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인접한 입사 전자 에너지에서 SEE 값을 비교하여 SEE의 변화를 입사 전자의 에너지로 설명하고 다음과 같이 정의합니다.

재료의 SEE는 \(R_{{{\text{SEE}}}}\) 값의 크기에 따라 세 영역, 즉 입사 전자의 낮은 에너지 영역(\(R_{{{ \text{SEE}}}} \ge 1.02\)), 입사 전자의 중간 에너지 영역(\(0.98 \le R_{{{\text{SEE}}}} <1.02\)) 및 고에너지 입사 전자의 영역(\({\text{R}}_{{{\text{SEE}}}} \ge 0.98\)). SEE 테스트에 사용하는 재료의 입사 전자 에너지 범위는 (100 eV, 1500 eV), x입니다. 입사 전자 에너지를 나타내고 b SEE 테스트에서 입사 전자 에너지의 단계 길이를 나타냅니다.

\({\text{Al}}_{2} {\text{O}}_{3}\) SEE는 기본적으로 그림 7과 같이 7nm 이후에 변하지 않습니다. 그림 7a, b에서 볼 수 있듯이, \({\text{Al}}_{2} {\text{O}}_{3}\)의 저에너지 영역은 100~250eV이며, \(R_{{{\text{SEE}} }}\)는 1.25에서 1.02로 감소하여 입사 전자 에너지가 증가함에 따라 SEE가 증가하고 최종적으로 안정화됨을 나타냅니다. 그림 7c, d에서 볼 수 있듯이 \({\text{Al}}_{2} {\text{O}}_{3}\)의 중간 에너지 영역은 250에서 500eV 사이이며, \( R_{{{\text{SEE}}}}\)는 [0.98, 1.02] 간격 내에서 일정한 것으로 간주됩니다. 즉, \(R_{{{\text{SEE}}}}\)는 거의 같습니다. 1로, 입사 전자 에너지가 증가함에 따라 SEE가 기본적으로 변하지 않음을 나타냅니다. 그림 7e, f에서 볼 수 있듯이 \({\text{Al}}_{2} {\text{O}}_{3}\)의 고에너지 영역은 모든 증가에 대해 500에서 1500eV 사이입니다. 200eV의 입사 전자 에너지에서 SEE는 약 0.9배 감소합니다.

입사 전자 에너지를 \(R_{{{\text{SEE}}}} =\frac{{\text{SEE(x + b)}}}{{\text{SEE(x)}}}로 나눈 후 \) a에 표시된 대로 , ㄷ , e Al₂의 변경 O₃ (실리콘 웨이퍼에서 xnm-Al₂ 성장 O₃ ) b에 표시된 대로 두께로 SEE , d , f

MgO SEE는 기본적으로 그림 9와 같이 20nm 후에도 변하지 않습니다. 그림 8a, b에서 볼 수 있듯이 MgO의 저에너지 영역은 100~500eV 사이이며, \(R_{{{\text{SEE} }}}\)는 1.3에서 1로 감소하여 입사 전자 에너지가 증가함에 따라 SEE가 증가하고 최종적으로 안정화됨을 나타냅니다. 그림 8c, d와 같이 MgO의 중간 에너지 영역은 500과 1000 eV 사이이고, \(R_{{{\text{SEE}}}}\)는 [0.98, 1.02 ], 즉 \(R_{{{\text{SEE}}}}\)는 대략 1과 같으며, 이는 입사 전자 에너지가 증가함에 따라 SEE가 기본적으로 변하지 않음을 나타냅니다. 그림 8e, f에서 볼 수 있듯이 MgO의 고에너지 영역은 1000~1500eV이며 입사 전자 에너지가 100eV 증가할 때마다 SEE가 약 0.94배 감소합니다.

