Tracking temperature compensation of an x/y stress independent resistor

An integrated circuit comprises a semiconductor substrate having a surface. A lateral resistor is arranged in a first plane parallel to the surface of the substrate. A vertical reference resistor comprises a layer arranged in a second plane parallel to the surface of the substrate and deeper than the first plane. This layer is doped to promote current flow in the second plane. The vertical reference resistor further comprises a first trench and a second trench coupled between the layer and the surface of the substrate. The first and second trenches are arranged in a vertical direction orthogonal to the first and the second planes and are doped to impede current flow in the vertical direction. A cross-section of the first and second trenches is two-fold rotationally symmetric around the vertical direction, and the lateral resistor and the first and second trenches have the same temperature coefficient.

BACKGROUND

Mechanical stress and temperature can cause changes to a semiconductor die, for example by changing the dimensions or mobility of devices on the die. Such changes can cause modifications to circuit parameters associated with the devices, such as the frequency of an integrated oscillator and resistivity of resistors, which in turn changes the behavior of the integrated circuits (ICs) including the devices. Some circuit parameters like bandgap voltage and oscillator frequency respond differently to stress in the x direction than stress in the y direction. Determining the magnitude and direction of stress components allows proper compensation for mechanical stresses on the chip, chip temperature, and the resulting influence on devices on the chip. Some stress sensing circuits include resistors in the x and y directions to determine stress components in each direction. The resistance from the sensing resistors is compared to the resistance from a reference resistor in the z direction. However, in some examples the reference resistor has a different temperature coefficient than the sensing resistors, which can cause improper adjustments due to confusion between temperature and stress effects. Further, in some examples the resistance of the reference resistor is influenced by in-plane stresses as well, masking the correct magnitude of stress components.

SUMMARY

In some implementations, an integrated circuit comprises a semiconductor substrate having a surface and a vertical reference resistor. The vertical reference resistor comprises a layer arranged in a plane parallel to the surface of the substrate. The layer is doped to promote current flow in the plane. The vertical reference resistor further comprises a first trench and a second trench coupled between the layer and the surface of the substrate. The first and second trenches are arranged in a vertical direction orthogonal to the plane and the surface, and are doped to impede current flow in the vertical direction. A cross-section of the first and second trenches is two-fold rotationally symmetric around the vertical direction.

In some examples, the integrated circuit also includes a lateral resistor arranged on a second plane parallel to the surface of the substrate. The second plane is shallower than the plane in which the vertical reference resistor's layer is arranged. The lateral resistor and the first and second trenches have the same temperature coefficients. In some examples, the lateral resistor and the first and second trenches have the same doping.

In some examples, the first and second trenches are separated by an isolating structure. In some examples, a cross-section of the second trench is pin-hole shaped. The integrated circuit can be used as a stress sensing circuit. In these examples, the integrated circuit includes the lateral resistor, a first current source coupled to the lateral resistor, a second current source coupled to the vertical reference resistor, and an amplifier coupled to the lateral resistor and the vertical reference resistor. The amplifier is configured to output a voltage difference signal between a voltage on the lateral resistor and a voltage on the vertical reference resistor. The voltage difference signal indicates a magnitude and direction of an in-plane stress on the integrated circuit.

DETAILED DESCRIPTION

Some stress sensing circuits include resistors arranged in a lateral plane parallel to a surface of the semiconductor die including the stress sensing circuits. The resistors in the lateral plane are arranged perpendicular to one another and are used to determine stress components in the respective directions within the lateral plane. The resistances from the sensing resistors are compared to the resistance from a reference resistor in a vertical direction perpendicular to the lateral plane. However, some stress sensing circuits include reference resistors having a different temperature coefficient than the sensing resistors, which can cause improper adjustments due to confusion between temperature and stress effects. Further, in some examples the reference resistor is influenced by in-plane stresses as well, masking the correct magnitude of stress components.

