Application of a Micromechanical Test Structure to the Measurement of Stress in an Electroplated Permalloy Film

Abstract

Suspended microrotating test structures designed to measure the stress in thin, surface micromachined films have been applied to the production of thick layers of electroplated permalloy (NiFe alloy). This process has particular significance to the production of magnetic MEMS components and devices. It is extremely important to characterise the stress in such materials, especially where these films are to be used on wafers with underlying integrated circuitry as it is well known that the matching of transistors can be affected by mechanical strains induced by interconnect features running above them. A new test chip has been designed and fabricated in order to determine the optimum dimensions for permalloy stress sensor structures.

Test Structures for the Wafer Mapping and Correlation of the Properties of Electroplated Ferromagnetic Alloy Films

“…6 the film thickness of the NiFe alloy is between 15 µm and 19 µm and so we define the maximum valid skin depth δ to be 7 µm. This value of δ is exceeded for results below [10][11][12][13][14][15] MHz and this in turn identifies the frequency below which the assumptions in the model are no longer valid. Consequently for the results in Fig.…”

“…For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org. Important measurement parameters for such materials includes the spatial characterisation of stress/strain [13], resistance [10], [18], alloy composition [14] and Young's modulus [16] and on-chip measurements of these parameters for wafer mapping have all been previously reported in the literature. The missing element that has not been available as a simple on-chip measurement has been the ability to spatially determine the permeability of magnetic materials electroplated on silicon wafers.…”

“…1 was designed to allow a range of physical parameters to be characterised. The design includes MEMS-based strain structures (in the centre) [13], high frequency permeability test structures (top row and second bottom row) and Greek cross electrical test structures (bottom row) [18] which enable a mix of optical (stress) and electrical (DC and RF) measurements on a single wafer. The microrotating strain measurement structures have a designed pointer and rotation arm width of 8 µm and three different expansion arm separations of 6, 8 and 10 µm.…”

Correlation of optical and electrical test structures for characterisation of copper self-annealing

“…The test structures used in this work have previously been used to study the stress in thick electroplated nickel and nickel iron alloys [8]. Figure 1 …”

Test structure to evaluate the impact of neighboring features on stress of metal interconnects

“…Publications in MEMS technologies report the use of thermal expansion to cause deflection of a free-floating needle [1][2][3][4][5][6][7]. This can be used to measure the stress applied by neighboring (and connected) features.…”