Testing method and testing system for current capability of polysilicon

Abstract

The present invention relates to a method and system for testing the current capacity of polysilicon, the method comprising: obtaining multiple samples of polysilicon with different widths; adjusting the temperature of the samples to a preset temperature, and testing the resistance value of each sample at the preset temperature; the preset temperature is the sum of the expected upper limit of the application temperature of the polysilicon and the upper limit of the Joule heat of the polysilicon; adjusting the temperature of the samples to the expected upper limit of the application temperature, and applying current to the samples respectively, and obtaining the current values ​​corresponding to the samples when they reach their respective resistance values; using each current value as the maximum root mean square current density, substituting it into the maximum root mean square current density model, and calculating the empirical parameters of the maximum root mean square current density model; substituting the width of the polysilicon into the maximum root mean square current density model, and calculating the maximum root mean square current density as the maximum safe current. The present invention does not require special testing equipment, has strong operability, and a short testing cycle.

Description

Technical Field

[0001] The invention relates to the testing of semiconductor components, in particular to a method for testing the current capability of polysilicon, and also to a system for testing the current capability of polysilicon. Background Art

[0002] As the size of semiconductor devices continues to shrink, the current density per chip continues to increase, which poses a huge challenge to the reliability of the current capacity of polysilicon interconnects and polysilicon resistors. The current and voltage signal transmission in the chip is achieved through metal and polysilicon interconnects. As the current density per chip continues to increase, the reliability of the overcurrent capacity of polycrystalline wiring and polycrystalline resistors is a challenging problem faced by the chip manufacturing industry. Therefore, it is necessary to evaluate the reliability of the current capacity of polysilicon resistors. Summary of the invention

[0003] Based on this, it is necessary to provide a method and system for testing the current capability of polysilicon.

[0004] A method for testing the current capability of polysilicon comprises: obtaining multiple samples of a first type of polysilicon, different samples having different widths; adjusting the temperature of the multiple samples to a preset temperature, and testing the resistance value of each sample at the preset temperature; the preset temperature is the sum of the upper limit of the expected application temperature of the polysilicon and the upper limit of the Joule heat of the polysilicon; adjusting the temperature of the multiple samples to the upper limit of the expected application temperature, and applying current to the multiple samples respectively, and obtaining the current values ​​corresponding to each sample when the sample reaches the respective resistance value; substituting each current value as a maximum root mean square current density into a maximum root mean square current density model, and calculating the empirical parameters of the maximum root mean square current density model; substituting the width of the first type of polysilicon to be evaluated having a first width into the maximum root mean square current density model, and calculating the maximum root mean square current density as the maximum safe current of the first type of polysilicon with the first width.

[0005] In one embodiment, the maximum root mean square current density model is a function with the width of polysilicon as a variable, and the empirical parameters include a constant term, a negative first-order term coefficient, and a negative second-order term coefficient.

[0006] In one embodiment, the function is: J=A+B / WD+C / (WD*WD);

[0007] Among them, J is the maximum root mean square current density, A is the constant term, B is the negative first-order term coefficient, and C is the negative second-order term coefficient; the step of obtaining multiple samples of the first type of polysilicon is to obtain 3 samples.

[0008] In one embodiment, in the step of testing the resistance value of each sample at the preset temperature, the test current is smaller than the current applied in the step of applying current to the multiple samples respectively.

[0009] In one embodiment, the step of obtaining a plurality of samples of the first type of polysilicon includes obtaining polysilicon resistor samples with widths of 1 micron, 5 microns, and 10 microns.

[0010] In one embodiment, in the step of obtaining multiple samples of the first type of polysilicon, the length of the obtained samples is 50 microns.

[0011] In one of the embodiments, the first type of polysilicon is polysilicon with a positive temperature coefficient or a negative temperature coefficient.

[0012] In one embodiment, the first type of polysilicon includes polysilicon and a metal silicide layer on the polysilicon and exhibits a positive temperature coefficient characteristic; or the first type of polysilicon does not have a metal silicide layer and exhibits a negative temperature coefficient characteristic.