The disclosed reference resistor for use in stress sensing circuits has the same temperature dependency as the associated sensing resistor because the two resistors have substantially the same dopings. This ensures the sensing and reference resistors have the same temperature coefficient and respond to temperature changes in substantially the same way, preventing confusion of different temperature responses for actual stress on the sensing resistor. The disclosed reference resistor is also symmetric in the x and y directions, ensuring the resistance of the reference resistor is largely independent of the direction of in-plane stresses.

An example reference resistor includes a buried layer that is highly doped to promote current flow and exhibit low resistance. The buried layer is arranged in a lateral plane parallel to the surface of a semiconductor die including the example reference resistor. The example reference resistor also includes deep vertical paths arranged perpendicular to the lateral plane including the buried layer. The deep vertical paths are side wall doped to exhibit high resistance in the vertical direction, perpendicular to the buried layer, with substantially the same temperature coefficient as an associated sensing resistor. The depth and the doping of the vertical paths and the doping and thickness of the buried layer cause the example reference resistor to experience substantially all vertical current flow. A cross-section of the deep vertical paths is two-fold rotationally symmetric, such as a pinhole or a toroid inside a larger toroid. The vertical current flow and symmetric layout of the deep vertical paths in the x and y directions reduce stress direction dependencies of the example reference resistor.

FIG. 1shows a graph demonstrating how an oscillator responds differently to stress in two orthogonal directions. The graph shown inFIG. 1is from the reference, “Electrical Compensation of Mechanical Stress Drift in Precision Analog Circuit”, M. Motz, U. Ausserlechner, Springer, 2016, and demonstrates the percentage drift in output frequency of a resistor/capacitor (RC) relaxation oscillator as stress is applied to the integrated circuit (IC) on which the oscillator is implemented. The oscillator responds differently to stress in one direction than in another, experiencing different percentage drift in output frequency depending on the direction of stress applied to it. In this example, stress along the x-axis, σxx, causes a much larger shift in the oscillator's output frequency than stress along the y-axis, σyy. Further, stress along the x-axis causes an increase in output frequency, while stress along the y-axis causes a decrease in output frequency.

Temperature can also affect components of an IC.FIG. 2shows a graph demonstrating the effect of temperature on an example reference resistor210and an example sensing resistor220with different dopings. The different doping between the reference resistor210and the sensing resistor220causes the two resistors to have different temperature dependencies. If the reference resistor210and the sensing resistor220are used in a stress sensing circuit, ambiguity will arise from the different temperature dependencies. At high or low temperatures, the reference and sensing resistors210and220will behave differently without any stress applied, yet the stress sensing circuit will identify the difference between the reference and sensing resistors210and220and treat it as a stress applied to the IC. The mistakenly identified stress will prompt unnecessary and inaccurate adjustments in other circuits on the IC.

FIG. 3illustrates an example stress sensing element300. A semiconductor wafer305, e.g., a silicon wafer, is sawn from a single crystal rod such that the wafer surface is associated to a crystallographic plane. Miller indices, indicated by curly brackets { }, are used to determine the respective plane in a cubic crystal.FIG. 3depicts a top view of stress sensing element300on a p-type semiconductor substrate305that is cut in the {100} plane and provided with a notch along the [100] direction. Although not a limitation, the examples described herein utilize p-type semiconductor wafers cut in the {100} plane. The wafer305shown inFIG. 3is an example; the stress sensing elements and reference resistors described herein are not limited to a wafer with a [100] notch, a {100} plane, or p-type doping. In other examples, an n-type semiconductor substrate is used. The n-type semiconductor wafer can be cut in any appropriate plane, such as {100}.

