Passivation of the sensor

Chapter 4: The surface of the magnetic biosensor

nm in an argon/oxygen plasma mixture, resulting in the formation of a thin tantalum-oxide layer at the interface which assures good adhesion (Ref. 167).

nm in an argon/oxygen plasma mixture, resulting in the formation of a thin tantalum-oxide layer at the interface which assures good adhesion (Ref. 167).

The thermal expansion coefficient of tantalum (6.6·10^-6/K, Ref. 156) is in-between the ones of typical sensor materials like iron or copper (12..17·10^-6/K; Ref. 168) and the expansion coefficient of SiO2 (0.45·10^-6/K, Ref. 168), thus reducing the mechanical stress at the interface. It is further decreased by a thin polymer layer on top of the SiO2. Together, these two layers form laminated glass, which is less brittle than SiO2

alone. The polymer is spin-coated from a 2.5 % (by weight) dioxane solution at 4000 rpm, resulting in a thickness of about 70 nm. Its exact composition is discussed in the following subchapter. The entire sensor passivation is sketched in Figure 27.

The thermal expansion coefficient of tantalum (6.6·10^-6/K, Ref. 156) is in-between the ones of typical sensor materials like iron or copper (12..17·10^-6/K; Ref. 168) and the expansion coefficient of SiO2 (0.45·10^-6/K, Ref. 168), thus reducing the mechanical stress at the interface. It is further decreased by a thin polymer layer on top of the SiO2. Together, these two layers form laminated glass, which is less brittle than SiO2

alone. The polymer is spin-coated from a 2.5 % (by weight) dioxane solution at 4000 rpm, resulting in a thickness of about 70 nm. Its exact composition is discussed in the following subchapter. The entire sensor passivation is sketched in Figure 27.

wafer SiO2

Ta-layer sensor element

polymer 70 nm

150 nm

Figure 27: Cross-section of the sensor passivation Figure 27: Cross-section of the sensor passivation

The passivation can only protect from oxidation if there are no residual particles on the sensor surface prior to the deposition of the protection layer. Otherwise, it is not tight enough and liquid can enter through the imperfections. Dust and other unintentional particles (like metal shreds after lift-off processes) are avoided by careful treatment and processing in a cleanroom. However, there is always the possibility of resist remnants at the edges of exposed patterns (both for our optical and e-beam resists), and these residues can also cause leakage through the passivation layer. This is demonstrated in Figure 28, which shows the partial oxidation of a TMR sensor element after leakage of solvents through imperfections in the passivation layer. The resist residues display a bright contrast, and the oxidized parts of the sensor element appear dark due to reduced secondary electron generation compared to the metallic parts.

The passivation can only protect from oxidation if there are no residual particles on the sensor surface prior to the deposition of the protection layer. Otherwise, it is not tight enough and liquid can enter through the imperfections. Dust and other unintentional particles (like metal shreds after lift-off processes) are avoided by careful treatment and processing in a cleanroom. However, there is always the possibility of resist remnants at the edges of exposed patterns (both for our optical and e-beam resists), and these residues can also cause leakage through the passivation layer. This is demonstrated in Figure 28, which shows the partial oxidation of a TMR sensor element after leakage of solvents through imperfections in the passivation layer. The resist residues display a bright contrast, and the oxidized parts of the sensor element appear dark due to reduced secondary electron generation compared to the metallic parts.

resist residues

Figure 28: Oxidation of a TMR sensor element after leaking of liquids through imperfections caused by resist residues

Figure 28: Oxidation of a TMR sensor element after leaking of liquids through imperfections caused by resist residues

Chapter 4: The surface of the magnetic biosensor The surface of the magnetic biosensor

43 er 4:

43 Mainly, such residues are generated when employing the resist itself as a mask for ion beam etching. Since the sample is rotating and tilted relative to the incident beam, the resist at the edges of the patterns is directly exposed to the beam throughout its entire thickness. Apparently, the energy deposited into the resist by the Ar-ions causes a chemical transformation of the polymer (most probably chain crosslinking) which prevents its subsequent removal. These remnants at the pattern edges are almost impossible to get rid of, which is demonstrated by the series of images in Figure 29, each showing the same spot after various attempts to remove the residues. Even prolonged exposure to an oxygen plasma does not remove the remnants completely, and such a harsh treatment is certainly not desirable concerning the sensor properties. Lift-off processes, on the other hand, are less susceptible to this problem, and the resist residues can be removed by not too harsh treatments with the designated chemicals. An exception is SiO2 lift-off, which also produces quite stable residues along the pattern edges.

Mainly, such residues are generated when employing the resist itself as a mask for ion beam etching. Since the sample is rotating and tilted relative to the incident beam, the resist at the edges of the patterns is directly exposed to the beam throughout its entire thickness. Apparently, the energy deposited into the resist by the Ar-ions causes a chemical transformation of the polymer (most probably chain crosslinking) which prevents its subsequent removal. These remnants at the pattern edges are almost impossible to get rid of, which is demonstrated by the series of images in Figure 29, each showing the same spot after various attempts to remove the residues. Even prolonged exposure to an oxygen plasma does not remove the remnants completely, and such a harsh treatment is certainly not desirable concerning the sensor properties. Lift-off processes, on the other hand, are less susceptible to this problem, and the resist residues can be removed by not too harsh treatments with the designated chemicals. An exception is SiO2 lift-off, which also produces quite stable residues along the pattern edges.

remover AR 300-70 for 75 min at 80°C in ultrasonic bath

20 min in oxygen plasma (30 W, 130^.10^-3 mbar oxygen pressure) remover

1-methyl-2-pyrrolidinone for 90 min at 80°C in ultrasonic bath 80 min ion etching

remover AR 300-70 for 40 min at 80°C in ultrasonic bath

Figure 29: Resist remnants after various attempts to remove them Figure 29: Resist remnants after various attempts to remove them

To avoid the problem of resist remnants, all processes involving etching steps are carried out using tantalum hard masks instead of the resist itself. This makes lithography somewhat more complex, since the thickness of the hard mask has to be tuned to the required etching dosage. Otherwise, the mask will either remain in parts on the sensor pattern, or the top layers of the sensor stack will be etched away. The conceptual difference between the two methods is sketched in Figure 30.

To avoid the problem of resist remnants, all processes involving etching steps are carried out using tantalum hard masks instead of the resist itself. This makes lithography somewhat more complex, since the thickness of the hard mask has to be tuned to the required etching dosage. Otherwise, the mask will either remain in parts on the sensor pattern, or the top layers of the sensor stack will be etched away. The conceptual difference between the two methods is sketched in Figure 30.

etching process with a hard mask etching process with a resist mask

ion beam

3) ion etching (resist mask) resist

sensor wafer 1) pattern exposure

2) developing 4) removing of resist

ion beam Ta

resist

sensor wafer

4) lift-off 5) ion etching (Ta mask) 1) pattern exposure

2) developing 3) Ta-sputtering

Figure 30: Steps of negative lithography using a resist mask or a hard mask Figure 30: Steps of negative lithography using a resist mask or a hard mask