bond strength measurements in bonded wafers. When the crack propagation technique is performed manually, the wafer often shifts significantly, causing the blade to enter the bonded wafer at varying angles and with varying force each time a crack is introduced. As described above, this in turn affects the measurement of surface energy or bond strength, introducing additional error and variance in the measurement. The apparatus described herein provides a receptacle for the bonded wafer that closely fits the size of the bonded wafer, securing the wafer in position and reducing shift of the wafer. This increases the accuracy of the bond strength measurement and the reproducibility of those measurements by stabilizing the angle and force with which the blade enters the bond interface and allowing the same angle and force to be reproduced in subsequent measurements.

The apparatus also includes two adjustment devices that substantially increase the accuracy of the bond strength measurement. First, the apparatus includes a Z-axis adjustment device, which in turn includes a precision actuator such as a micrometer, that allows the blade to be lined up along the bond interface at the bonded wafer's edge. The precision of the adjustment device allows a person to more accurately align the blade, and to align the blade in the same position in each successive experiment. The Z-axis adjustment device may also include screws that may be tightened to hold the blade in place once it is aligned, allowing the blade to be inserted at a steady angle and thereby increasing the accuracy of the bond strength measurement.

The apparatus also includes an X-axis adjustment device for radially introducing the blade inward and cutting into the bonded wafer at the bond interface. The X-axis adjustment device also includes a precision actuator such as a micrometer. The precision actuator is attached to a spring that is in turn attached to a support holding the blade. The spring and the precision actuator allow a person performing the measurement to precisely control the amount of force applied to the bond interface which substantially enhances the ability to reproduce the measurement on other bonded wafers in order to better compare the bond strengths of different bonded wafers. The X-axis adjustment device also enliances the ability to insert the blade at a steady angle, increasing the accuracy of the bond strength measurement.

The apparatus described herein can be used in conjunction with an infrared imaging system sometimes employed in the crack propagation technique to inspect and measure the crack length. An infrared source may be placed under the wafer receptacle to illuminate the bonded wafer. An infrared sensitive camera records the image which is then downloaded to a computer. The infrared image allows for a more precise and accurate measurement of the crack length versus measuring the length of the crack in the bonded wafer with the naked eye. In addition, a microscope may be used to magnify the infrared image before the image is recorded.

The above and further objects and advantages of the invention will be better understood from the following detailed description of at least one preferred embodiment of the invention, taken into consideration with the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a perspective view of an apparatus for measuring bond strength of bonded wafers.

Fig. 2 is a side view of an apparatus for measuring bond strength of bonded wafers. Fig. 3 is a perspective view of an apparatus for measuring bond strength of bonded wafers without a support for a wafer receptacle. Fig. 4 is a top plan view of the apparatus illustrating a wafer receptacle, an X-axis adjustment device, a blade support and blade.

Fig. 5 is a top plan view of an X-axis adjustment device consisting of a precision actuator connected to a spring which is connected to a support for a blade and a blade.

Fig. 6 is a cross-section of a wafer receptacle and bonded wafer extending across a central opening in the receptacle. Fig. 7 is a diagram of an apparatus for measuring bond strength of bonded wafers used in conjunction with an infrared source, camera, and computer to capture an infrared image of a bonded wafer extending across a central opening in a wafer receptacle of the apparatus. Detailed Description

The Apparatus

Turning to the drawings in detail, there is described a specific, exemplary embodiment of the apparatus discussed above. In Figs. 1, 3, 4, and 6 is shown an apparatus 30 for accurately measuring the bond strength of a bonded wafer. The apparatus consists of a wafer receptacle 2 having a wafer-receiving opening 3 of only slightly larger diameter than that of the bonded wafer 28 being measured (Fig. 6) allowing the bonded wafer 28 to be securely held in place during the crack propagation procedure. The wafer receptacle 2 is constructed to have an inwardly projecting support surface 4 or ledge upon which the wafer 28 rests. A minimal amount of the perimeter of the bonded wafer 28 is used to support the wafer so that reflectance or transmission of infrared images 38 of the bonded wafers during the crack propagation procedure can be captured without obstruction, as shown in Fig. 7. Fig. 6 is a cross-section of the wafer receptacle 2 illustrating the minimal amount od perimeter of the wafer 28 that rests upon the inwardly projecting support surface 4. The apparatus illustrated in these figures is made for 4-inch wafers, but may be easily modified to accommodate bonded wafers of all sizes by machining a new wafer receptacle 2 to the appropriate size and then attaching the existing adjustment mechanisms 8 and 10 described below.