[0013] In one embodiment, the step of adjusting the temperature of the plurality of samples to a preset temperature and the step of adjusting the temperature of the plurality of samples to the upper limit of the expected application temperature are performed by heating the samples on a heating plate of a probe station.

[0014] A testing system for current capability of polysilicon comprises: a probe station including a heating plate; a tester for testing the resistance value of a polysilicon sample; a control module for controlling the heating plate to heat the polysilicon sample on the heating plate to a preset temperature, and controlling the tester to test the resistance value of the polysilicon sample at the preset temperature, wherein the preset temperature is the sum of an expected upper limit value of application temperature of the polysilicon and an upper limit value of Joule heat of the polysilicon; and further for controlling the heating plate to heat the polysilicon sample on the heating plate to the expected upper limit value of application temperature, and controlling the tester to apply current to the multiple samples respectively. flow, and obtain the current value corresponding to the sample when the resistance value is reached; it is also used to substitute the current values ​​of multiple polysilicon samples into the maximum root mean square current density model as the maximum root mean square current density, and calculate the empirical parameters of the maximum root mean square current density model; it is also used to substitute the width of the first type of polysilicon to be evaluated with a first width into the maximum root mean square current density model, and calculate the maximum root mean square current density as the maximum safe current of the first type of polysilicon with the first width; wherein the multiple polysilicon samples are all the first type of polysilicon, and different samples have different widths.

[0015] The above-mentioned method and system for testing the current capacity of polysilicon start from the actual application conditions of polysilicon, monitor the current of the polysilicon generating the upper limit of Joule heat under the condition of the expected upper limit of application temperature according to the upper limit of expected application temperature given by polysilicon, and substitute the current obtained by monitoring samples with different widths and the width of polysilicon into the maximum root mean square current density model, so as to obtain the empirical parameters of the maximum root mean square current density model. By determining the maximum root mean square current density model with empirical parameters, the maximum root mean square current density of polysilicon of the same type with any given width can be obtained, and it is used as the maximum safe current of polysilicon corresponding to the actual application conditions. The test method does not require special test equipment, has strong operability and a short test cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to better describe and illustrate the embodiments and / or examples of the inventions disclosed herein, reference may be made to one or more drawings. The additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed inventions, the embodiments and / or examples currently described, and the best modes of these inventions currently understood.

[0017] Figure 1 is a flow chart of a method for testing the current capability of polysilicon in one embodiment;

[0018] Figure 2a It is a schematic diagram of the structure of a polysilicon resistor with CoSi2 thin film. Figure 2b It is a schematic diagram of the structure of a polysilicon resistor without CoSi2 film. DETAILED DESCRIPTION

[0019] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0021] It should be understood that when an element or layer is referred to as being "on, adjacent to, connected to or coupled to other elements or layers, it may be directly on, adjacent to, connected to or coupled to other elements or layers, or there may be intervening elements or layers. In contrast, when an element is referred to as being "directly on, directly adjacent to, directly connected to or directly coupled to other elements or layers, there may be no intervening elements or layers. It should be understood that, although the terms first, second, third, etc. may be used to describe various elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or part from another element, component, region, layer or part. Therefore, without departing from the teachings of the present invention, the first element, component, region, layer or part discussed below may be represented as a second element, component, region, layer or part.

[0022] Spatially relative terms such as "under," "below," "below," "under," "above," "above," etc., may be used herein for ease of description to describe the relationship of an element or feature shown in the figures to other elements or features. It should be understood that in addition to the orientations shown in the figures, the spatially relative terms are intended to include different orientations of the device in use and operation. For example, if the device in the accompanying drawings is flipped, then the elements or features described as "under other elements" or "under" or "under" will be oriented as "on" the other elements or features. Therefore, the exemplary terms "under" and "under" may include both upper and lower orientations. The device may be oriented otherwise (rotated 90 degrees or other orientations) and the spatial descriptors used herein are interpreted accordingly.