입사 전자 에너지를 \(R_{{{\text{SEE}}}} =\frac{{\text{SEE(x + b)}}}{{\text{SEE(x)}}}로 나눈 후 \) a에 표시된 대로 , ㄷ , e b와 같이 두께에 따른 MgO의 변화(실리콘 웨이퍼에서 xnm-MgO 성장) SEE , d , f

그림 9에서 볼 수 있듯이 \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO와 MgO의 SEE는 유사한 입사 전자 에너지 파티션을 가지며, \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO는 기본적으로 3nm 이후에 변하지 않습니다. 그림 9a, b에서 볼 수 있듯이 \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO의 저에너지 영역은 100~450eV이며, \(R_{{{\text{SEE}}}}\) 는 1.4에서 1.05로 감소하여 입사 전자 에너지가 증가함에 따라 SEE가 증가하고 최종적으로 안정화됨을 나타냅니다. 그림 9c, d와 같이 \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO의 중간 에너지 영역은 500에서 1000 eV 사이이며, \(R_{{{\text{SEE}}}}\)는 [0.98, 1.02] 간격 내에서 일정한 것으로 간주됩니다. 즉, \(R_{{{\text{SEE}}}}\)는 거의 1과 같으며, 입사 전자 에너지가 증가함에 따라 SEE가 기본적으로 변하지 않음을 나타냅니다. 그림 9e, f에서 볼 수 있듯이 \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO의 고에너지 영역은 1000에서 1500 eV 사이입니다. 입사 전자 에너지가 100V 증가할 때마다 SEE는 약 0.95배 감소합니다. \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO의 SEE는 중간 에너지 영역에서 안정적이므로 입사 전자 에너지를 변수로 제외할 수 있습니다. 요인.

입사 전자 에너지를 \(R_{{{\text{SEE}}}} =\frac{{\text{SEE(x + b)}}}{{\text{SEE(x)}}}로 나눈 후 \) a에 표시된 대로 , ㄷ , e Al₂의 변경 O₃ /MgO(실리콘 웨이퍼에서 xnm-Al₂ 성장) O₃ 그런 다음 b와 같이 두께로 9nm-MgO) SEE를 성장시킵니다. , d , f

그림 10과 같이 MgO/\({\text{Al}}_{2} {\text{O}}_{3}\) 및 \({\text{Al}}_{ 2} {\text{O}}_{3}\) 유사한 입사 전자 에너지 파티션을 가지며, MgO/\({\text{Al}}_{2} {\text{O}}_{3 }\) 기본적으로 3nm 이후에는 변경되지 않습니다. 그림 10a, b에서 볼 수 있듯이 MgO/\({\text{Al}}_{2} {\text{O}}_{3}\)의 저에너지 영역은 100~300 eV이며, \(R_{{{\text{SEE}}}}\) 는 1.8에서 1로 감소하여 입사 전자 에너지가 증가함에 따라 SEE가 증가하고 최종적으로 안정화됨을 나타냅니다. 그림 10c, d와 같이 MgO/\({\text{Al}}_{2} {\text{O}}_{3}\)의 중간 에너지 영역은 300~500eV이며, \(R_{{{\text{SEE}}}}\)는 \({\text{Al}}_{2} {\text{O}}일 때 [0.98, 1.02] 간격 내에서 상수로 간주됩니다. _{3}\) 는 얇고 \(R_{{{\text{SEE}}}}\) 는 1에서 벗어나며, 다른 입사 전자 에너지에서 SEE의 차이는 분명합니다. \({\text{Al}}_{2} {\text{O}}_{3}\)가 두꺼운 경우 \(R_{{{\text{SEE}}}}\)는 1에 가까움 , 그리고 그 차이는 분명하지 않습니다. 그림 10e, f에서 볼 수 있듯이 MgO/\({\text{Al}}_{2} {\text{O}}_{3}\)의 고에너지 영역은 500에서 1500 eV 사이입니다. \({\text{Al}}_{2} {\text{O}}_{3}\)는 가늘고, \(R_{{{\text{SEE}}}}\)는 1에 가깝고, 다른 입사 전자 에너지에서 SEE의 차이는 분명하지 않습니다. \({\text{Al}}_{2} {\text{O}}_{3}\)가 두꺼울 때 \(R_{{{\text{SEE}}}}\)는 1에서 벗어나고, 그리고 그 차이는 분명합니다. 입사 전자 에너지가 200V 증가할 때마다 SEE는 약 0.9배 감소합니다.

