E-beam evaporation

E-beam Evaporation Modern machines increasingly use electron-beam evaporators. In this process an e-beam is focused on the evaporation material which, as a result of the energy input from the beam, is heated and evaporates (Fig. 8.2). 

E-beam evaporators have the benefit of higher evaporation temperatures than resistance evaporators and it is also possible to evaporate materials, for example Ni, B, and Co, which alloy or react chemically when using resistance evaporators [3], A disadvantage of e-beam evaporators is that substantially more complex instrumentation is required for process control. 

Evaporation is suitable for all metals (the most refractory metals cannot be thermally evaporated, but can be e-beam evaporated), a few sublimable organic and organometallic compounds such as the phthalocyanines, and a relatively small number of inorganic dielectric materials, such as Mgp2 and SnC>2. Nearly all polymers decompose in one manner or another before a high enough temperature is obtained for them to become mobile in the vapor phase. 

The cathode is typically a low-to-medium workfunction () metal such as Ca (0 = 2.87 eV), A1 ( = 4.3 eV),15 or Mgo.gAgo.i (for Mg,

buffer layer between the top organic layer and the metal cathode improves the device performance considerably. This issue is discussed in some detail in Sec. 1.5.8 below. 

Such an approach is usually followed for physical vapor deposition (PVD) (i.e. sputter deposition or e-beam evaporation). In figure 2.7 we have given the situation for a two dimensional case. Inside the contact the deposition rate can be described by ... 

In the next experiment, an oxide interlayer was deposited between the Cu(TCNQ) layer and the An top contact (0.5 x 2 mm ). The AI2O3 layer had a thickness of 200 nm and was e-beam evaporated at a rate of 2-3 A/s before deposition of the An pad. In spite of a 200 nm AI2O3 dielectric layer no expeeted insulating behaviour was observed. The rough surface profile (up to 1 pm) of a Cu(TCNQ) layer seemed not to be eompletely covered with insulating AI2O3. 

The planar structure was fabricated as follows the glass substrate was carefully cleaned as described above. A 300 nm metal layer was deposited onto the substrate by e-beam evaporation. Next, the metal layer was patterned by standard photolithography. After metal etching, the device structure contained two parallel metal stripes that served as contacts. The distance between the two stripes was 10 pm. Then, a Cu-stripe with a thickness of 70 nm was e-beam evaporated onto the contacts. Finally, the substrate was immersed into the TCNQ/acetonitrile solution. The device was kept in the TCNQ/acetonitrile solution until the Cu layer was completely converted into Cu(TCNQ). Finally, the substrate was rinsed with acetone and dried with nitrogen. Figure 27.10 shows a schematic drawing of the device. 

The extended device was fabricated as follows. After substrate cleaning a 100 nm Au-layer was deposited by e-beam evaporation. After photolithographic patterning an Au middle contact stripe of 5 am was patterned. Through the next deposition, photolithography and etching steps two A1 contact stripes were formed. The two A1 stripes were positioned parallel to the previous Au... 

Planar SDs were fabricated using standard photolithography on 0, 3,4 and 5 min SiN samples (undoped). Before metallization, all samples were cleaned in acetone, methanol, and deionized (DI) water in an ultrasonic bath, followed by boiling aqua regia cleaning for 20 min and 5 min DI water rinse. Ti/Al/Ti/Au (30/100/30/100 nm) ohmic contacts were deposited by e-beam and thermal evaporation, followed by a 60 s rapid thermal annealing (RTA) at 900 °C in nitrogen ambient. Finally, 200 pm diameter Ni/Au (30/120 nm) SDs were deposited by e-beam evaporation. The distance between SDs and ohmic contacts was 50 pm. 

Zinc pretreatment baths are prepared by varying the amount of zinc oxide in a strong alkaline bath. A commercial zincation bath is also analyzed for the purpose of comparison. Three different types of substrates are used CMOS wafer chips with multiple Al bond pads, sputtered silicon wafers, and silicon wafers coated with e-beam evaporated Al. Morphologies of the 3 types of substrates vary in terms of grain size and roughness (Fig. 2). Thickness of the Al films ranges from 5000 A to 1 pm. 

The e-beam evaporated A1 films are deposited by Mr. Walter Lim in the Microelectronics Laboratory of the Department of Electrical Engineering at the National University of Singapore. 

Fig. 5. AFM of CMOS wafer chip after single zincation of (a) 5 s, (b) 20 s, and (c) 30 s AFM of sputtered A1 after single zincation of (d) 5 s and (e) 30 s AFM of e-beam evaporated A1 after single zincation of (f) 30 s.

X-ray damage Only with e-beam evaporation All types of radiation and particle damage are possible... 

The samples have been prepared by e-beam evaporation of a dielectric layer followed by thermal evaporation of the silver fraction, which builds the island film, while the sandwich is completed by a further dielectric film. In every sample, intentionally the same amount of silver (corresponding to an average thickness of 4 nm, as recorded by quartz monitoring) has been embedded in a 6 nm thick dielectric film, formed from either Mgp2, LaFs, Si02, or AI2O3. The optical transmittance T and reflectance R of all films have been measured by a Perkin Elmer Lambda 19 spectrophotometer. To correlate the optical properties with the sample morphology, transmission electron microscopy (TEM) has been applied. 

Electron beam (e-beam) evaporation is a technique suitable for evaporating high temperature evaporation materials such as metals, glasses, and ceramics. It is not used for evaporation of organic semiconductors, which have substantially lower evaporation temperatures. 

Fig. 4.6. Schematic cross-section of an electron-beam gun for e-beam evaporation.

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