Name: In Situ Time-dependent Dielectric Breakdown in the Transmission Electron Microscope: A Possibility to Understand the Failure Mechanism in Microelectronic Devices (Video) | JoVE Journal
Uploaded: 2015-06-26T13:00:00.000+00:00
Duration: 9 min 26 s
Description: Learn in situ time-dependent dielectric breakdown testing using transmission electron microscopy to study failure mechanisms in copper ultra low K interconnect stacks. This method reveals real-time degradation kinetics.

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5 minutes

The authors would like to thank R&#252;diger Rosenkranz and Sven Niese (Fraunhofer IKTS-MD) for their assistance in sample preparation, and Ude Hangen, Douglas Stauffer, Ryan Major and Oden Warren (Hysitron Inc.) for their technical support on the PI95 TEM holder. The support of the Center for Advancing Electronics Dresden (cfaed) and the Dresden Center for Nanoanalysis (DCN) at Technische Universit&#228;t Dresden is acknowledged as well.

1. Preparing the Sample for the Focused Ion Beam (FIB) Thinning (Figure 1)
<ol>
	<li>Cleave the full wafer into small chips (~ 10 mm by 10 mm) with a diamond scribe.</li>
	<li>Mark the positions of the &#8220;tip-to-tip&#8221; structure on the chips.</li>
	<li>Saw the chip with a dicing machine to obtain bars of 60 &#181;m by 2 mm size. The bar includes the &#8220;tip-to-tip&#8221; structure in the center.</li>
	<li>Glue the target bar on a Cu half ring using the super glue. Next, glue the bar on a Cu sample stage also using the super glue. Then, use silver paste to set the conduction between the half ring and the copper sample stage. 
	Note: When handling the sample, make sure to always wear an antistatic wrist strap to prevent electrostatic discharges, which may damage the sensitive structure in the sample.</li>
</ol>
2. FIB Thinning in the Scanning Electron Microscope (Figure 2)
<ol>
	<li>Put the sample obtained in step 1 on an SEM sample stage and place the stage carefully into the SEM.</li>
	<li>Chose the deposition mode, and set up the dimensions (area and thickness) of the needed Pt protection layer. Always use a 30 kV ion beam to maintain the highest precision. Tune the current to get the satisfied efficiency, dependent on the dimensions of the needed Pt layer.
	<ol>
		<li>Deposit a Pt line to contact one pad to the Cu stage (ground potential). Subsequently, deposit a thick Pt layer on top of the &#8220;tip-to-tip&#8221; structure, which is very important to minimize the ion damage during the FIB thinning process and to reinforce the thin lamella. This is a standard procedure used in FIB preparation.</li>
		<li>Take caution not to introduce any conductive paths between the two pads on top of the &#8220;tip-to-tip&#8221; structure through the Pt layer when performing the Pt deposition. Any conductive path will short the electrical circuit (Figure 2A and B).</li>
	</ol>
	</li>
	<li>FIB milling
	<ol>
		<li>Use a voltage of 30 kV and current of 10 pA for the final cut. Thin the target bar into an H-bar TEM lamella with a thickness between 150 and 180 nm.</li>
		<li>Cut a notch close to the pad (V+ pad) which will be touched by a transducer tip in the TEM. Use the notch as a marker to identify the correct pad in the TEM.</li>
	</ol>
	</li>
</ol>
3. Sample Transfer from the SEM to the TEM
<ol>
	<li>Put on the antistatic wrist strap before touching the sample.</li>
	<li>Dismount the prepared H-bar sample from the SEM stage. Keep the sample on the Cu stage when removing it from the SEM.</li>
	<li>Fix the Cu stage onto the TEM holder. Move the transducer tip of the TEM holder close to the test structure (a few hundreds of micrometers away from the test structure) under the optical microscope.
	<ol>
		<li>Insert the TEM holder into the TEM carefully. Do not utilize any cleaning treatment (e.g., plasma cleaning) during the transfer process, otherwise the lamella may be influenced.</li>
	</ol>
	</li>
	<li>Keep the time for the sample transfer within 15 min or shorter to avoid too much exposure to ambient moisture and oxygen.</li>
</ol>
4. Establishing the Electrical Connection (Figure 3)
<ol>
	<li>Connect the TEM holder to its control system and the SourceMeter. Then switch on the control system and the SourceMeter.</li>
	<li>Monitor the transducer tip in the TEM when doing the coarse approach of the transducer tip to the test structure by tuning the knobs on the TEM holder.
	<ol>
		<li>Move the transducer tip of the TEM holder close to the V+ pad (&#8804; 500 nm). Bring the transducer tip to the same level (Z: height) as the pad. Tune the position of the tip and make the tip face the center of the V+ pad.</li>
	</ol>
	</li>
	<li>Contact the transducer tip to the V+ pad. Set a very low voltage on the tip (0.5 V to about 1 V) while approaching the pad. Monitor the current simultaneously to make sure the contact is established.</li>
</ol>
5. In Situ TDDB Experiment
<ol>
	<li>Use an accelerated voltage of 200 kV in the TEM. Move the electron beam to the area of interest; choose a proper magnification and focus the image.</li>
	<li>Use low illumination steps (&#8804; 8) to reduce the beam damage on the test structure. Use a condenser aperture to localize the illumination area only within the thin part of the H-bar sample.</li>
	<li>Apply a constant voltage (&#8804; 40 V) on the &#8220;tip-to-tip&#8221; structure using the SourceMeter while recording the TEM images in situ (2-3 frames/sec). Record the images automatically by using a self-scripted code, e.g., using the DigitalMicrograph software.</li>
	<li>Pause the experiment when seeing an apparent diffusion of metal into the ULK dielectrics and do the Electron Spectroscopic Imaging (ESI) chemical analysis.
	<ol>
		<li>Insert the filter slit aperture into the Omega energy filter in the TEM.</li>
		<li>Tune the width of the filter silt aperture to get a proper energy width (10&#8211;20 eV) in the electron energy loss spectrum (EELS).</li>
		<li>Shift the energy to the copper M-edge adsorption peak in the EELS.</li>
		<li>Go back to the imaging mode to acquire an energy filtered TEM image at the Cu M-edge absorption peak.</li>
		<li>Shift the energy to the pre-edge of the copper M-edge and get another energy filtered TEM image.</li>
		<li>Correct the drift of the sample between the two images.</li>
		<li>Divide the first image by the second one to get the jump ratio image of Cu.</li>
	</ol>
	</li>
	<li>Continue the TDDB experiment: reapply a constant voltage (&#8804; 40 V) on the &#8220;tip-to-tip&#8221; structure using the SourceMeter and record the TEM images.</li>
</ol>
6. Computed Tomography
<ol>
	<li>Perform TEM computed tomography when the TDDB experiment is finished, to get 3D distribution information about the diffused particles.</li>
	<li>Tilt the sample and record a tilt series of 138&#176;. Use a tilt step of 1&#176;, and record the image during every step in the bright field (BF) STEM mode.</li>
	<li>Reconstruct the series (includes aligning images, determining tilt axis, reconstructing volume and segmentation to form the 3D tomographic volume).</li>
</ol>