입사 전자 에너지를 \(R_{{{\text{SEE}}}} =\frac{{\text{SEE(x + b)}}}{{\text{SEE(x)}}}로 나눈 후 \) a, c, e와 같이 , MgO/Al₂의 변화 O₃ (실리콘 웨이퍼에서 35nm-MgO를 성장시킨 다음 xnm-Al₂을 성장시킵니다. O₃ ) b에 표시된 대로 두께로 SEE , d, f

\({\text{Al}}_{2} {\text{O}}_{3}\) SEE는 중간 에너지 영역에서 안정적이기 때문에 입사 전자 에너지는 가변 요인으로 제외될 수 있습니다. 실험 공식인 \({\text{Al}}_{2} {\text{O}}_{3}\)의 SEE 수준을 측정하기 위한 표준으로 중간 입사 전자 에너지 300eV를 선택했습니다. \({\text{Al}}_{2} {\text{O}}_{3}\)의 두께와 최상의 SEE는 그림 11a(표 1)와 같이 피팅하여 얻어집니다.

재료의 최상의 2차전자방출계수와 막두께의 관계 a shows the information of Al₂ O₃ (on the silicon wafer, grow xnm-Al₂ O₃ ), b shows the information of MgO (on the silicon wafer, grow xnm-MgO), c shows the information of Al₂ O₃ /MgO (on the silicon wafer, grow xnm-Al₂ O₃ , and then grow 9 nm-MgO), and d shows the information of MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow xnm-Al₂ O₃ )

Because the MgO SEE is stable in the medium energy region, the incident electron energy can be excluded as a variable factor. We choose the medium incident electron energy 700 eV as the standard to measure the SEE level of MgO, the empirical formula for the thickness of alumina material and the best SEE is obtained by fitting as shown in Fig. 11b.

Because the SEE of \({\text{Al}}_{2} {\text{O}}_{3} /{\text{MgO}}\) is stable in the medium energy region, the incident electron energy can be excluded as a variable factor. We choose the medium incident electron energy 700 eV as the standard to measure the SEE level of \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO, the empirical formula for the thickness of alumina material and the best SEE is obtained by fitting as shown in Fig. 11c.

Because the SEE of MgO/\({\text{Al}}_{2} {\text{O}}_{3}\) is stable in the medium energy region, the incident electron energy can be excluded as a variable factor. We choose the medium incident electron energy 450 eV as the standard to measure the SEE level of MgO/\({\text{Al}}_{2} {\text{O}}_{3}\), the empirical formula for the thickness of alumina material and the best SEE is obtained by fitting as shown in Fig. 11d.

According to formulas 1 and 2, the SEE level of 9 nm MgO is 1.755 times higher than that of 30 nm \({\text{Al}}_{2} {\text{O}}_{3}\).

We deposit 0–30 nm \({\text{Al}}_{2} {\text{O}}_{3}\) and redeposit 9 nm MgO on the Si wafer as the film, as shown in Fig. 12a. formulas 1 and 3 show that the SEE level of 9 nm MgO grown on 3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) is 1.973 times higher than that of \({\text{Al}}_{2} {\text{O}}_{3}\). formulas 2 and 3 show that the SEE level of 9 nm MgO grown on 3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) is 1.124 times higher than that of 9 nm MgO.

