experimental validation using accelerated life tests. ... Fatigue lifetime of TPVs in glass is predicted .... The glass used in this study is provided by Asahi Glass.
Thermomechanical and Electrochemical Reliability of Fine-Pitch Through-Package-Copper Vias (TPV) in Thin Glass Interposers and Packages Kaya Demir, Koushik Ramachandran, *Yoichiro Sato, Qiao Chen, Vijay Sukumaran, Raghu Pucha, Venkatesh Sundaram and Rao Tummala 3D Systems Packaging Research Center, Georgia Institute of Technology, Atlanta, GA, USA *Asahi Glass Company (AGC), Japan Abstract This paper reports reliability of copper-plated throughpackage-vias (TPVs) in glass interposer by modeling and experimental validation using accelerated life tests. In this paper, both thermomechanical reliability and electrochemical reliability of fine-pitch TPVs in glass interposer were investigated. Thermomechanical reliability was investigated by developing finite element models to calculate the thermomechanical stresses and strains inside TPVs during thermal cycling tests with several glass and polymer liner combinations. Fatigue lifetime of TPVs in glass is predicted based on these simulation results and then validated using experiments. Test samples with daisy chains of TPVs are fabricated with different glass and polymer material combinations and subjected to accelerated temperature cycling tests to assess the thermomechanical reliability of TPVs in glass interposer. Resistance of each daisy chain is monitored using 4-point probe during cycling. It is observed that majority of test samples passed 1000 thermal cycles without any significant changes in electrical resistance. Crosssectioning of TPV daisy chains that showed significant changes in resistance, revealed that failures were related to defects induced during copper plating in TPV side walls. Electrochemical migration reliability of TPVs in glass was investigated to study conductive anodic filament (CAF) resistance of glass at very small via spacing. Test samples with different material combinations were subjected to biased and highly accelerated stress temperature-humidity test (HAST) to assess electrochemical migration reliability of TPVs. After biased-HAST for 100 hours at 130oC, 85% relative humidity (RH) and 5 V DC, no CAF failures were detected in either of the two material combinations, indicating good insulation reliability under high temperature and humidity conditions. Key Words TPV (Through Package Via), Reliability, Glass Interposer, Temperature Cycling Test (TCT), biased-HAST (Highly Accelerated Stress testing), Mechanical modeling Introduction As organic packages reach limits in I/O density, silicon interposers are being developed to address these limits. However, silicon interposers have their own limits in cost and electrical performance. Glass appears to be an ideal candidate for interposers that addresses the limits of both organic and silicon substrates [1]. Glass is an attractive material for interposers due to its silicon matched-CTE, excellent surface flatness, dimensional stability, high electrical resistivity and availability in thin and large panels at low cost [2]. Georgia Tech PRC has proposed and begun to demonstrate glass interposer as an electrically-superior and lower cost
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alternative to wafer-based silicon interposers to enable 3D integration of logic and memory devices [3]. Such a 3D concept using glass interposer allows for integration of ICs on both sides, while enabling testability, scalability, better thermal management at low cost and without disruption to form TSVs(Through-Silicon-Via) in wafer fabs. However, the main concern with thin glass is its brittleness and mechanical strength, especially with arrays of through vias with copper metallization. Laminating the glass with polymer on both sides has previously been demonstrated as a successful method to enhance the material strength and handling during processing [3]. However, two other major challenges must be overcome to achieve reliable glass interposers with fine pitch through vias; (a) defects created on the glass surfaces and via walls, during via formation, and (b) the mismatch in coefficient of thermal expansion (CTE) between glass, copper and polymer liner resulting in high stress on the via walls. High insulation resistance between adjacent TPVs under bias voltage, high temperature and high humidity conditions are also essential to enable the use of glass for high I/O density packages in harsh environmental conditions. Reliability of through-silicon-vias is extensively studied both from modeling and experimental aspects [4, 10].However, very limited data is available for reliability of fine-pitch TPVs in thin glass interposer [11]. Guidelines for glass interposer design, fabrication and TPV formation were previously reported [3], but TPV reliability, the biggest barrier to glass interposers and packages, has not yet been addressed. This paper presents the first comprehensive study on the thermomechanical and electrochemical migration reliability of laser-ablated fine pitch through-package-copper vias (TPVs) in thin glass interposers using finite element modeling, fabrication and validation by accelerated testing. Thermomechanical Reliability of TPV in Glass Thermomechanical reliability of TPV in glass is investigated through finite element modeling and experimental validation by thermal cycling test. The TPV cross-section shown in the figure 1 is used for finite element modeling and experimental verification. Finite Element Modeling The CTE mismatch between the materials constituting the TPV results in stresses during thermal loading. In order to understand the effect of CTE mismatch on reliability, axisymmetric finite element models of TPVs were created and simulated in AnsysTM. Due to the symmetry of the structure, only quarter of a TPV is modeled to reduce simulation time while maintaining the accuracy. Standard temperature cycling
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2013 Electronic Components & Technology Conference
between -55oC and 125oC was applied as thermomechanical load to TPV.