Stress sensing element300includes a reference resistor310and a sensing resistor320. In this example, reference resistor310and sensing resistor320are n-type resistors. In some examples, reference resistor310and sensing resistor320are p-type resistors. In other examples, reference resistor310and sensing resistor320utilize different doping types, such as mixed doping. Sensing resistor320includes a first resistor element320A aligned in the y direction and a second resistor element320B aligned in the x direction. The alignment of resistor elements320A and320B causes most current flow through resistor elements320A and320B to be either longitudinal or transverse to the [100] crystal axis. Current flows through resistor element320A longitudinal to the crystal axis. Current flows through resistor element320B transverse to the [100] crystal axis direction, in the [010] direction. In other examples, resistor elements320A and320B are aligned such that current flow through resistor elements320A and320B is either longitudinal or transverse to the [110] crystal axis. Other resistor orientations are possible as well. For example, resistor elements320A and320B can be rotated 45 degrees from alignment with the x and y axes. In another example, one of resistor elements320A and320B includes multiple lines of resistive elements, such that the ratio of resistor element320A to resistor element320B is not one to one. Reference resistor310is a vertical resistor aligned in the z direction.

FIG. 4illustrates an example stress sensing circuit400including the stress sensing element300shown inFIG. 3. Stress sensing circuit400also includes two current sources and a difference circuit. Current source415is coupled to a supply voltage node405and to reference resistor310. In this example, reference resistor310is further coupled to common mode node410. Current source415applies a current to reference resistor310, causing a voltage drop across reference resistor310. This voltage Vref420is provided to difference circuit440from a point between current source415and reference resistor310. Current source425is coupled to a supply voltage node405and to sensing resistor320. Sensing resistor320is further coupled to common mode node410. Current source425applies a current to sensing resistor320, causing a voltage drop across sensing resistor320. This voltage Vsense430is provided to difference circuit440from a point between current source425and sensing resistor320. In other examples, reference resistor310and sensing resistor320are coupled to a supply voltage at node410. In this example, sensing resistor320includes both resistor element320A and resistor element320B coupled together in series. In other examples, a separate stress sensing circuit400is included for each of resistor element320A and resistor element320B.

Difference circuit440outputs a voltage difference signal Vdiff450. In some examples, difference circuit440is an amplifier. Vdiff450represents the difference between Vsense430and Vref420, and can be used to determine values for stress components in the transverse and longitudinal directions to the [100] crystal axis. In turn, the values for these stress components can be used to determine appropriate adjustments to operation in other circuits on the IC. To determine correct values for stress components in the transverse and longitudinal directions, reference resistor310is substantially independent from the direction of in-plane stresses. Further, reference resistor310and sensing resistor320have the same temperature coefficients.

For example, sensing resistor320and reference resistor310have expected resistance values and corresponding expected values for Vsense and Vref, and by extension an expected value of Vdiff. Because reference resistor310and sensing resistor have the same temperature coefficient, any difference between the actual and expected values for Vsense and Vref due to the temperature of the semiconductor die including stress sensing circuit400is cancelled out. Thus, a difference between the actual and expected values of Vdiff are likely due to in-plane stresses on the semiconductor die.

Reference resistor310is substantially independent from the direction of in-plane stresses, and so the difference between the actual and expected values of Vdiff are likely due to changes in Vsense from changes in the resistance of sensing resistor320. The perpendicular arrangement of resistor element320A and resistor element320B allows the difference between the actual and expected Vdiff to be apportioned according to the direction and magnitude of in-plane stress components on the semiconductor die. The direction and magnitude of in-plane stress components can be provided to other circuits on the semiconductor die, and used to determine appropriate calibration parameters for devices on the die.

FIGS. 5A-Billustrate an example reference resistor500for use in a stress sensing element (e.g., to implement reference resistor310). Reference resistor500is an n-type, vertical resistor. In other examples, reference resistor500is a p-type resistor.FIG. 5Aillustrates a cross section of reference resistor500. A deep n-well560is implanted into p-type substrate570, which can include an epitaxial layer (not specifically shown). In other examples, substrate570is an n-type substrate. Deep n-well560forms a buried layer and is highly doped to promote current flow and exhibit low resistance. Trenches550are side wall doped deep trenches contacting opposite ends of deep n-well560, and are highly doped for horizontal current flow and lower doped for vertical current flow. This causes trenches550to have a first piezo-resistive coefficient for current flow in lateral directions and a second, higher piezo-resistive coefficient for current flow in vertical directions. Moreover, a greater portion of the resistive path is in the vertical direction, along trenches550, while a smaller portion of the resistive path is in the lateral direction.