[0023] The purpose of the terms used herein is only to describe specific embodiments and is not intended to be limiting of the present invention. When used herein, the singular forms "one", "an" and "said / the" are also intended to include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "consisting of" and / or "comprising", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0024] An exemplary method for reliability assessment of polysilicon current capability is to use the electromigration test method. The study of electromigration began with metal interconnects, which refers to the phenomenon that the electron flow generated by the interconnect under the action of current and temperature collides with atoms and causes atomic migration. The exemplary method for testing electromigration requires the use of special testing equipment and adopts a high-temperature accelerated test method: keep the temperature constant and pass a constant current through both ends of the polysilicon resistor to test the change in the resistance of the polysilicon resistor. The median failure time (MTF) is often used to describe the failure caused by electromigration. The median failure time refers to the time taken for 50% of the interconnects to fail under the same DC test current conditions. The basis for failure judgment is a 10% increase in lead resistance. The formula that describes the median time of electromigration failure in DC mode is the Block formula: MTF = AJ -n exp(Ea / kT)

[0025] Where: A is a constant, J is the current density, N is the current density index, Ea is the diffusion activation energy, k is the Boltzmann constant, and T is the absolute temperature. MTP is closely related to current density and temperature, so the maximum allowable current density can be obtained.

[0026] The above method of obtaining the current capability of polysilicon by evaluating the reliability of polysilicon electromigration requires special testing equipment and a relatively long testing cycle (usually several weeks).

[0027] Figure 1 The flowchart of a method for testing the current capability of polysilicon in one embodiment includes:

[0028] S110 , obtaining a plurality of samples of first type polysilicon, where different samples have different widths.

[0029] A plurality of samples of a first type of polysilicon resistor are obtained, and different samples have different widths. In one embodiment of the present application, each sample is a first type of polysilicon resistor, which means that each sample has the same material, the same doping impurities and doping concentration, and the same resistivity. This embodiment is described by taking the sample as a polysilicon resistor as an example. In other embodiments, the sample may also be a polysilicon interconnect.

[0030] Based on the inventor's research on the temperature characteristics of polysilicon materials, the test method of the present application needs to utilize the temperature characteristics of polysilicon resistors, that is, the characteristics of the resistance value changing with temperature. Therefore, the first type of polysilicon resistor is a polysilicon resistor with a positive temperature coefficient or a negative temperature coefficient. Typical polysilicon resistors include those with a Silicide (metal silicide) structure and those without a Silicide structure. An exemplary Silicide structure is a CoSi2 thin film, such as Figure 2a and Figure 2bAs shown. Polysilicon resistors with Silicide structure usually have metallic characteristics and positive temperature coefficient characteristics; polysilicon resistors without Silicide structure usually have negative temperature coefficient characteristics.

[0031] S120, adjusting the temperature of the plurality of samples to a preset temperature, and testing the resistance value of each sample at the preset temperature.

[0032] The preset temperature is the sum of the expected application temperature upper limit of the polysilicon resistor and the Joule heat upper limit of the polysilicon resistor. In one embodiment of the present application, the sample can be heated to the preset temperature using the heating plate of the probe station. Since Fab is usually equipped with a probe station, there is no need to purchase additional test equipment for heating using the heating plate of the probe station. In one embodiment of the present application, a tester with a resistance value test function can be used to test the resistance value of each sample and record it. The tester with a resistance value test function is also a common device in Fab. In one embodiment of the present application, a low voltage is used to test the resistance value to ensure that the Joule heat caused by applying the test voltage / test current during the resistance value test can be ignored, and the resistance value tested under the low voltage is stable. Due to the low voltage test resistance value generated by the use of the Joule heat that can be ignored, the temperature of the sample of the polysilicon resistor during the test can be maintained at the preset temperature.