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<li>Liao, Z. Q., et al. In-situ Study of the TDDB-Induced Damage Mechanism in Cu/Ultra-low-k Interconnect Structures. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aLiao%2C+ZQ+%22Microelectron.+Eng%22">Microelectron. Eng</a>. In Press, (2014).</li>
<li>Liao, Z. Q., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((A+New+In+Situ+Microscopy+Approach+to+Study+the+Degradation+and+Failure+Mechanisms+of+Time-Dependent+Dielectric+Breakdown%3A+Set-Up+and+Opportunities%5BTitle%5D)+AND+%22Adv.+Eng.+Mater%22%5BJournal%5D)">A New In Situ Microscopy Approach to Study the Degradation and Failure Mechanisms of Time-Dependent Dielectric Breakdown: Set-Up and Opportunities.</a> Adv. Eng. Mater. 16 (5), 486-493 (2014).</li>
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No competing financial interests.

The prerequisite of success in the TDDB experiment is good sample preparation, especially in the FIB milling process in the SEM. Firstly, a thick Pt layer on top of the &#8220;tip-to-tip&#8221; structure has to be deposited. The thickness and the size of the Pt layer can be adjusted by the SEM operator, but have to follow three principles: (1) The thickness and the size are enough to protect the target area from possible ion beam damage during the whole milling process; (2) There is still a relatively thick Pt layer (&#8...