N-wells535are implanted into the surface of substrate570to contact trenches550, followed by the implantation of p-well540. Dielectric layer520is then formed to cover the surface of substrate570. N contacts530are implanted in n-wells535, and p contact525is implanted in p-well540. Inter-level dielectric515is deposited before vias510are formed to n contacts530and p contact525. Then metallization layer505is formed over vias510.

Current580flows from n-well535A, through trench550A to deep n-well560. Current580flows the length of deep n-well560to trench550B, up trench550B and through n-well535B. The depth and lower doping concentration of trenches550for current flow in the vertical direction causes reference resistor500to exhibit higher vertical resistance and larger voltage differences across trenches550. In contrast, the thickness and higher doping concentration of deep n-well560results in a lower resistive path for current flow in the lateral directions, causing reference resistor500to exhibit lower lateral resistance and a smaller voltage difference across deep n-well560.

Although described as “trenches” inFIG. 5A, any appropriate deep vertical path can be used. The disclosed reference resistors include two deep vertical paths with a lower doping concentration for current flow in the vertical direction, which cause the disclosed reference resistors to exhibit higher vertical resistance and larger voltage differences across the deep vertical paths. The two deep vertical paths are coupled together by a deep well with a higher doping concentration for current flow in the lateral directions, which cause the disclosed reference resistors to exhibit lower lateral resistance and a smaller voltage difference across the deep well. The primarily vertical current flow causes the disclosed reference resistors to be largely independent of the direction of in-plane stresses.

FIG. 5Billustrates a top down view590and an angled view595of a cross section of trenches550of reference resistor500. The cross-section of trenches550is symmetric in the x and y directions, in this example a square shape. In other examples, the inner trenches550B are in a pinhole or other symmetric shape. Trenches550and deep n-well560are two-fold rotationally symmetric around the z-axis of the reference resistor500, but not symmetric when rotated in the z direction. The x and y symmetric cross-section of trenches550and the primarily vertical current flow through reference resistor500reduces stress direction dependencies such that reference resistor500is largely independent from the direction of in-plane stress on the IC. Reference resistor500is primarily resistant in the vertical direction. The doping in trenches550is the same as doping within an associated sensing resistor, such that the reference resistor500and the associated sensing resistor have the same temperature coefficient. It will be understood that the drawings of resistor500are not necessarily drawn to scale.

FIG. 6illustrates alternative cross-sections of trenches550of reference resistor500. Example layout610shows trench550A in a square shape and trench550B in a smaller square shape inside the square formed by trench550A. Example layout620shows trench550A in a circular shape and trench550B in a smaller circular shape inside the circle formed by trench550A. Example layout630shows trench550A in a square shape rotated 45 degrees from the x and y axes and trench550B in a smaller square shape rotated 45 degrees from the x and y axes inside the rotated square formed by trench550A. Example layout640shows trench550A in a circular shape and trench550B in a circular pinhole shape inside the circle formed by trench550A. Example layout650shows trench550A in a square shape and trench550B in a square pinhole shape inside the square formed by trench550A. In each of example layouts610,620,630,640, and650, trenches550are symmetric and experience substantially the same amount of stress in the x direction as in the y direction. The example layouts shown inFIG. 6are not the complete set of possible layouts for trenches550. Other symmetric layouts can be used as well.

The examples described herein utilize p-type semiconductor wafers cut in the {100} plane. However, the reference resistors described herein are not limited to a wafer with a [100] notch, a {100} plane, or p-type doping. In other examples, an n-type semiconductor substrate is used. The n-type semiconductor wafer can be cut in any appropriate plane, such as {100}.

In this description, the term “couple” or “couples” means either an indirect or direct wired or wireless connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. The recitation “based on” means “based at least in part on.” Therefore, if X is based on Y, X may be a function of Y and any number of other factors. Unless otherwise stated, in this description, “the same” or “substantially” or “largely” the same means the two are within ten percent of each other, “substantially” or “largely” unaffected means less than a ten percent change, and “substantially” all means ninety percent or more.