[0033] Polysilicon is usually used as a resistor or interconnect in a semiconductor IC (integrated circuit). IC applications have certain requirements for the application temperature and resistance change of polysilicon resistors or connections. Because we can start from the client application. For example, the customer demand is that the application operating temperature of the IC is required to be lower than 125 degrees Celsius, and the Joule heat generated by the polysilicon resistor passing current is required to be lower than 10 degrees Celsius. We believe that Joule heat has a direct relationship with the change in resistance, so one inventive idea is to monitor the current that generates 10 degrees of Joule heat in the polysilicon resistor at a temperature of 125 degrees according to the application requirements of the client. The following is a specific description of the solution based on this application condition. It can be understood that 125 degrees is the upper limit of the expected application temperature of the aforementioned polysilicon resistor, and 10 degrees is the upper limit of the Joule heat of the aforementioned polysilicon resistor. Therefore, in the corresponding embodiment, the preset temperature is 125+10=135 degrees.

[0034] S130, adjusting the temperature of the plurality of samples to an expected upper limit of the application temperature, and obtaining the current value corresponding to each sample when the sample reaches the respective resistance value.

[0035] In one embodiment of the present application, the temperature of the heating plate can be reduced from the preset temperature to the upper limit of the expected application temperature, and a current with a monotonically changing magnitude is applied to each sample. During the application of the current, the temperature of the heating plate is kept constant. The resistance value of the sample will change with the change of the current magnitude, and it is necessary to record the current value corresponding to each sample when it reaches the resistance value measured in step S120. Specifically, the I ramp (current ramp) test method can be adopted. When the resistance value of each sample reaches the resistance value measured in step S120, the corresponding current value is recorded; this current can be considered as a current that causes the sample to generate 10 degrees of Joule heat. It can be understood that since the current applied in step S130 will generate Joule heat, the current is greater than the test current of step S120, and the voltage generated on the polysilicon resistor sample in step S130 is greater than the test voltage of step S120.

[0036] S140, taking each of the current values ​​as the maximum root mean square current density, substituting them into a maximum root mean square current density model, and calculating empirical parameters of the maximum root mean square current density model.

[0037] The current value recorded in step S130 is used as the maximum root mean square current density (JRMS, Max) of the polysilicon resistor and substituted into the maximum root mean square current density model of the polysilicon resistor. Since the maximum root mean square current density is related to the width of the polysilicon, the mathematical expression of the maximum root mean square current density model can be calculated / fitted based on the maximum root mean square current density measured by samples with different widths, and the empirical fitting parameters of the maximum root mean square current density model can be obtained accordingly.

[0038] S150, substituting the width of the polysilicon into the maximum root mean square current density model, and calculating the maximum root mean square current density as the maximum safe current.

[0039] Substituting the width of the first type of polysilicon resistor to be evaluated with the first width into the maximum root mean square current density model, the maximum root mean square current density is calculated as the maximum safe current of the first type of polysilicon resistor with the first width. According to the mathematical expression of the maximum root mean square current density model of the polysilicon resistor, the maximum root mean square current density of other polysilicon resistors of the same type with any given width can be derived.

[0040] The above-mentioned test method for the current capability of the polysilicon resistor starts from the actual application conditions of the polysilicon resistor, and monitors the current of the polysilicon resistor under the condition of the expected application temperature upper limit and the Joule heat upper limit given by the polysilicon resistor, and substitutes the current obtained by monitoring the samples with different widths and the width of the polysilicon resistor into the maximum root mean square current density model, and the empirical parameters of the maximum root mean square current density model can be obtained. By determining the maximum root mean square current density model with empirical parameters, the maximum root mean square current density of the polysilicon resistor of the same type with any given width can be obtained, and it is used as the maximum safe current of the corresponding polysilicon resistor under the actual application conditions. This test method does not require special test equipment, has strong operability and a short test cycle. The above-mentioned test method can define the expected application temperature upper limit and the Joule heat upper limit according to the different needs of customers, and conduct corresponding tests to obtain the maximum safe current.