Change of secondary electron emission coefficient with different incident electron energy, a shows the information of Al₂ O₃ /MgO (on the silicon wafer, grow xnm-Al₂ O₃ , and then grow 9 nm-MgO), b shows the information of MgO/Al₂ O₃ and deliquescent MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 1 nm-Al₂ O₃ ), c shows the information of MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 0.3 nm-Al₂ O₃ ), and d shows the information of Al₂ O₃ /MgO (on the silicon wafer, grow 3 nm-Al₂ O₃ , and then grow 5 nm-MgO)

The SEE level of MgO after deliquescent drops significantly as shown in Fig. 1. Then, we deposit 35 nm MgO and redeposit 1 nm \({\text{Al}}_{2} {\text{O}}_{3}\) on the Si wafer as the film. We found the SEE of this film exposed to the air 7 months is close to the SEE without exposed to the air as shown in Fig. 12b. Formulas 1 and 3 show that the SEE level of 1 nm \({\text{Al}}_{2} {\text{O}}_{3}\) grown on MgO is 1.743 times higher than the SEE of \({\text{Al}}_{2} {\text{O}}_{3}\) and can be long-term maintain a high SEE level (no obvious deliquescence in 7 months).

We deposited 35 nm MgO on the Si wafer and re-deposited 0.3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) as a thin film as shown in Fig. 12c. It can be seen from formulas 1, 2 and 4 that the SEE level of 0.3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) grown on MgO is 1.967 times higher than that of \({\text{Al}}_{2} {\text{O}}_{3}\) and 1.12 times higher than that of MgO;

The emission layer of the electron multiplier pursues thinner and higher SEE level, so we sacrificed some SEE level to make the film thinner. We deposited 3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) on the Si wafer and re-deposited 5 nm MgO as a thin film as shown in Fig. 12d.

We propose to grow 2–3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) as a buffer layer, grow 5–9 nm MgO as the main layer, and grow 0.3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) as an enhancement layer or 1 nm \({\text{Al}}_{2} {\text{O}}_{3}\) as a protective layer as the \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO/\({\text{Al}}_{2} {\text{O}}_{3}\) emissive layer of electron multipliers as shown in Fig. 13. SEE level of \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO/\({\text{Al}}_{2} {\text{O}}_{3}\) emission layer (\({\text{Al}}_{2} {\text{O}}_{3}\)/MgO/\({\text{Al}}_{2} {\text{O}}_{3}\) = 3 nm/9 nm/0.3 nm) is shown in Fig. 12a. And, we tested a traditional microchannel plate with good gain and then grew \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO/\({\text{Al}}_{2} {\text{O}}_{3}\) emission layer on microchannel wall of microchannel plate, and the gain result obtained by the test was significantly improved. Then, another piece of the first convention microchannel plate with close gain is grown with \({\text{Al}}_{2} {\text{O}}_{3}\) emission layer. Compared with the gain results obtained by the test, the \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO/\({\text{Al}}_{2} {\text{O}}_{3}\) emission layer structure is more superior as shown in Fig. 14.

Schematic diagram of sandwich structure (Al₂ O₃ /MgO/Al₂ O₃ )

Relationship between the voltage and gain of the three microchannel plates (conventional microchannel plate, microchannel plate for growing Al₂ O₃ emission layer, microchannel plate for growing Al₂ O₃ /MgO/Al₂ O₃ emission layer)

XPS Characterization and Transition Layer Concept

SEE data usually uses Dionne model for fitting analysis [26, 27]. The current double-layer model based on Dionne model does not consider the existence of a transition layer between the two materials. Through the design of the emission layer structure this time, the SEE difference between \({\text{Al}}_{2} {\text{O}}_{3}\)/MgO and \({\text{Si}}\)/MgO can be clearly observed. Under the same SEE level, MgO exhibits a very large thickness difference. Sample (0.3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) grown on MgO) can get a higher SEE than MgO. Sample (1 nm \({\text{Al}}_{2} {\text{O}}_{3}\) grown on MgO) maintain a high SEE level. The current double-layer model [28] can no longer explain the above phenomenon, so we put forward the concept of transition layer, there are two kinds of materials at the interface, forming two processes:the process of destroying the bottom material and the process of building the top material. The following are two X-ray photoelectron spectroscopy (XPS) test experiments to prove and the concept of transition layer to understand the SEE phenomenon of multilayer materials.