Attached to the wafer receptacle 2 are the two adjustment devices, an X-axis adjustment device 8, and a Z-axis adjustment device 10, as shown in Figs. 1 through 3. The Z-axis adjustment device 10 includes a precision actuator 18, that may be a micrometer for example, that can be adjusted such that the height of the blade 6 is aligned along the bond interface 29 (Fig. 8) of the bonded wafer. The Z-axis adjustment device 10 may also include one or more screws 16 that can be tightened to hold the blade 6 securely in position. The X-axis adjustment device 8 is illustrated in Figs. 1 through 5. The X-axis adjustment device 8 includes a precision actuator 12, that could be a micrometer for example, attached to a spring 24, which is in turn attached to a support 14 for a blade 6. The precision actuator 12 can be tuned or tightened, pushing on the spring 24 which applies a force to the blade support 14 and blade 6, pushing the blade 6 inward to enter the bonded wafer 28 and crack the bond. The precision actuator 12 in combination with the attached spring 24 allows a person performing the measurement to precisely control the force applied to the blade 6 and the bonded wafer 28, resulting in more accurate bond strength measurements and allowing reproduction of measurements on other bonded wafers under essentially similar conditions, thus allowing a better comparison of bond strengths between bonded wafers.

The receptacle 2 is connected to the adjustment devices 8 and 10. The receptacle 2 and adjustment devices 8 and 10 can be placed on legs or any other supports 20 as demonstrated in Figs. 1 and 2, to facilitate the use of infrared imaging of the bonded wafers during blade insertion.

Fig. 7 illustrates the apparatus in conjunction with the items needed to record an infrared image 38 of the bonded wafer 28. An infrared source 32 is placed under the bottom side of the wafer receptacle 2 in the central opening of the receptacle to illuminate the bonded wafer 28 resting in the wafer receptacle 2. A camera 34 is positioned above the top side of the wafer receptacle 2 to record an infrared image 38 of the bonded wafer. The infrared image 38 is downloaded to a computer 36. The infrared image 38 produces a clear picture of the bonded wafer and crack, allowing for more precise measurements of crack length. In addition, a microscope may be used to magnify the infrared image 38 before the image is captured by the camera 34. The Method

The apparatus described in Figs. 1 2, and 7 can be used in a method for measuring bond strength in bonded wafers. In this method, a bonded wafer 28 is placed in a central opening of a wafer receptacle 2 and rested on the inwardly projecting support surface 4 of the receptacle 2 allowing only a minimal amount of a perimeter of a bonded wafer 28 to rest on the support surface 4. Then a precision actuator 18 of a Z-axis adjustment device 10, which is attached to a blade support 14 and blade 6, is tuned or adjusted to align the height of the blade 6 along the bonded interface of the bonded wafer. The position of the blade 6 may be secured by tightening screws 16 that can be placed on the Z-adjustment device 10 for the purpose of securing the height of the blade 6.

The X-axis adjustment device 8 is tuned or adjusted causing an attached spring 24 to apply force to the blade support 14 and blade 6 in an inwardly radial direction, as shown in Fig. 5. The X-axis adjustment device 8 is adjusted until the blade 6 causes a crack in the bond of the bonded wafer. The resulting crack is then used to measure the surface energy or bond strength of the bonded wafer. Once the crack occurs no further pressure will be applied by the actuator 12, so the crack can be observed as it is initially created and without interference by further entry of the blade 6. An infrared imaging system may be used to capture a more precise and/or larger image 38 of the bonded wafer 28 and resulting crack. As generally shown in Fig. 7, the bonded wafer 28 is illuminated by an infrared source 32, the resulting infrared image 38 is captured by a camera 34 and then downloaded to a computer 36. A microscope may also be used to magnify the infrared image before it is captured. The foregoing description of at least one preferred embodiment and one preferred method are exemplary and not intended to limit the claimed invention. Obvious modifications that do not depart from the spirit and scope of the invention as claimed will be apparent to those skilled in the art.