[0041] In one embodiment of the present application, the maximum RMS current density model of the polysilicon resistor is a function with the width of the polysilicon resistor as a variable, and the empirical parameters of the maximum RMS current density model include a constant term, a negative first-order term coefficient, and a negative second-order term coefficient. Specifically, the function is:

[0042] J=A+B / WD+C / (WD*WD) (1)

[0043] Among them, J is the maximum RMS current density, A is a constant term, B is the coefficient of the negative first-order term, and C is the coefficient of the negative second-order term. Since the function has three unknown terms, A, B, and C, A, B, and C can be calculated by the maximum RMS current density corresponding to the three widths, and accordingly, three samples with different widths are prepared in step S110.

[0044] In one embodiment of the present application, four types of polysilicon resistors are obtained and the current capabilities of the polysilicon resistors are tested. Three samples of different widths are used for testing each type of polysilicon resistor, as shown in Table 1. In Table 1, Structure is the structural type of the polysilicon resistor, Width is the width (strip width) of the polysilicon resistor, and Length is the length of the polysilicon resistor.

[0045] Structure Width length 7.5ohm / square CO-SI2 polysilicon resistor 1um / 5um / 10um 50um 290ohm / square without CO-SI2 polysilicon resistor 1um / 5um / 10um 50um 1.1kohm / square no CO-SI2 polysilicon resistor 1um / 5um / 10um 50um 1.9kohm / square no CO-SI2 polysilicon resistor 1um / 5um / 10um 50um

[0046] Table 1

[0047] The samples corresponding to Table 1 were tested using the above-mentioned test method for the current capability of the polysilicon resistor, and the obtained maximum root mean square current density (ie, the current value obtained in step S130 ) is shown in Table 2.

[0048]

[0049] Table 2

[0050] By substituting the JRMS,Max values ​​of different widths in the above table into formula (1), three empirical fitting parameters A, B, and C can be derived, as shown in Table 3.

[0051]

[0052] Table 3

[0053] The above-mentioned test method for the current capability of polysilicon resistors can quickly test the current capability (maximum allowable current) of polysilicon resistors according to the actual customer application conditions. In addition to the tested samples, the current capability of polysilicon resistors of different widths can be derived. This test method can be tested using a probe station and a tester with resistance value test function, without the need for special test equipment; the test cycle is short (less than one day), and the current capability of polysilicon interconnects and polysilicon resistors can be quickly evaluated, which is simple and easy to operate.

[0054] The present application also provides a system for testing the current capability of a polysilicon resistor, comprising:

[0055] Probe station, including heated stage plate.

[0056] A tester is used to test the resistance value of polysilicon samples.

[0057] A control module, for controlling the heating plate to heat the polysilicon sample on the heating plate to a preset temperature, and controlling the tester to test the resistance value of the polysilicon sample at the preset temperature, wherein the preset temperature is the sum of the expected upper limit of the application temperature of the polysilicon resistor and the Joule heat upper limit of the polysilicon resistor; for controlling the heating plate to heat the polysilicon sample on the heating plate to the expected upper limit of the application temperature, and controlling the tester to apply current to the multiple samples respectively to obtain the current values ​​corresponding to the samples when the resistance value is reached; for substituting the current values ​​of the multiple polysilicon samples as maximum root mean square current density into a maximum root mean square current density model to calculate the empirical parameters of the maximum root mean square current density model; and for substituting the width of a first type of polysilicon resistor to be evaluated having a first width into the maximum root mean square current density model to calculate the maximum root mean square current density as the maximum safe current of the first type of polysilicon resistor with the first width.

[0058] The plurality of polysilicon samples are all polysilicon resistors of the first type, and different samples have different widths.

[0059] In the description of this specification, the description with reference to the terms "some embodiments", "other embodiments", "ideal embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example.