XPS test experiment 1:

First, the sample (0.3 nm \({\text{Al}}_{2} {\text{O}}_{3}\) grown on MgO) in the air for 1 year are tested for XPS as shown in Fig. 15a. We use an Ar ion gun to etch the surface of the material, and then test the various elements in the material by XPS. The two are alternately performed. The etching depth is controlled by controlling the etching time, and the relative atomic concentration percentage changes of various elements are obtained by XPS. Al element is almost undetectable after 8 s of etching as shown in Fig. 16a. The etching rate of \({\text{Al}}_{2} {\text{O}}_{3}\) is known, \({\text{Etching}}\,{\text{rate}}_{{{\text{Al}}_{2} {\text{O}}_{3} }} =0.7{\text{{\AA}/s}}\),

Schematic diagram of XPS test experiment sample, a shows the information of MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 0.3 nm-Al₂ O₃ ), b shows the information of deliquescent MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 0.3 nm-Al₂ O₃ ), c shows the information of deliquescent MgO (on the silicon wafer, grow 11 nm-MgO), d shows the information of deliquescent MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 1 nm-Al₂ O₃ )

Atomic concentration percentage of C, Al, Si elements relative to Mg element obtained by XPS. 아 Shows the Al element information of deliquescent MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 0.3 nm-Al₂ O₃ ), a shows the C element information of deliquescent MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 0.3 nm-Al₂ O₃ ), c shows the C element information of deliquescent MgO (on the silicon wafer, grow 11 nm-MgO), d shows the Si element information of deliquescent MgO (on the silicon wafer, grow 11 nm-MgO), e shows the C element information of deliquescent MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 1 nm-Al₂ O₃ ). 에 shows the Al element information of deliquescent MgO/Al₂ O₃ (on the silicon wafer, grow 35 nm-MgO, and then grow 1 nm-Al₂ O₃ )

Therefore, it shows that \({\text{Al}}_{2} {\text{O}}_{3}\) must exist in the MgO part, that is, \({\text{Al}}_{2} {\text{O}}_{3}\) destroys the lattice state of the MgO surface. \({\text{Al}}_{2} {\text{O}}_{3}\) forms a finite solid solution in MgO [30]. At this time, the experimentally measured SEE level increased. As we all know, the higher the SEE level, the better the insulation of the material. Due to the destruction of the surface lattice, the surface layer of MgO is more insulating, which further confirms the process of destroying the underlying material in the concept of the transition layer.

According to the results of the SEE experiment, the SEE level has dropped significantly. A small amount of \({\text{Al}}_{2} {\text{O}}_{3}\) in the top layer cannot protect the MgO in the bottom layer. MgO is still deliquescent in the air. The air contains \({\text{O}}_{2} ,{\text{H}}_{2} {\text{O}},{\text{CO}}_{2} ,{\text{CO}},{\text{N}}_{2}\), etc. When air enters MgO, the reaction of MgO and \({\text{CO}}_{2}\) and \({\text{H}}_{2} {\text{O}}\) proceeds at the same time.