(c) (d) Figure 2. (a) Quarter TPV model, b) Shear stress between material surfaces (c) Axial Stress in Glass and (d) Plastic Strain in Copper
The glass used in this study is provided by Asahi Glass Company (AGC) with product names EN-A1 and CF-XX in 180μm thickness and in two different CTEs (low CTE and high CTE). Polymer liners used in this study are ZEONIF ZS6 (ZIF) from ZEON Corporation and RXP-4 from Rogers Corporation.The key material properties of glass, polymer and copper are described in table 1.
From simulation results, it was observed that CF-XX/ZIF combination results in lowest shear stress and lowest tensile axial stress in glass resulting in lower risk of glass cracking. Additionally, CF-XX/ZIF combination gives the lowest equivalent plastic strain implying higher lifetime with respect to other combinations. With all the constituting material combinations, copper-polymer and copper-polymer-glass material junctions are critical parts due to high stresses and strains.
Table 1. Material properties Elastic Modulus(GPa)
Poisson’s Ration
CTE(ppm/oC)
Stress Free T(oC)
Glass (EN‐A1)
77
0.22
3.8
25
Glass(CF‐XX)
74
0.23
9.8
25
Polymer Liner(RXP‐4)
1.83
0.3
67
232
Polymer Liner(ZIF)
6.9
0.3
31
120
Copper (Cu)
121
0.3
17.3
25
Maximum Shear Stress(MPa)
Maximum Shear Stress(Mpa)
Maximum Axial Stress in Glass(Mpa)
180
60
175
50
170
40
165
30
160
20
155
10
150
0 EN‐A1/RXP‐4
Thermomechanical simulations were performed to compare stress/strain of different TPVs with two different glasses and two different polymers. Sample simulation results after 10 thermal cycles at low extreme temperature are shown in Figure 2.
EN‐A1/ZIF
CF‐XX/RXP‐4
CF‐XX/ZIF
Glass/Polymer Combinations
Maximum Plastic Strain in Copper
(a) 0.025 Plastic Strain in Copper 0.02 0.015 0.01 0.005 0 EN‐A1/RXP‐4
EN‐A1/ZIF
CF‐XX/RXP‐4
CF‐XX/ZIF
Glass/Polymer Combinations
(b) (a)
(b)
Figure 3. (a) Shear Stress and Axial stress in glass and (b) Equivalent plastic Strain in copper for different combinations.
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Maximum Axial Stress in Glass(MPa)
(a) (b) Figure 1. (a), (b) Cross-sections of analyzed TPV.
The expected lifetime of the TPVs was calculated based on copper fatigue failure. As indicated by simulations, CTE mismatch between copper, glass and polymer results in plastic deformation in copper during every cycle. The plastic deformation leads to fatigue failure of TPVs. Accumulated plastic strain per cycle can be calculated from simulations. Using a Coffin-Manson type equation, lifetime of TPVs in different combinations can be roughly estimated. N
c f
Test Vehicle Details The process flow for fabricating the test samples are shown in figure 5.The details of each fabrication step were presented in previous papers [3]. The low-cost and high throughput fabrication process basically consists of two major steps: 1) TPV hole formation, and 2) TPV and surface metallization through semi-additive process.
0.75 f p
c : Fatigue ductility exponent of copper (-0.6) ' : Fatigue ductility coefficient of copper (0.3) f
N f : Number of cycles to failure
p : Accumulated Plastic strain in each cycle
Table 2. Fatigue life of TPV for different material combinations. Glass/polymer combination EN-A1/RXP-4 EN-A1/ZIF CF-XX/RXP-4 CF-XX/ZIF
(a) (b) Figure 5. (a) TPV hole formation and (b) TPV and test vehicle metallization.
