Electron beam inspection technology plays an important role in IC chip failure analysis, fault diagnosis, and internal structure and function testing. Its working principle is mainly based on the potential contrast, that is, when the electron beam illuminates the IC chip, the potential of its internal metal wire will affect the movement of secondary electrons emitted from the surface, thereby forming secondary electron signals with different intensities in the collector. For the device under test covered with an insulating protective film, although the high-energy electron beam can penetrate the insulating film, it will cause radiation damage to the device and affect the inherent working state of the device.
Therefore, a common method is to use low-energy electrons below a few keV to utilize the capacitance, especially the capacitive contrast voltage (CCVC). In principle, the contrast at this time is different from the potential contrast. The former is caused by the positive charging phenomenon of the insulating film, caused by the difference in capacitance between the electron beam irradiation point and the chip substrate, and there is a maximum value with the irradiation conditions. In this paper, we first study the effect of the local electric field formed by the electron beam on the insulating film on the trajectory of the exiting secondary electrons; on this basis, the surface charge accumulation process caused by the electron beam irradiation is equivalent to capacitance charging Process, thus determining the transition process of the surface potential of the insulating film and the secondary electron signal current with the irradiation conditions, and systematically establish the SCC theory. Then, the numerical calculation is used to analyze the relationship between the maximum SCC value and the corresponding optimal irradiation conditions and the internal topography material parameters of the IC chip, and compare with the experimental values ​​of the optimal irradiation conditions.
2 Local electric field and the law of movement of secondary electrons 2.1 Surface potential distribution and equation of secondary electron movement When an insulator is irradiated with a low-energy electron beam, if the secondary electron emission coefficient W is greater than 1, local positive charging phenomena will occur on its surface | 81. When irradiated by a fixed-point or slow-scanning electron beam, the potential in the vicinity of the irradiated micro-area is generally regarded as a central high-platform distribution (CoreModel) with rotational symmetry characteristics | 9,1 |. Select the reverse direction of the z-axis as the incident direction of the electron beam, and the potential distribution on the surface of the insulating film is approximated by the following rectangular model where a represents the radius of the electron beam irradiated micro-region. Further, with the surface potential and the zero potential value in the calculated field (r = 5Ca, z = 5Ca) as the boundary conditions, the spatial potential distribution and field strength ErEz can be obtained using the finite difference method and the bilinear interpolation method, respectively. In the cylindrical coordinate system, the secondary electron motion equation when the field strength is ignored is the initial parameter (r, UO, W) of the introduced secondary electron, where rnW is the initial position and initial energy, and the exit angle U is the initial velocity and The angle between the z-axis and the initial azimuth angle O is the angle between the projection of the initial velocity on the z = plane and the r direction. Then, the initial condition of (2) is 0). Finally, using the Runge-Kutta method, the secondary electron trajectory can be obtained.
2.2 Calculation result of secondary electron trajectory The calculated equipotential line (dashed line, potential interval is 0.3V) and secondary electron trajectory (solid line) at 3.0V. The contour lines show that the closer to the surface, the stronger the electric field; there is a hemispherical situation barrier that prevents secondary electrons from leaving the surface above the illuminated micro-region. The initial parameters of trajectories 1 ~ 4 are: r = 0, U from 20 to 80, interval 20 = 0, W = 25eV (eVS), where the electrons of trajectories 5 ~ 8 can cross the barrier and leave the surface, while U is larger The electron (trace 9) returns to the surface.
When the initial energy W is less than a certain value, it will return to the surface. We call the maximum initial energy of secondary electrons that can return to the surface as the critical energy Wm (r0, U). Wm is only related to r0U, not to the initial azimuth. For the calculation of the different r, the relationship with ".. It can be seen that the image contrast in the SEM of the static capacitance contrast theory of U 3 is determined by the difference in the secondary electron signal current corresponding to the different irradiation points. Here, We define the secondary electron return rate T as the ratio of the number of secondary electrons returning to the surface and the total number of emitted secondary electrons. Let the irradiated electron flow be Ib, and the secondary electron emission coefficient on the surface of the insulating film be W to exit the secondary electron flow as If the secondary electron flow returning to the surface is AWT and it is assumed that the secondary electrons that do not return to the surface can be collected, then the secondary electron signal current corresponding to the irradiation point is therefore, the key to obtaining Is is to determine T using the above section. The initial position, exit angle and energy distribution of the secondary electrons can be obtained by substituting equation (7) into equation (8), and defining the area density of the irradiated charge e = t / b / S (t is the cumulative irradiated area S Time), you can get. IZ =-1-w ~ Vs / Vm ~ 1 (9) So, solve equation (9) by numerical method to get the surface potential Vs corresponding to a certain e, and then substitute V into (7) The secondary electron return rate T can be obtained by formula (4). If the shape of the lower part of the two points AB on the surface of the insulating film of the IC chip is different, the corresponding effective capacitance per unit area CACob will also be different. This will cause the rate of change of V with e to be inconsistent, resulting in secondary electrons corresponding to the irradiation point The signal current / SA / SB is different to form a contrast. For this, as an evaluation standard for the static capacitance contrast of the SEM image between two points AB, we define from the above SCC theoretical model that it can be seen that the SEM image is affected by / and SCC There are three factors: the charge density of the electron beam irradiation surface, the effective capacitance per unit area at the irradiation point c, and the secondary electron emission coefficient W. Here, macroscopically, e = Ts / b / Sic, where Ts is the irradiation time of the electron beam scanning , Sic is the area of ​​the range of the IC chip irradiated by the electron beam. Therefore, in practice, the junction of the 15 parts can be approached by adjusting the T irradiation> 70 °. The Msh beam can be near the fox. Influencing factors of static capacitance contrast 4.1 Electron beam irradiation conditions Now, we use the above theory to analyze the situation in which the surface potential Vs secondary electron signal current Is and SCC change with the electron beam irradiation surface charge density e in an example. In the following calculations, unless otherwise specified, the secondary electron emission coefficient of 1.2 is a partial cross-sectional view of the LSI covering the Si2 insulating film we used in the experiment. The thickness of the insulator between point A and the substrate dA = 3.24m, the effective capacitance per unit area Coa = X / dA = 1.05 Since the area of ​​the aluminum wiring and the substrate is much larger than the area of ​​the irradiated micro area, C2C1, CbC1, B The effective capacitance ratio per unit area between the substrate and the substrate is Cob / Ca = 64. In addition, in the experiment, we changed e by adjusting the SEM magnification.