The above four chemical reactions occur, the deliquescent reaction of air and MgO is mainly the reaction of MgO and \({\text{CO}}_{2}\) and \({\text{H}}_{2} {\text{O}}\) to produce \({\text{MgCO}}_{3}\) and \({\text{Mg}}_{2} \left( {{\text{OH}}} \right)_{2} {\text{CO}}_{3}\). As long as the prepared MgO is exposed to the air, \({\text{Mg}}\left( {{\text{OH}}} \right)_{2}\) will be produced. After being placed in the air for 28 days, \({\text{MgCO}}_{3}\) is the main product [31]. Because the tested MgO sample needs to be transferred to the SEE test equipment, the actual test is the SEE level of MgO–\({\text{Mg}}\left( {{\text{OH}}} \right)_{2}\). Main reason for the decrease in SEE level is the \({\text{Mg}}_{2} \left( {{\text{OH}}} \right)_{2} {\text{CO}}_{3}\) and MgCO₃ produced by deliquescent. Therefore, when using XPS, C can be selected as the calibration element for the deliquescent depth of MgO in the air. As shown in Fig. 16b, after 8 s of etching, no Al content is detected, but C content is still detected, indicating that the MgO in the bottom layer continues to deliquesce and is not protected by a small amount of \({\text{Al}}_{2} {\text{O}}_{3}\) as shown in Fig. 15b.

XPS test experiment 2:

First, the MgO sample in the air for 1 year are tested for XPS. After 1 min of etching, there was almost no C element as shown in Fig. 16c, indicating that the thickness of the dense \({\text{Mg}}_{2} \left( {{\text{OH}}} \right)_{2} {\text{CO}}_{3}\) film formed was the thickness of 1 min of etching.

After etching for 3 min, the sample begins to show Si element as shown in Fig. 16d, the etching rate of MgO and the thickness of \({\text{Mg}}_{2} \left( {{\text{OH}}} \right)_{2} {\text{CO}}_{3}\) film can be calculated through these data.

The 3.85 nm \({\text{Mg}}_{2} \left( {{\text{OH}}} \right)_{2} {\text{CO}}_{3}\) film layer acts as an air barrier layer to prevent further deliquescent of deep MgO as shown in Fig. 15c.

When 1 nm \({\text{Al}}_{2} {\text{O}}_{3}\) is grown on MgO, the XPS test data show that there is basically no C content and no Al content in the sample after the etching time of 14 s as shown in Fig. 16e, f.

It can be known by testing the C content that the depth of air penetration into the material is about 1 nm at this time. According to the concept of the transition layer, there are two kinds of materials at the interface to form the process of destroying the bottom layer material and constructing the top layer material. At the interface, \({\text{Al}}_{2} {\text{O}}_{3}\) destroys the crystal lattice on the surface of MgO. In order to prevent excessive infiltration of air, a complete \({\text{Al}}_{2} {\text{O}}_{3}\) atomic level is formed at least at 1 nm. When a complete \({\text{Al}}_{2} {\text{O}}_{3}\) atomic layer is not formed, the infiltration of air into the material cannot be prevented as in Example 1 above. The \({\text{Al}}_{2} {\text{O}}_{3}\) and \({\text{ Mg}}_{2} \left( {{\text{OH}}} \right)_{2} {\text{CO}}_{3}\) in the inner layer are mixed to help MgO form a dense air barrier layer in advance as shown in Fig. 15d.

The concept of transition layer understands the SEE phenomenon of multilayer materials:

The schematic diagram shown in Fig. 17a shows the concept of the transition layer, The thickness of the top layer material is \(d_{1}\), the thickness of the bottom layer material is \(d_{2}\) and the thickness of the transition layer is \(d_{1\sim 2}\).The schematic diagram is shown in Fig. 17b, c when there is enough thick \({\text{Al}}_{2} {\text{O}}_{3}\) or MgO, the incident electron depth is \(d_{{{\text{max}}\_1}}\), and there is no transition layer between \({\text{Al}}_{2} {\text{O}}_{3}\) and \({\text{Al}}_{2} {\text{O}}_{3}\) (there is no transition layer between MgO and MgO), that is, the thickness of the transition layer is 0. Through XPS test experiment 2, we get that the thickness of the transition layer between MgO and \({\text{Al}}_{2} {\text{O}}_{3}\) is 1 nm as shown in Fig. 17d, e.