Approximate fatigue life 4750 6250 4000 7500
Number of cycles to failure can be solved roughly and approximate fatigue life expectations are summarized in table 2. It is seen that fatigue life of TPV is estimated to be over 3500 thermal cycles for all material combinations. Test Vehicle Design The test vehicle design used for investigating the thermomechanical reliability of TPVs is shown in figure 4. Each test coupon consists of 64 vias arranged in an 8x8 array and metalized to form daisy chains. The diameter of the TPV is 60µm and the pitch is 120µm. Each array is connected to 4 ports to accurately measure using 4-probe resistance by applying DC on to the 2 pads while measuring the voltage on the other 2 pads.
The bare glass surface is first treated with silane coupling agents to improve adhesion to the polymer and then glass is laminated with polymers on both sides via vacuum lamination. Before laser ablation, polymer surface is metalized by electroless and electrolytic plating to form a thin (approximately 1µm) copper layer. This copper layer acts as a protection by preventing glass debris to attach to polymer surface during laser ablation. The TPV formation is performed by 193nm excimer laser ablation and the polymer surface is then cleaned by etching the protective copper layer. After laser ablation, TPV and polymer surfaces are again metalized by electroless copper. The surface is patterned by photoresist deposition and lithography. Electrolytic plating is followed by removing the photoresist and remaining copper seed layer to complete the fabrication process. Three different glass-polymer liner variations were used in this study. These different combinations are outlined in table 3. Table 3. Different material combinations used in thermal cycling test. Glass EN-A1 (low CTE Glass) EN-A1 (low CTE Glass) CF-XX (high CTE Glass)
Center part
Polymer Liner RXP4 ZIF RXP-4
(a) The thickness of the glass was 180μm, and the thickness of polymer liner was 20μm. The plated copper thickness inside the TPV and on the surface was approximately 10μm, with a typical surface line width of 90μm. The fabricated test vehicles with various combinations of glass and polymer are shown in figure 6.
90μm 60μm
120μm Glass (180μm)
(b)
Figure 4. (a) Test vehicle layout and (b) TPV arrays and cross-section.
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resistance, as also indicated by finite element modeling simulation. Table 5. Temperature cycling test results.
(a) (b) (c) Figure 6. (a), (b), and (c). Top side image of EN-A1/RXP4, EN-A1/ZIF and CF-XX/RXP-4 samples respectively. Test setup and accelerated testing Electrical resistance of the TPV daisy chains was monitored periodically using 4-point probe measurement to identify the initiation of cycling-induced failures. By using 4 point probe as shown in figure 7, the resistance contribution coming from copper lines and contact resistance between probes and pads was minimized. The 4-Probe system is calibrated with two reference resistors to maintain sensitivity and the calibration results are shown in table 4.
Glass/polymer combination ENA1/RXP4 ENA1/ZIF CF-XX/RXP-4
No. of pass/total no. of test coupons 35/40 37/44 48/48
Failure analysis was done by cross-sectioning. In figure 8 and figure 9 a passed TPV array and a failed TPV array corresponding to the same material combination is shown in a comparative manner.
(a)
(a) (b) Figure 7. (a) Nanovoltmeter and precision current source (b) Probe station.
(b) Figure 8. (a)Passed EN-A1/RXP-4 TPV chain and ( b) Failed EN-A1/RXP-4 TPV chain.
Table 4. Calibration of 4-point probe measurement system. Measured Resistance 5.0mΩ 0.5mΩ
5.0081mΩ 0.5052mΩ
Accuracy 0.16% 1.01%
The test vehicles were subjected to a 24h bake at 125oC to, moisture sensitivity level-3 (MSL-3) preconditioning( 60C, 60% RH and 40 hours) ,followed by 3-times reflow at a peak temperature of 260oC to simulate the lead-free board assembly processes and to investigate the effect of moisture absorption on reliability. The test vehicles were then subjected to thermal cycles between -55oC and 125oC with a dwell-time of 15 minutes at each temperature (JEDEC JESD22-A104 condition B test standards Results and Discussion Failures were identified based on changes in electrical resistance during cycling. A 10% change in initial resistance value of the TPV daisy chain is selected as the failure criterion. The results of thermal cycling test are given in Table 5. The failed samples were cross-sectioned and characterized to understand the TPV failure mechanism. The experimental results revealed that the majority of TPVs survived 1000 thermal cycles without any significant change in electrical
(a)
(b) Figure 9. a) Passed EN-A1/ZIF TPV chain and (b) Failed EN-A1/ZIF 4 TPV chain. It is observed that for both the combinations, the main cause of failure is related to copper plating process defects. Problems associated with plating high aspect ratio structures resulted in failures in metalizing single TPVs as well as TPV arrays.