Since Cu interconnects were firstly introduced into the ultra-large-scale integration (ULSI) technology in 1997 1, low-k and ultra-low-k (ULK) dielectrics have been adopted into the back-end-of-line (BEoL) as the insulating materials between on-chip interconnects. The combination of new materials, e.g., Cu for reduced resistance and low-k/ULK dielectrics for lower capacitance, overcomes the effects of increased resistance-capacitance (RC) delay caused by interconnect dimensional shrinkage 2, 3. However, this benefit was encroached by the continuing aggressive scaling of microelectronic devices in recent years. The use of low-k/ULK materials results in various challenges in the manufacturing process and for the product reliability, particularly if the interconnect pitch reaches about 100 nm or less 4-6.
TDDB refers to the physical failure mechanism of a dielectric material as a function of time under an electric field. The TDDB reliability test is usually carried out under accelerated conditions (elevated electrical field and/or elevated temperature).
The TDDB in on-chip interconnect stacks is one of the most critical failure mechanisms for the microelectronic devices, which has already raised intense concerns in the reliability community. It will continue to be in the spotlight of reliability engineers since ULK dielectrics with even weaker electrical and mechanical properties are being integrated into the devices in advanced technology nodes.
Dedicated experiments have been performed to investigate the TDDB failure mechanism 7-9, and a significant amount of effort has been invested to develop models which describe the relationship between electric field and lifetime of the devices 10-13. The existing studies benefit the community of reliability engineers in microelectronics; however, many challenges still exist and many questions still need to be answered in detail. For example, proven models to describe the physical failure mechanism and degradation kinetics in the TDDB process and the respective experimental verification are still lacking. As a particular need, a more appropriate model is needed to substitute the conservative &#8730;E-model 14.
As a very important part of the TDDB investigation, typical failure analysis is facing an unprecedented challenge, i.e., providing comprehensive and hard evidence to explain the physics of failure mechanisms and degradation kinetics. Apparently, inspecting millions of vias and meters of nanoscale Cu lines one by one and ex situ imaging the failure site is not the appropriate choice to hurdle this challenge, because it is very time consuming, and only limited information about the kinetics of the damage mechanism can be provided. Therefore, an urgent task has emerged to develop and to optimize experiments and to get a better procedure to study the TDDB failure mechanisms and degradation kinetics.
In this paper, we will demonstrate an in situ experimental methodology to investigate the TDDB failure mechanism in Cu/ULK interconnect stacks. A TEM with the ability of high quality imaging and chemical analysis is used to study the kinetic process at dedicated test structures. The in situ electrical test is integrated into the TEM experiment to provide an elevated electrical field to the dielectrics. A customized &#8220;tip-to-tip&#8221; structure, consisting of fully encapsulated Cu interconnects and insulated by a ULK material, is designed in the 32 nm CMOS technology node. The experimental procedure described here can also be extended to other structures in active devices.

<table><tbody><tr><td>Automatic Dicing Saw</td><td>DISCO Kiru-Kezuru-Migaku Technologies</td><td></td><td></td></tr><tr><td>Scanning Electron Microscope</td><td>Zeiss</td><td>Zeiss Nvision 40</td><td></td></tr><tr><td>Picoindentor</td><td>Hysitron</td><td>Hysitron Pi95</td><td></td></tr><tr><td>Keithley SourceMeter</td><td>Keithley</td><td>Keithley 2602/237</td><td></td></tr><tr><td>Transmission Electron Microscope</td><td>FEI</td><td>FEI Tecnai F20</td><td></td></tr><tr><td>Transmission Electron Microscope</td><td>Zeiss</td><td>Zeiss Libra 200</td><td></td></tr></tbody></table>

in situ time-dependent dielectric breakdown in the transmission electron microscope: a possibility to understand the failure mechanism in microelectronic devices

In Situ Time-dependent Dielectric Breakdown in the Transmission Electron Microscope: A Possibility to Understand the Failure Mechanism in Microelectronic Devices