[0060] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0061] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for testing the current capability of polysilicon, comprising: obtaining a plurality of samples of a first type of polysilicon, different samples having different widths; Adjusting the temperature of the plurality of samples to a preset temperature, and testing the resistance value of each sample at the preset temperature; the preset temperature is the sum of the expected upper limit of the application temperature of polysilicon and the temperature change value caused by the Joule heat upper limit of polysilicon; Adjusting the temperature of the plurality of samples to the expected upper limit of the application temperature, and applying current to the plurality of samples respectively, to obtain the current value corresponding to each sample when the resistance value reaches the respective sample; Substituting each current value as a maximum root mean square current density into a maximum root mean square current density model to calculate empirical parameters of the maximum root mean square current density model; Substituting the width of the first type of polysilicon to be evaluated with a first width into the maximum root mean square current density model, the maximum root mean square current density is calculated as the maximum safe current of the first type of polysilicon with the first width.

2. The method for testing the current capability of polysilicon according to claim 1, characterized in that: The maximum root mean square current density model is a function with the width of polysilicon as a variable, and the empirical parameters include a constant term, a negative first-order term coefficient, and a negative second-order term coefficient.

3. The method for testing the current capability of polysilicon according to claim 2, characterized in that: The function is: J=A+B / WD+C / (WD*WD); Among them, J is the maximum root mean square current density, A is the constant term, B is the negative first-order term coefficient, C is the negative second-order term coefficient, WD is the width of polysilicon; the step of obtaining multiple samples of the first type of polysilicon is to obtain 3 samples.

4. The method for testing the current capability of polysilicon according to claim 1, characterized in that: In the step of testing the resistance value of each sample at the preset temperature, the test current is smaller than the current applied in the step of applying current to the multiple samples respectively.

5. The method for testing the current capability of polysilicon according to claim 1, characterized in that: The step of obtaining a plurality of samples of the first type of polysilicon includes obtaining polysilicon resistor samples with widths of 1 micron, 5 microns and 10 microns.

6. The method for testing the current capability of polysilicon according to claim 5, characterized in that: In the step of obtaining a plurality of samples of the first type of polysilicon, the length of the obtained samples is 50 microns.

7. The method for testing the current capability of polysilicon according to claim 1, characterized in that: The first type of polysilicon is polysilicon with a positive temperature coefficient or a negative temperature coefficient.

8. The method for testing the current capability of polysilicon according to claim 1, characterized in that: The first type of polysilicon includes polysilicon and a metal silicide layer on the polysilicon and exhibits a positive temperature coefficient characteristic; or The first type of polysilicon has no metal silicide layer and exhibits a negative temperature coefficient characteristic.

9. The method for testing the current capability of polysilicon according to claim 1, characterized in that: The step of adjusting the temperature of the plurality of samples to a preset temperature and the step of adjusting the temperature of the plurality of samples to the upper limit of the expected application temperature are to heat the samples on the heating plate of the probe station.

10. A system for testing the current capability of polysilicon, characterized in that: include: Probe station, including heated stage plate; A tester, used to test the resistance value of polysilicon samples; A control module, for controlling the heating plate to heat the polysilicon sample on the heating plate to a preset temperature, and controlling the tester to test the resistance value of the polysilicon sample at the preset temperature, wherein the preset temperature is the sum of the expected upper limit of the application temperature of the polysilicon and the temperature change value caused by the Joule heat upper limit of the polysilicon; for controlling the heating plate to heat the polysilicon sample on the heating plate to the expected upper limit of the application temperature, and controlling the tester to apply current to multiple polysilicon samples respectively to obtain the current values ​​corresponding to the samples when the resistance value is reached; for substituting the current values ​​of the multiple polysilicon samples as maximum root mean square current densities into a maximum root mean square current density model to calculate the empirical parameters of the maximum root mean square current density model; for substituting the width of the first type of polysilicon to be evaluated having a first width into the maximum root mean square current density model to calculate the maximum root mean square current density as the maximum safe current of the first type of polysilicon with the first width; The plurality of polysilicon samples are all polysilicon of the first type, and different samples have different widths.

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