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Electrochemical Migration Reliability of TPVs in Glass The next section of the paper deals with the electrochemical migration reliability of fine-pitch TPVs in glass interposer. A sub-surface failure mode, known as conductive anodic filament (CAF), was investigated in this study. This type of failures has been known to commonly occur in glass fiber reinforced organic substrates on the resinglass fiber interface [5].The CAF failures are reported to be accelerated by drilling process induced interfacial degradation, thermal stresses resulting from lead-free assembly processes and moisture induced chemical degradation of silane coupling agents in organic substrates. Small conductor spacing results in reliability issue from previously reported quantitative mean time to failure model describe a dependence of L2 or L4, where L is the conductor spacing (wall to wall spacing between two TPVs) [6]. The glass interposer test vehicle used in this study also consists of a polymer-glass fiber interface with very fine-pitch via spacing, which is prone to similar type of electrochemical migration failures in the presence of temperature, humidity and voltage. Figure 10 shows the schematic of glass interposer used in this study with potential electrochemical migration failure sites.
reliability of fine-pitch TPVs in glass interposers has not yet been reported. This is the main motivation behind studying reliability of TPVs in glass interposers in high temperature, humidity and bias conditions. Test vehicle design The test vehicle design used in this study is shown in figure 11. Each test coupon consists of 64 vias arranged in 8 x 8 arrays, 8 rows of which are positively biased and the rest 8 are negatively biased. The diameter of the TPVs is 60 µm and the pitch is 120 µm, resulting in via wall-wall spacing of 60 µm. The TPVs were connected in a configuration as shown in figure10. This type of via arrangement increases the number of possible failure sites, enabling the TPVs to fail in both directions as opposed to linearly connected TPVs. Such a TV design results in 112 potential failure sites in one test coupon with 64 vias.
Center part
Potential failure interface sites
90μm 60μm
120μm
Copper plated TPV
Glass (180μm)
Glass
Figure 11. Test vehicle design (alternate rows of anode and cathode) used for electrochemical migration test.
Polymer Positive biased TPV
Negative biased TPV
Figure 10. Schematic showing possible electrochemical migration failure sites in glass interposer with polymer liner. The electrochemical migration reliability of TPVs was investigated in this study using biased highly accelerated temperature and humidity stress test (biased-HAST). BiasedHAST has previously been used to investigate the electrochemical reliability of through-vias in glass fiber reinforced organic package substrates with different bias voltages [7]-[9]. The CAF failures in biased-HAST have been reported in organic package substrates such as FR4 and BT at a through-via spacing of 100 µm and 150 µm respectively [8, 9]. In addition to process induced degradation, the CAF failures are also influenced by the substrate material properties such as moisture absorption, glass transition temperature and CTE. The magnitude of such failures increases significantly with smaller spacing between the through-vias, as a result of large electric fields between the conductors, thus resulting in dramatic reduction in failure time. Such failures lead to poor via reliability in high temperature and humid conditions, acting as a barrier to miniaturization of organic packages. Ultra-fine pitch TPVs have been previously demonstrated in glass interposers [3]. Such fine-pitch TPVs are expected to result in large electric fields between the vias, thereby potentially reducing the mean time to failures. However, electrochemical migration
Test vehicle details The test vehicles used in this study consisted of a stack up similar to the test vehicles previously described for thermal cycling test. Two different glass-polymer combinations were used in this study. For each polymer-glass combination, four coupons were tested resulting in a total of eight test coupons for the two different variations. The positive anode of the test coupons vehicle were connected together and the pads connected to the negative bias were individually connected to the voltage source as shown in Figure 12. Each test coupon consisted of four contact pads for measurement to isolate the failures. The test vehicles utilizing different glass and polymer liner combinations were fabricated using the same process as previously described in figure 5, shown in figure 12. Figure 12 shows image of top surface of the test vehicle used in this study. Test setup and accelerated testing The test setup consists of an HAST chamber, to produce 130C and 85% required for the test. A source/measurement unit (Keithley 236) was used to apply the voltage and measure the output current. The test setup used in this study is shown in fig D. The test vehicles were first subjected to a 24 hour bake at 125C, MSL-3 preconditioning (60C, 60% RH and 40 hours), followed by 3-times reflow at 260C, and then subjected to biased-HAST (130C, 85% RH and 5V DC bias) for 100 hours as per JEDEC JESD22-A110 test standard.
