热退火对硅通孔中铜微观结构的影响

ABSTRACT  

In this paper, we have studied the microstructure evolution  of one-year room-temperature-aged Through-Silicon Via  (TSV) copper after annealing the TSV samples at 300 °C, 400  °C and 500 °C for 180 minutes. Hardness and elastic modulus  values are obtained by using nano-indentation technique. The  hardness and elastic modulus values decrease as annealing  temperature increases. The microstructure of copper (Cu) is examined to obtain grain size and texture, using electron  backscatter diffraction (EBSD). Copper grain growth, if any, is studied under different annealing temperatures. There was no observable grain growth for the annealing temperatures studied in this work. Moreover, microstructure variation at  different locations within a Cu TSV is also studied.

INTRODUCTION  

Copper through-silicon via (TSV) is an enabling  technology for 3D integration, playing an important role in  connecting stacked dice. Increasingly, TSVs with a diameter  ranging from 100 µm, 50 µm, 10 µm, or less are being  pursued by industry. The microstructure of electroplated  copper will be dependent on the TSV diameter, as the  diameter shrinks in size. Annealing the TSVs at high  temperatures typically changes the grain size and orientation,  resulting in additional changes in material properties such as  hardness. Annealing the Cu TSV reduces the residual stress as  well as Cu pumping and associated reliability issues in  subsequent thermal excursions.  

The performance of transistors is sensitive to stresses  induced by the integration of Cu TSV in a chip. In metaloxide-semiconductor field-effect transistor (MOSFET)  devices, a stress of 100 MPa can change the carrier mobility  by over 7% [28]. There are a few studies done on the  microstructure and mechanical behavior of Cu TSV after  annealing [1, 26, 27], while there are a lot of studies available on the mechanical behavior and reliability aspects of Cu TSVs [2-5, 12-24]. There is only limited study on the long-term  aging of Cu TSVs. In this paper, the effect of long-term aging  followed by high-temperature annealing on the microstructure and mechanical properties of aged Cu TSV is experimentally  studied.

SAMPLE PREPARATION 

Cu TSV samples were fabricated using a conventional  cleanroom process. The silicon wafer was 500 µm thick and  had uniformly distributed 50-µm diameter blind TSVs with  250 µm depth. Copper was electroplated from a seed layer  deposited on the side wall of a blind TSV. Upon electroplating, the silicon substrates were diced into a number of samples for  molding and microstructure analysis. The over-plated Cu was not polished off. The Cu TSV samples were stored in room  temperature (e.g. 25 ºC) and cross-sectioned to be  characterized after one year. The cross-sectioned TSV  samples were polished using colloidal silica solution and  mechanical planarization to obtain a smooth planar surface.  The electron backscatter diffraction (EBSD) was performed to  characterize grain size and texture, while nano-indentation was performed to characterize the hardness and elastic  modulus.

EXPERIMENTAL PROCEDURE  

Since Cu recrystallizatio Annealing effects were studied by EBSD and Nanon occurs at 250  °C [6, 7], annealing temperature was chosen above 250 °C.  This ensures that the temperature is high enough to let  microstructure recrystallization. Three annealing temperatures  were 300 °C, 400 °C and 500 °C with 180 minutes of  annealing for each temperature.  Nano-indentations were carried out using a Nano-indenter  (MTS XP System® equipped with the continuous stiffness  measurement attachment). During the indentation process, the  applied load and the displacement were continuously recorded.  A three-sided pyramidal diamond Berkovich tip to a  maximum depth of 1000 nm. There are six indentations,  starting from 5 µm from the top edge with a spacing of 20 µm along the axis of the TSVs (Figure 1). The spacing was  selected so as to minimize the influence from other  neighboring indents as well as the influence of surrounding silicon walls. The elastic modulus and hardness values were  obtained from the test by using the Oliver-Pharr relation [8].

Fig 1

EBSD measurements were carried out using a TESCAN  SEM® equipped with an EBSD detector. The samples were  angled 70° towards the electron-scanning microscopy (SEM)  detector while taking the EBSD measurements. All EBSD  images were rectangular in shape with 120 um length along  the axis of the TSV and 60 um width along the transverse  width or radial direction of the TSV. EBSD measurements  were first taken on the cross-sectioned samples, and then the  samples were subjected to high-temperature annealing  treatments. It is important to study the cross-sectioned  samples under vacuum condition or flowing argon gas to  prevent copper oxidization. Therefore, the annealing  treatments were accomplished using Cambridge Fiji Plasma®  atomic layer deposition (ALD) tool that is capable of  providing such operating conditions. The cross-sectioned Cu  TSVs were placed in the chamber with a ramp of 13  °C/minute under vacuum and flowing argon gas at 20 sccm  throughout the annealing process. After letting the Cu TSVs  cool down under vacuum environment, the second postannealing EBSD measurement was conducted on the same  TSV. By comparing the two EBSD images, before and after  annealing, the crystal structure changes due to annealing could  be determined.