IEBL weekly progress report

Student Name: Renjie ChenProject I: Ultra-short Channel InGaAs Transistors and In situ TEM Study of Ni-InGaAs

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Week of 08/12/2013 – 08/18/2013

Summary: In this week, both the fabrication procedure on TEM membrane and HSQ e-beam writing were modified. It’s found that by employing MMA/PMMA double layer and write the Ni pads before fin layer, the surface of the TEM membrane is much cleaner than samples of last week. However, writing the Ni pads before fin brings another problem of HSQ dose influence near the metal edge. Hence, systematic study on HSQ dose test and developing modification were made, and it’s found that with higher dose and concentrate TMAH developer, the fins were cleared clearly defined with less influence by the Ni pads. This will be introduced in the TEM sample preparation, and hope the sample could be ready for TEM study soon.

1. Modifications in fabrication on Si TEM frame:In last week, a lot of problems are faced during fabrication of fins on TEM membrane: 1) the surface become very messy after three steps of EBL writing, especially when there’s lift-off problem after the 1st EBL step; 2) holes are found after Cl-ICP etch and membrane broke during cycling HF/O2 plasma treatment; 3) Ni and SiO2 removal from the backside hasn’t been tested, which might lead to the further damage of the membrane.

Figure 1: Problems faced in last week. (a) improper lift-off and messy surface. (b) damaged HfO2 layer after Cl-ICP.

In order to solve these problems, several modifications are done in this week. First of all, the lift-off problem was solved by employing MMA EL6 /PMMA C3 double layer e-beam resist, and the surface (shown in Fig 2a) was clean after three EBL steps. Secondly, the HfO2 quality was checked this week, Daisy checked the CV property and Minh did the TEM observation, and they all found the HfO2 is still the same as previous growth. I checked with Wei Tang on his previous experience on ALD deposited HfO2, and he suggested that the condition we currently used was ok. I think the holes might be due to the over-etching during Cl-ICP step. With controlled Cl-etch time, the surface seems ok this time.

(a) (b)

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Figure 2: Modified process. (a) By adopting MMA/PMMA double layer, the lift-off problem was solved. (b) With controlled Cl-etch time, no severe surface damage was found this time.

Thirdly, the HNO3 dip (20min in 1:10 diluted HNO3) followed by F-ICP (50s) was done from the backside. The microscope image (Fig 3a) shows that the membrane became quite transparent after these etching steps. However, SEM image (Fig 3b) does show that the polymer blocked some pocket, which is probably the PMMA that goes to the backside of the TEM grid when I tried to protect the top surface. So currently, I’m not so sure whether the dialectic dielectric layer has been totally removed from the backside.

Figure 3: Backside etching. (a) Microscope image shows the nearly transparent structure on top surface, and the remaining discontinued red color layer might be Al2O3/HfO2. (b) Some polymer reached the backside of membrane and blocked the etching step.

Beside the uncertainty of backside etching, another problem recently been realized is that the fin structures are not clearly written, and that there’re always some “dirt” on it. (shown in Fig 4) I though it might be due to PMMA residue or improper surface cleaning, but finally realize that it’s due to the HSQ dosage influence by Ni pad. The proof and solutions will be discussed in detail later.

IEBL weekly progress report

Figure 4: A lot of “dirt” is found after Fin writing, which is due to the Ni pad influenced dosage of HSQ.

2. Ni diffusion study in radial patterned Fins:Last week, I tested the Ni diffusion in designed radial Fin structures, and there’re several problems encountered: 1) The fin structures with 50um length showed poor adhesion to the substrate, especially the thinner fins; 2) Visible diffusion was not found even after 2.5h annealing at 250’C.

Figure 5: Problems faced in last week. (a) poor adhesion of narrow fins to the substrate. (b) 250’C annealing for 2.5h gives very little diffusion. There’re two possible reasons: one is the lack of Ni source, and the other is the poor contact between Ni and InGaAs.

To solve the 1st problem, I designed a circular structure to hold the outer edge of the narrow fins, and I also realized from reference that people used CD 26 developer (2.4% TMAH in water) to rinse the substrate before HSQ spin coating – there seems no need for HF dip. I tried both of these modifications and it worked out to be good.

(a) (b)

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Figure 5: Radial fin structures with modified HSQ writing. The stain marks on the surface are due to the acetone/IPA rinse and self-drying after developing.