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Insulation resistance was recorded periodically and a drop in resistance to 50 kΩ was used as the failure criteria.
(a)
(b)
(c) Figure 12.(a),(b),(c) Fabricated test vehicles EN-A1/ZIF ,EN-A1/RXP-4 and CF-XX/ZIF samples respectively and (d) Wired sample for HAST. Results and discussion The insulation measurement results after biased-HAST test are summarized in table A. It was observed that all the 4 test coupons with EN-A1/RXP-4 and EN-A1/ZIF survived the biased-HAST. Table 6. Biased-HAST results of different glass-polymer combinations. Glass Polymer Combination EN-A1/RXP-4 EN-A1/ZIF
No. of Pass/ Total No. Of Test coupons 4/4 4/4
The absence of failures in the two test vehicle combination indicates the possibility of limited or no process-induced degradation between glass and polymer liner. Process induced degradation during via formation can cause copper plating in the cracks, further decreasing the via wall-wall spacing. This results in a very small length for copper migration resulting in drastic reduction in failure times, sometimes as soon as application of bias. The path formation for copper migration has been known to be the rate-limiting step for CAF failures. Thus, the insulation measurement results indicate a suitable via drilling and plating processes preventing electrochemical path even at small via spacing. Additionally, material properties such as moisture absorption and thermal stability have also been known to influence CAF failures. Low moisture absorption property and high thermal stability are preferred to prevent chemical degradation and thermal stress related degradation and to achieve high CAF resistance of substrates [7]. Glass has negligible absorption of moisture and
the moisture absorption of polymer liners that were used in the test vehicles is quite low. Also, adhesion of polymer to glass was found to be sufficiently high enough to survive high reflow temperature (260C) and HAST (130C and 85% RH) condition without interfacial delamination. The higher glass transition temperature (Tg) of glass used in this study also indicates superior thermal stability, which is a key requirement for lead-free assembly resulting in smaller dimensional changes during assembly. This in turn results in smaller stress at the glass-polymer liner interface during assembly. A backlight optical microscope was used to analyze the test samples after biased-HAST and no signature of subsurface migration could be detected. The biased-HAST test is being continued beyond 100 hours to predict mean time to failure in glass interposers. Summary and Conclusions In summary, this paper presents the first comprehensive study on the reliability of fine-pitch copper-plated through package vias (TPVs) in thin glass interposers. Thermomechanical reliability of TPV is studied through finite element modeling of different material combinations. Modeling results are validated by temperature cycling tests. It was found that TPV holes in glass formed with laser ablation passed reflow and 1000 standard thermal cycles (temperature extremes-55oC and 125oC). Identified failures were platingprocess related defects. Thus if via formation methods are successful in forming holes in thin glass with minimal surface defects and if the adhesion of materials are high enough to endure thermal stresses due to CTE mismatch, reliability of TPVs seem to be high. The other section of the paper deals with electrochemical migration reliability of TPVs in glass interposers.Test vehicles of different material combinations are fabricated and subjected to biased-HAST. The insulation measurement results and failure analyses revealed no indication of CAFrelated failures. The results suggest that via formation method in thin glass by laser ablation creates minimal degradation on glass surfaces and material interfaces. Additionally, the material properties of the glass interposer test vehicle used in this study such as low moisture absorption and high thermal stability help in preventing CAF failures in biased-HAST. Acknowledgements This research was supported by the Silicon and Glass Interposer Industry Consortia. The authors would also like to thank Tobias Materne and Prakhar Srivastava for their assistance with figures and cross-sections. References 1. Tummala, Rao R., et al. "Trend from ICs to 3D ICs to 3D systems." Custom Integrated Circuits Conference, 2009. CICC'09. IEEE. IEEE, 2009. 2. Sukumaran, Vijay, et al. "Design, fabrication and characterization of low-cost glass interposers with finepitch through-package-vias." Electronic Components and Technology Conference (ECTC), 2011 IEEE 61st. IEEE, 2011. 3. Sukumaran, Vijay, et al. "Through-package-via formation and metallization of glass interposers." Electronic
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