Characterisation of Through Silicon Via (TSV) …

1 1 Characterisation of Through Silicon Via (TSV) processes utilising Mass Metrology Liam Cunnane, Adrian Kiermasz PhD, Gary Ditmer Metryx Ltd., Bristol UK Abstract Implementation of the Through Silicon Via (TSV) process in the IC wafer fab is a fabrication methodology being developed to achieve 3D-IC design architectures. The TSV process flow shares many common process steps with standard BEOL processing, however significant challenges are presented due to scale and aspect ratio while still demanding highly reliable connections between the stacked chips. Mass1 metrology is an innovative, non-destructive, on-product measurement technique implemented to monitor individual TSV process steps or a module sequence.

2 Mass change in any process step encompasses all physical parameters, thereby providing a process fingerprint, with any change in the ultra-thin microelectronic layers recognised with atomic layer accuracy. Existing metrology techniques face challenges in the TSV process and these challenges are effectively addressed by mass metrology. The characteristic results of the Silicon Via etch including CD, depth and profile; all impact the mass change of this process. Residual polymer and the effectiveness of its cleaning can be measured. Mass change reveals the conformal step coverage of the Via Liner dielectric. The Barrier/Cu seed process, as well, requires monitoring of a conformal deposition in the high-aspect ratio vias, confirming stack integrity.

3 Use of mass metrology to non-destructively characterise the selective bottom-fill may be applied in Copper Fill process development, as well as providing a monitoring capability in manufacturing. TSV fabrication challenges will intensify with calls for improved product performance, requiring increased interconnect density and a corresponding increase in aspect ratio, thereby placing new demands on metrology. The high aspect ratio features limits what information is available from conventional non-destructive metrology. Mass metrology provides a comprehensive solution allowing processes to be brought from development to production, providing effective monitoring on product wafers in a high-volume manufacturing environment.

4 Introduction The application of a Through Silicon Via process achieves increased functionality and performance in device manufacture. As these processes are introduced into production; the development activity continues for more advanced structures. Adoption into high volume manufacturing (HMV) will demand that the reliability of the TSV process is established, even as via densities increase and the dimensions are reduced. TSV processes are unique in that the scale of the features range from sub-micron up to processes where many microns of material are removed or deposited. The cost of the Cu fill is a clear example where TSV fabrication creates a cost issue directly related to via size.

5 The structures created are generally large, however the accepted degree of variation in a successful process will be relatively, very small. Metrology capable of scaling across the process range with the essential sensitivity is necessary for implementation into the production environment. Mass metrology provides a unique capability in non-destructively monitoring the physical nature (dimensions) of the process, on the product wafer. The technique is also shown to be an effective contributor in the development of process technologies utilised in the TSV fabrication sequence. 2 TSV Si Etching The etching of the TSV is a critical step in the process flow, in which the depth and profile must be well controlled.

6 With higher Aspect Ratio (AR) features the sidewall taper or profile is particularly important, as the common approach for barrier deposition remains a PVD-based process. Therefore, a metrology requirement exists for both TSV depth and sidewall angle control. Two TSV profile variants, as shown in , are compared. TSV-A is a straight walled via with minimal taper, while TSV-B is a high aspect ratio via with tapered profile. The experiment is carried out with a number of wafers etched for process times longer and shorter than the process of record (PoR). The mass change associated with the etch process is measured using Metryx metrology.

7 Cross-sectional SEM analysis is then used to measure the via depth. The etch time, depth, and mass change are expressed as a percent deviation from the PoR. The relationship between etch time or cycle time (Bosch process) and via depth is shown in For both TSV-A and TSV-B a reducing etch rate (depth) as a function of etch time is observed. Mass change, plotted against etch time in Fig 3., yields similar information non-destructively. Note also the significant difference in behaviour observed between TSV-A & TSV-B when mass change is the measured response. The difference is in the rate of change of mass loss. TSV-A conforms to a linear fit, that is to say the rate of change in mass is zero.

8 TSV-B exhibits a decreasing rate of mass loss as a function of etch time. In the case of TSV-B this is the result of the highly tapered profile. As the TSV-B etch proceeds there is less mass loss due to the shrinking diameter at the via base. Mass change indicates the volume of Silicon removed, and the relationship between via depth and mass loss is linear for a constant via diameter. Using absolute mass loss and rate of change in the mass response provides a more comprehensive understanding of the process. The value of studying rate of mass change with processing time has been reported in the case of Plasma doping 2 TSV BTSV ARADA Fig.

9 1 TSV profile variations TSV By = + + = Ay = + + = Time % (Cycles)Depth % of PORD epth TSV ADepth TSV B Fig. 2 Etch depth vs Cycle time TSV By = + + = Ay = + = Time % (cycles)Mass Loss % of PoRMass TSV AMass TSV B Fig. 3 Mass loss vs Cycle time 3 TSV Polymer Formation and cleaning The extent of polymer formation will also impact the via profile. In the case of a highly tapered via, a negative rate of change in mass is observed with respect to increasing cycle time. When the process is in a regime of inadequate sidewall polymer formation, the mass loss as a function of etch depth or time will exhibit a positive rate of change, as the mass change accelerates due to the increasing rate of change in the surface area of the via.

10 The impact on via volume as a result of changing sidewall angle has a clear impact on mass response. As a practical matter, via etch processes operate in a regime where etch depth is a near linear function of etch time. However, a change in side wall angle will have a profound impact on via volume. The volume of a 25 m deep via will be reduced by approximately 30% when the sidewall angle changes from 90deg to 88deg. Since the mass loss directly represents the volume of Silicon etched, it provides a metrology response that is sensitive to both depth and sidewall angle. The mass change due to Silicon etch is monitored by taking an initial mass measurement of the wafer before masking and then after etch and resist strip, isolating the mass change associated with the Silicon etch.