md EngineeringA41-482(200)582-589 Passive element T aws Laser anneale miM Non-anneale Annealed ntegration of multiple functionalities in a same substrate sure time.these effects remain negligible fo most cases.On thin-films,the oxide thickness of laser-annealed hasheen introduced durins ess After anneal specimens in normal air was measured for various laser irradi ances s and sca nning speco /.Itwa t the o restored. [B536t ca be performed by direct joule hea Wang et al.38]have performed a systematic analysis of the microstructure found for various scanning parameter and laser was foun d that the microstructure is homogeneous in path matches the desired mechanical element that needs to be wasshown that for fim deposited annealed ona Sisubstrate.stress generated during deposition and anneal- appropriate:in that wavelength range,essentially heat is trans ling micro-de ice.stress ferred to the material.In that wavelength range the Ni-Tiabsorbs used to introduce a two-way effect without the need to intro devices are afew W/mm Irradiance levels depend on the mate duce a:during local ser exposure.the device car er comp rial thickness.geometry and substrate.Process feedback can be into a tensile stress on cooling [391. nsors or by monitoring the change Alaser-annealed micro-gripper is shown in Fig.13[40].This demonstrated on based aer aled micro switch device was designed for the ma pulation of s (39].Inthe annealing temperature ranges,Ni-Tioxidizes rapidly a Nd YAG slab laser.The raw material,as received,does not exhibi a phase transformation.This can be seen on the differential Thet calor 900 aser-affected one.mited t Non annealed 700 ss rest om the machining proc heat producing micr asers laser-machining with water-jet) ct of a serve Smaller 200 (515cs min) aaioaiseirhalequiredadtioal9hOCgTecime From a design point of view,this device is similar to the oiaeds uator conngura n.As theraw material isa 0.00 0.06 0.08 between the contact pads durins sembly As mentioned earlier Strain(%) this pre-strain can be bypassed for laser-annealed thin film-based Fig.12.Tensile characteristics of annealed and no ed binary Ni-Ti. micro-devices.Y. Bellouard / Materials Science and Engineering A 481–482 (2008) 582–589 587 Fig. 11. Schematic of monolithic integration. By locally annealing the material, mechanical properties are distributed across the material. has been introduced during the forming process. After anneal￾ing, the material re-crystallizes and its functional properties are restored. Local annealing can be performed by direct joule heating [35,36]: electrical current flows between two contact points and dissipates heat. This method assumes that the electrical path matches the desired mechanical element that needs to be annealed. Another approach is to use laser annealing [36]. For anneal￾ing application, near-infrared lasers (in the 600–1000 nm) are appropriate: in that wavelength range, essentially heat is trans￾ferred to the material. In that wavelength range the Ni–Ti absorbs about 30–35% of the incident radiation (for a thin film, with near normal incidence). Typical irradiances to anneal SMA micro￾devices are a few W/mm2. Irradiance levels depend on the mate￾rial thickness, geometry and substrate. Process feedback can be achieved using infrared sensors or by monitoring the change of electrical resistivity during annealing. The latter has been demonstrated on thin-film based laser-annealed micro-switch [39]. In the annealing temperature ranges, Ni–Ti oxidizes rapidly in the presence of oxygen: a protective atmosphere may be required for certain application. However, due to the short high￾Fig. 12. Tensile characteristics of annealed and non-annealed binary Ni–Ti. Fig. 13. Laser-annealed monolithic micro-gripper [40]. This example illustrates the integration of multiple functionalities in a same substrate. temperature exposure time, these effects remain negligible for most cases. On thin-films, the oxide thickness of laser-annealed specimens in normal air was measured for various laser irradi￾ances and scanning speed [37]. It was found that the oxide thick￾ness does not exceed 250 nm for the laser irradiances of interest. Wang et al. [38] have performed a systematic analysis of the microstructure found for various scanning parameter and laser power. It was found that the microstructure is homogeneous in the laser spot region. In the previous paragraph, it was shown that for film deposited on a Si substrate, stress generated during deposition and anneal￾ing can be used to achieve a reversible motion. Similarly, in the case of laser annealing of free-standing micro-device, stress induced during annealing and thermal expansion can also be used to introduce a two-way effect without the need to intro￾duce a pre-strain: during local laser exposure, the device can locally be put under compressive stress due to asymmetric ther￾mal expansion; this stress is relaxed during annealing and turns into a tensile stress on cooling [39]. A laser-annealed micro-gripper is shown in Fig. 13 [40]. This device was designed for the manipulation of scaffold parts used in bone reconstruction therapy. The raw material is a cold-rolled Ni–Ti–Cu (5 at.% Cu) sheet. The device is laser-cut using a Nd￾YAG slab laser. The raw material, as received, does not exhibit a phase transformation. This can be seen on the differential scanning calorimetry presented in Fig. 14 (raw material). The laser-cut itself induced a partial annealing effect local￾ized in the laser-affected zone. This annealing effect is limited to the so-called heat-affected-zone (typically 8–10 m). Note that this annealing process resulting from the machining process is not desirable: it can be significantly reduced by choosing a less heat producing micro-manufacturing method (like femtosecond lasers or combined laser-machining with water-jet). The effect of laser annealing on the DSC signal is clearly visible. A sharp, well-defined, transformation peak is observed. (Smaller secondary peaks are related to the DSC specimen preparation itself that required additional laser cutting.) From a design point of view, this device is similar to the bias-spring/actuator configuration. As the raw material is a cold￾rolled sheet, the pre-strain is achieved by introducing an offset between the contact pads during assembly. As mentioned earlier, this pre-strain can be bypassed for laser-annealed thin film-based micro-devices.