The time-dependent dielectric breakdown (TDDB) in on-chip interconnect stacks is one of the most critical failure mechanisms for microelectronic devices. The aggressive scaling of feature sizes, both on devices and interconnects, leads to serious challenges to ensure the required product reliability. Standard reliability tests and post-mortem failure analysis provide only limited information about the physics of failure mechanisms and degradation kinetics. Therefore it is necessary to develop new experimental approaches and procedures to study the TDDB failure mechanisms and degradation kinetics in particular. In this paper, an in situ experimental methodology in the transmission electron microscope (TEM) is demonstrated to investigate the TDDB degradation and failure mechanisms in Cu/ULK interconnect stacks. High quality imaging and chemical analysis are used to study the kinetic process. The in situ electrical test is integrated into the TEM to provide an elevated electrical field to the dielectrics. Electron tomography is utilized to characterize the directed Cu diffusion in the insulating dielectrics. This experimental procedure opens a possibility to study the failure mechanism in interconnect stacks of microelectronic products, and it could also be extended to other structures in active devices.

Figure 4 shows bright field (BF) TEM images from an in situ test. There are partially breached TaN/Ta barriers and pre-existing Cu atoms in the ULK dielectrics before the electrical test (Figure 4A) due to extended storage in ambient. After only 376 sec at 40 V, the dielectric breakdown started and was accompanied with two major migration pathways of copper from the M1 metal, having a positive potential with reference to the ground side 15-16. The diffused Cu particle...

The time-dependent dielectric breakdown (TDDB) in on-chip interconnect stacks is one of the most critical failure mechanisms for microelectronic devices. This paper demonstrates the procedure of an in situ TDDB experiment in the transmission electron microscope, which opens a possibility to study the failure mechanism in microelectronic products.

The time-dependent dielectric breakdown (TDDB) in on-chip interconnect stacks is one of the most critical failure mechanisms for microelectronic devices. ...