Claims

We claim:

1. Apparatus for evaluating a bond between semiconductor wafers includes: (a) a generally circular wafer receptacle of a size to closely fit the perimeter of a bonded wafer pair, (b) an inwardly projecting support surface in the receptacle for engaging the wafer pair at its periphery, the receptacle and inwardly projecting support surface defining a central opening through the receptacle, (c) infrared imaging means including an infrared illumination source positioned to illuminate a wafer pair extending across the central opening, (d) a knife support, (e) a knife in the knife support having an edge engageable with an edge of the wafer pair, (f) a Z-axis positional adjuster operative to move the knife axially across the edge of the wafer pair to locate the knife edge at the junction of the wafers of the pair, (g) a radial positional adjuster operative to move the knife edge under force into contact with the wafer pair at the junction of the wafers of the pair to cause at least partial cleaving of the pair, whereby the degree of cleaving that occurs is visible in an infrared image of the wafer pair to give an indication of the degree to which the wafer pair were appropriately bonded.

2. An apparatus for evaluating a bond at an interface between bonded wafers comprising: (a) a wafer receptacle of a size to closely fit the perimeter of a bonded wafer such that the said receptacle secures said bonded wafer, (b) an inwardly projecting support surface in the receptacle for engaging the bonded wafer at its periphery, the receptacle and inwardly projecting support surface defining a central opening through the receptacle, (c) a blade support, (d) a blade on the blade support having an edge engageable with an edge of the bonded wafer at the bond interface, (e) a Z-axis adjustment device including a precision actuator operative to move the blade axially of the bonded wafer to locate the blade edge along the bonded wafer's interface, and (f) an X-axis adjustment device including a precision actuator connected to a spring, operative to move the blade support and blade under force into contact with the bonded wafer interface to cause at least partial cracking of the bonded wafer.

3. The apparatus of claim 2, wherein each of the X-axis and Z-axis precision actuators are micrometers.

4. The apparatus of claim 2, wherein the blade is supported for movement inwardly of the central opening, the spring being interposed between the blade and the X-axis precision actuator.

5. The apparatus of claim 2, wherein the wafer receptacle and the central opening are generally circular and the blade is mounted to allow movement radially inward of the central opening under urging of the spring and the X-axis precision actuator.

6. The apparatus of claim 2 or 3, wherein the Z-axis adjustment device includes screws that can be tightened to prevent substantial movement of the blade once the blade is aligned at the proper height with the bond interface.

7. The apparatus of claim 2 or 3, further comprising a support for the wafer receptacle.

8. The apparatus of claim 7, further compromising an infrared source positioned to illuminate a bonded wafer extending across the central opening, a camera for capturing a resulting infrared image of the bonded wafer, and a computer for downloading, storing and displaying the infrared image.

9. The apparatus of claim 8, further comprising a microscope for enlarging the infrared image.

10. A method evaluating a bond at an interface between bonded wafers, comprising the steps of: (a) placing a bonded wafer in a central opening of a wafer receptacle, on an inwardly projecting support surface in the receptacle, (b) adjusting a Z-axis adjustment device connected to a blade support for a blade so that the blade aligns along the bonded wafer's interface, (c) adjusting an X-axis adjustment device, including a precision actuator connected to a spring, to move the blade support and blade under force into contact with the bonded wafer at the bond interface to cause cracking of the bond, and (d) measuring the length of the resulting crack to determine the strength of the bond.

11. The method of claim 10, further comprising the steps of: (a) illuminating the bonded wafer by means of an infrared source, (b) capturing an infrared image of the bonded wafer and crack with a camera, and (c) downloading and displaying the infrared image from the camera to a computer whereby the length of the resulting crack can be precisely monitored and measured.

12. The method of claim 11 further comprising the step of magnifying the infrared image with a microscope before the infrared image is captured.