아 Schematic diagram of the transition layer of the double layer structure, b schematic diagram of the Al₂ O₃ transition layer and incident electron depth, c schematic diagram of the MgO transition layer and incident electron depth, d schematic diagram of the Al₂ O₃ /MgO transition layer, e schematic diagram of the MgO/Al₂ O₃ transition layer

When the top layer material in the double-layer structure is MgO, the thickness of the MgO that reaches the saturated SEE level is different when the bottom layer material is different. If electrons are incident on the bottom layer material, the SEE level of the bottom layer material is low and cannot reach the saturated SEE level. Therefore, to reach the saturation SEE level, a complete MgO incident electron path needs to be formed. When the bottom layer material is different, such as Si or \({\text{Al}}_{2} {\text{O}}_{3}\), the thickness of the transition layer will be different, so the top layer MgO shows a different thickness.

It is found through experiments that a sample that grows 2 nm \({\text{Al}}_{2} {\text{O}}_{3}\) on a Si wafer and then grows 15 nm MgO can reach the SEE level of MgO saturation. Knowing that the thickness of the MgO–\({\text{Al}}_{2} {\text{O}}_{3}\) transition layer is 1 nm, it can be inferred that the thickness of the \({\text{Al}}_{2} {\text{O}}_{3}\)–Si transition layer is 1 nm, and the maximum depth of incident electrons of MgO is 14 nm as shown in Fig. 18a. It is found through experiments that the sample of 20 nm MgO grown on the Si wafer can reach the SEE level of MgO saturation. It has been inferred that the maximum depth of incident electrons of MgO is 14 nm, so the thickness of the MgO–Si transition layer can be calculated to be 6 nm as shown in Fig. 18b. Therefore, it can be explained that the SEE level of growing 2 nm \({\text{Al}}_{2} {\text{O}}_{3}\) on Si wafer and then growing 9 nm MgO is higher than the SEE level of 9 nm MgO growing on Si wafer. This is because the thickness of the MgO–\({\text{Al}}_{2} {\text{O}}_{3}\) transition layer is thinner than that of the MgO–Si transition layer. The actual MgO thickness of 8 nm involved in incident electrons is much thicker than 3 nm as shown in Fig. 18c, d.

Schematic diagram of the thickness of each layer of a multilayer structure, a shows the thickness of Al₂ O₃ /MgO (on the silicon wafer, grow 2 nm-Al₂ O₃ , and then grow 15 nm-MgO), b shows the thickness of MgO (on the silicon wafer, grow 20 nm-MgO), c shows the thickness of Al₂ O₃ /MgO (on the silicon wafer, grow 2 nm-Al₂ O₃ , and then grow 9 nm-MgO), d shows the thickness of MgO (on the silicon wafer, grow 9 nm-MgO)

It can be seen through experiments that growing 7 nm \({\text{Al}}_{2} {\text{O}}_{3}\) on Si wafers can reach the SEE level of \({\text{Al}}_{2} {\text{O}}_{3}\) saturation, so it can be calculated that the maximum depth of incident electrons of \({\text{Al}}_{2} {\text{O}}_{3}\) is 6 nm; growing 7 nm \({\text{Al}}_{2} {\text{O}}_{3}\) on 35 nm MgO can reach the SEE level of \({\text{Al}}_{2} {\text{O}}_{3}\) saturation, the thickness of the MgO-\({\text{Al}}_{2} {\text{O}}_{3}\) transition layer is 1 nm, and the maximum depth of incident electrons of \({\text{Al}}_{2} {\text{O}}_{3}\) is calculated again to be confirmed by 6 nm, as shown in Fig. 19a, b.

Schematic diagram of the thickness of each layer of a multilayer structure, a shows the thickness of Al₂ O₃ (on the silicon wafer, grow 7 nm-Al₂ O₃ ), b shows the thickness of MgO/Al₂ O₃ (on the silicon wafer, grow 20 nm-MgO, and then grow 7 nm-Al₂ O₃ )