The overall goal of the following experiment is to improve the procedure. Used to study the time dependent dielectric breakdown failure mechanisms and degradation kinetics in copper, ultra low K interconnect stacks. This is achieved by designing and fabricating a dedicated tip to tip structure consisting of fully encapsulated copper interconnects and insulated by an ultra low K material inside a full wafer.As a second step, prepare an intact H bar sample using focused ion beam thinning in the scanning electron microscope. Next, establish the electrical connection and perform in situ electrical tests in a transmission electron microscope. To study the degradation kinetics and failure mechanisms of the copper ultra low K interconnect stacks.The results show the degradation kinetics and the time dependent dielectric breakdown failure mechanisms in copper, ultra low K interconnect stacks. Failure analysis is typically done after the electrical test, which is done at the wafer level, always packaged parts. This only gives a very limited amount of information on the true failure mechanism.The main advantage of our technique is the fact that we're able to follow the failure mechanism in situ in the TEM, the procedure will be shown by Yvo Duncan, our technician, Mula, our TM expert, and John a PhD student in our lab. To begin, put on an antistatic wrist strap to prevent damage to the sample. Next, mark the positions of tip to tip structure on the wafer.Then use a diamond scribe to cleave the full wafer into small chips that are approximately 10 millimeters by 10 millimeters square. Fix the chips on a six inches silicon wafer with hot wax using a dicing machine. Saw each chip to obtain bars that are 60 microns by two millimeters and are centered on the tip to tip structure.Then glue the target bar on a copper half ring using super glue. Next, glue the bar on a copper sample stage. Also using the super glue.Then use silver paste to set the conduction between the half ring and the copper sample stage. Place the sample onto the SEM stage and place the stage carefully into the microscope. Next, choose the deposition mode to add a platinum protection layer using a 30 kilovolt ion beam.Tune the current to around 150 pico amps in order to get a satisfied efficiency for the desired platinum layer dimensions. Deposit a platinum line to contact one pad to the copper stage as the ground potential in a similar manner. Deposit a thick platinum layer on top of the tip to tip structure.This is very important as it minimizes the ion damage during the focused ion beam thinning process and reinforces the thin lamella. Take care not to introduce any conductive connections between the two pads of the tip to tip structure on top. Otherwise, you will destroy your structure.Use a voltage of 30 kilovolts and a current of 10 pico amps to thin the target bar into an H bar. TEM lamella with a thickness between 150 and 180 nanometers. Cut a notch close to the positive voltage pad using focused ion beam thinning.The notch will be used as a marker to identify the correct pad in the TEM and will be touched by a transducer tip in the transmission electron microscope. Put on the antistatic wrist strap before touching the sample. Then remove the prepared hbar sample from the SEM stage while still keeping it on the copper stage.Affix the copper stage onto the TEM holder and move the transducer tip of the TEM holder close to the test structure under an optical microscope. Next, insert the sample into the TEM Carefully keep the time for the sample transfer within 15 minutes or shorter. To avoid too much exposure to ambient moisture and oxygen, connect the TEM holder to its control system and the source meter.Then switch on both the control system and the source meter. Begin the course approach of the transducer tip to the test structure by tuning the knobs on the TEM holder. During the approach, monitor the transducer tip in the window.Next, move the transducer tip of the TEM holder to within 500 nanometers of the positive voltage pad. Bring the transducer tip to the same Z height as the pad, and then tune the position of the tip to make the tip face the center of the positive voltage pad. Set a potential of 0.5 volts to about one volt on the tip while approaching the positive voltage pad.Monitor the current simultaneously to know when contact is established. Move the electron beam to the area of interest. Choose a proper magnification and focus on the image.Use low illumination steps to reduce the beam damage on the test structure. Also use a condenser aperture to localize the illumination area only within the thin part of the hbar sample. Apply a constant voltage of less than or equal to 40 volts on the tip to tip structure using the source meter while recording two to three frames of the TEM images in C two.Pause the experiment when seeing an apparent diffusion of metal into the ULK dielectrics and perform electron spectroscopic imaging chemical analysis. To do this. First, insert the filter slit aperture into the omega energy filter in the TEM.Tune the width of the filter slit aperture to get a proper energy width of 10 to 20 electron volts in the electron energy loss spectrum. Next, shift the energy to the copper M Edge absorption peak in the electron energy loss spectrum. Go back to the imaging mode to acquire an energy filtered TEM image at the copper M Edge absorption peak.Then shift the energy to the pre edge of the copper M Edge and get another energy filtered TEM image, correct the drift of the sample between the two images, and then divide the first image by the second one to get the jump ratio image of copper. To get a three dimensional distribution of information about the diffused particles, tilt the sample and record a tilt series starting at 138 degrees. Use a tilt step of one degree and record images during every step in the bright field scanning mode.Finally, reconstruct the series of images using standard techniques to form a three dimensional tomographic volume. Here is a brightfield TEM image from an in CQ test. There are partially breached tantalum nitride, tantalum barriers, and preexisting copper atoms in the ultra locate dielectrics Before the electrical test due to extended storage in ambient conditions after only 376 seconds at 40 volts, the dielectric breakdown started and was accompanied with two major migration pathways of copper from the M1 metal having a positive potential with reference to the ground side.The diffused copper particles in the ultra locate dielectrics are also visible in a flawless sample. The tip to tip structure stays intact without any damage in the tanin nitrite tanin barrier for more than 50 minutes until the breakdown occurred. It appears that metal atoms migrated into the silicon oxide from the bottom corner of the M1 metal having a positive potential indicated by a red arrow.The electron spectroscopic chemical analysis shown here proves that there is a migration path of copper at the fracture interface between the layers. This could not be detected from the contrast of the bright field TEM images. The main advantage of our technique is that we're able to follow the TDB mechanism in situ and the TEM.What we saw is that in the case of a breached barrier, that massive copper diffusion can occur, and even in the case that there's an intact barrier, alum nitrite, tantalum based, we saw that at the very thinnest positions, the solution can occur and then diffusion follows as well. And this gives a very, very strong emphasis on the fact that the barrier process has to be controlled, especially if we think forward to downscaling of non-electronic products.

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In Situ Time-dependent Dielectric Breakdown in the Transmission Electron Microscope: A Possibility to Understand the Failure Mechanism in Microelectronic Devices (Video) | JoVE Journal

In Situ Time-dependent Dielectric Breakdown in the Transmission Electron Microscope: A Possibility to Understand the Failure Mechanism in Microelectronic Devices

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