It is the relationship between the Vs and Is at the AB point calculated by using the above CaCob value and e with e. It can be seen that the effective capacitance of the irradiation point will cause the VsIs to vary with e, so the SCC phenomenon occurs. Here, Ca shows that the SCC gradually increases with e and reaches the maximum value SCG- and then gradually decreases to 0. This is consistent with the phenomenon observed in Experiment 17. In addition, the maximum irradiated surface charge corresponding to the maximum contrast SCCmx Density value e, t = 24 4.2 Morphology, material parameters (9) The formula shows that the effective capacitance per unit area Co determines the rate of change of Vs with e. When other conditions remain unchanged, C becomes k times the original, that is C = kC, at e = ke, the corresponding Vs / s values ​​are equal, so if the CaCob at the two points of AB are changed k times at the same time, then the SCC when the irradiated surface charge density is ke and the irradiated surface charge density before the change are The SCC at e is the same, that is, SCG- is only determined by the ratio of effective capacitance, and e varies with the effective capacitance lingHouse.Allrightsreserved, http: // k times.
(A) The calculation result of the change of SCCmax with the effective capacitance ratio Cob / Coa is given. Obviously, the increase of SCCCob / Coa increases, and the change of SCCmax is relatively slow when Cb / CA> 5.
Therefore, when Cob / Coa is large, we can get SCC images with large contrast and relatively uniform. (B) is the calculated relationship curve with CoaCob, in which the straight line indicated by the dotted line is the result of Cob 3. It can be seen that it is not determined by Cob / Ca, but becomes larger with the increase of any capacitance value in CaCob. Here, in the charging process corresponding to electron beam irradiation, as the effective capacitance of the irradiation point increases, the transition process of Vs and Is changing with e will become slow, and the charge density value e of the irradiation surface that causes the maximum value of SCC will be So it gets bigger.
(A) The relationship between SCC, a and the ratio of effective capacitance; (b) The relationship between% and effective capacitance is the relationship curve between SGGnax and% and the secondary electron emission coefficient W of the insulating film, where the value of CACOb is still taken from 4.1 Examples in the section. From the calculation results, it can be seen that increasing W will increase SGGnax and decrease ept. The variation range of epl is (1 ~ 1 (T4G / m2. In practice, if the insulation material is fixed, W can be adjusted by irradiating electrons Accelerate the voltage to change.
5 Conclusion This paper studies the theoretical model of the capacitance contrast of the secondary electrons when the surface of the insulating film is positively charged, and the calculated value of the best irradiated charge density obtained from this is consistent with the experimental value. The main conclusions of this paper are as follows: when the electron beam irradiates the insulating film, the local electric field formed by the positive charge on the surface returns the secondary electrons whose initial energy is less than the critical energy to the surface of the insulating film.
The change of the surface potential of the insulating film of the IG chip irradiated by the electron beam can be determined from an equivalent capacitor charging process, and its effective capacitance is determined by the morphology and material parameters between the irradiated micro-region and the substrate. The effective capacitance corresponding to the irradiation point will be different, which will cause the surface potential change process to be different. By affecting the secondary electron flow returning to the surface, the collected secondary electron signal current will be changed to form SGG. SGG changes with the charge density of the irradiation surface There is a maximum contrast value SGGmax. The parameters that affect SGGnax and the corresponding optimal irradiated surface charge density epi are the effective capacitance per unit area and the secondary electron emission coefficient WSGGmax at the corresponding point of the image as the effective capacitance ratio increases, ept only increases with The effective capacitance value increases and becomes larger. SGGnax increases with increasing W, while epi decreases with increasing W.
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