Previously, a center merging part was observed after HSQ writing, and I tried to design the Ni pad with a slightly larger radius in order to reach the fins. Now, however, with the Ni layer written first, a new merging layer can be observed around the Ni pad, which is bad for observing the diffusion in fins. This problem will be analyzed and solved in the later discussion. To solve the 2nd problem I faced last week, I tried with thicker Ni (200 nm) this week, and wrote the Ni layer first before the fin writing, because I’m worried that the HSQ will not be removed completely even with cycling HF/O2 plasma treatment.

Figure 6: Diffusion observed after (a) 30min and (b) 2.5h annealing at 250’C.

Dose = 800uC/cm2

Width = 1um Width = 500nm Width = 200nm Width = 60nm

(a) (b)

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IEBL weekly progress report

IEBL weekly progress report

IEBL weekly progress report

Figure 7: Diffusion observation along different directions. The sample was annealed at 250’C for 2.5h.

No concrete conclusion can be made now on the preferred diffusion orientation or frontier from the SEM images above, because surface conditions/ surface roughness may also influence the diffusing frontier, which still needs further diffusion test and TEM study. Do you have some suggestion on this?

3. HSQ recipe modification:Since I realized the dirty parts on TEM grid and merged part on radial fin structures after HSQ exposure/developing, I planned to do a systematic study of HSQ writing with different shapes and triedy to optimize the recipe. Daisy’s recipe works well for most devices, which I quite agree, but the writing condition is different when I write the Ni pad first. The metal edge always has an influence on the dosage and leads to a poor development (or poor developing)ing.

Figure 8: Clear evidence of Ni pad influence on the HSQ dosage. (a) the situation for radial fin structure writing. The fins are clearly defined on the Ni pad, but merged when reaching the edge. (b) the situation of writing fins in between Ni lines (TEM sample condition). The lines are separated from 1um to 5um from each bottom to top, and 10 fins are separated with spacing of

(b)(a)

IEBL weekly progress report

300nm (top pack) and of 800nm (bottom pack). The HSQ cannot be fully developed between the Ni lines with spacing even up to 5um.

This problem may originated in from two situationsconstraints: 1) the close pack of fin structures; 2) close pack of Ni pads. The 1st situation constraint (in Fig 8a) can be relieved relaxed with less fin density (fig 9), but the 2nd situation constraint (in Fig 8a8b?) cannot.

Figure 9: the problem (in fig 8a) can be solved by writing the radial fins with less density. However, this approach losses the possibility to study the diffusion in most orientations, and especially when we are not sure about the low index orientations (like <100>).

Here lists the modifications for HSQ writing, and the outcome results:

Previous NowSurface Clean HF dip CD 26 developer rinseSpin-coating 3000rpm for 60s 6000rpm for 60sWriting dose 800uC/cm2 2000uC/cm2 ++ (write 4 times)Developing MF 319, 70s 25% TMAH (80’C), 30s

Rinse Water Water, acetone, IPA

(a) (b)

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Figure 10: Modified HSQ writing. (a) and (b) radial fins can be written with both large or small density. (c) and (d) in between the Ni lines with even 1um separation, the fins are clearly written, and the minimized minimum fin width is 21nm in this case.

It’s found that, with the concentrated TMAH developer, the HSQ requires much larger dosage, and the dose difference between small features and large features decrease.

4. Modification for entire process on TEM membrane:Here I summarize the modifications I plan to make for the process:

Previous Now

BondingSchematic

1st EBL step write markers and Ni lines only write the markers

2nd EBL step

Write fin structures Followed by cycling HF/O2 plasma to

remove HSQ(This will not only damage the marker or windows, bad for following EBL process, but also lead to a poor contact between Ni-InGaAs if HSQ is not completely removed.)

Write Ni lines (these Ni lines act as reaction source)

3rd EBL step Write Ni pad above the fins Write fins in between Ni lines, with modified HSQ writing condition

Backside remove

Remove Ni, SiO2 from the backside, 10nm HfO2/ 10nm Al2O3 will remain there

Remove Ni, SiO2 from backside, no need for HfO2/Al2O3 in the future, because the fins are written in last EBL step. With 6000rpm spin-coat, the top HSQ will be thin and even thinner after Cl-etch, hopefully will not influence the TEM observation.

(c) (d)

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Plan for Next Week: - TEM training on Aug 21th21st.- Group meeting preparation. - With modified HSQ writing process, hope the TEM sample can be done as soon

as possible.